Journal of Experimental Botany - Advance Access

Phylogenetically-based variation in the regulation of the Calvin cycle enzymes, phosphoribulokinase and glyceraldehyde-3-phosphate dehydrogenase, in algae

Maberly, S. C., Courcelle, C., Groben, R., Gontero, B. — Thu, 19 Nov 2009 05:39:08 PST

Aquatic photosynthesis is responsible for about half of the global production and is undertaken by a huge phylogenetic diversity of algae that are poorly studied. The diversity of redox-regulation of phosphoribulokinase (PRK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was investigated in a wide range of algal groups under standard conditions. Redox-regulation of PRK was greatest in chlorophytes, low or absent in a red alga and most chromalveolates, and linked to the number of amino acids between two regulatory cysteine residues. GAPDH regulation was not strongly-related to the different forms of this enzyme and was less variable than for PRK. Addition of recombinant CP12, a protein that forms a complex with PRK and GAPDH, to crude extracts inhibited GAPDH and PRK inversely in the Plantae, but in most chromalveolates had little effect on GAPDH and inhibited or stimulated PRK depending on the species. Patterns of enzyme regulation were used to produce a phylogenetic tree in which cryptophytes and haptophytes, at the base of the chromalveolates, formed a distinct clade. A second clade comprised only chromalveolates. A third clade comprised a mixture of Plantae, an excavate and three chromalveolates: a marine diatom and two others (a xanthophyte and eustigmatophyte) that are distinguished by a low content of chlorophyll c and a lack of fucoxanthin. Regulation of both enzymes was greater in freshwater than in marine taxa, possibly because most freshwaters are more dynamic than oceans. This work highlights the importance of understanding enzyme regulation in diverse algae if their ecology and productivity is to be understood.

Sporophytic control of pollen tube growth and guidance in maize

Lausser, A., Kliwer, I., Srilunchang, K.-o., Dresselhaus, T. — Thu, 19 Nov 2009 05:39:04 PST

Pollen tube germination, growth, and guidance (progamic phase) culminating in sperm discharge is a multi-stage process including complex interactions between the male gametophyte as well as sporophytic tissues and the female gametophyte (embryo sac), respectively. Inter- and intra-specific crossing barriers in maize and Tripsacum have been studied and a precise description of progamic pollen tube development in maize is reported here. It was found that pollen germination and initial tube growth are rather unspecific, but an early, first crossing barrier was detected before arrival at the transmitting tract. Pollination of maize silks with Tripsacum pollen and incompatible pollination of Ga1s/Ga1s-maize silks with ga1-maize pollen revealed another two incompatibility barriers, namely transmitting tract mistargeting and insufficient growth support. Attraction and growth support by the transmitting tract seem to play key roles for progamic pollen tube growth. After leaving transmitting tracts, pollen tubes have to navigate across the ovule in the ovular cavity. Pollination of an embryo sac-less maize RNAi-line allowed the role of the female gametophyte for pollen tube guidance to be determined in maize. It was found that female gametophyte controlled guidance is restricted to a small region around the micropyle, approximately 50–100 µm in diameter. This area is comparable to the area of influence of previously described ZmEA1-based short-range female gametophyte signalling. In conclusion, the progamic phase is almost completely under sporophytic control in maize.

Determinants of ozone fluxes and metrics for ozone risk assessment in plants

Fares, S., Goldstein, A., Loreto, F. — Wed, 18 Nov 2009 06:43:17 PST

Tropospheric ozone concentration is increasing and represents a threat to single plants and whole ecosystems. The deleterious ozone effects mainly occur when (i) ozone concentration in the air builds up; (ii) the pollutant enters the leaf through stomatal uptake, and (iii) ozone-produced reactive oxygen species are not efficiently scavenged by leaf antioxidants and then oxidize leaf tissues. The sensitivity of plants to ozone is species-specific, and a correct risk assessment should be based on a metric that correctly takes into account the ambient concentration of ozone, the physiological control on stomatal apertures, and the efficiency of leaf antioxidant system. Current methodologies have been analysed to evaluate ozone risk assessment, and, by phasing-in and phasing out sources and sinks of ozone, elements of improvements for the current metrics have been suggested.

An analysis of dormancy, ABA responsiveness, after-ripening and pre-harvest sprouting in hexaploid wheat (Triticum aestivum L.) caryopses

Gerjets, T., Scholefield, D., Foulkes, M. J., Lenton, J. R., Holdsworth, M. J. — Wed, 18 Nov 2009 06:43:10 PST

Embryo and caryopsis dormancy, abscisic acid (ABA) responsiveness, after-ripening (AR), and the disorder pre-harvest sprouting (PHS) were investigated in six genetically related wheat varieties previously characterized as resistant, intermediate, or susceptible to PHS. Timing of caryopsis AR differed between varieties; AR occurred before harvest ripeness in the most PHS-susceptible, whereas AR was slowest in the most PHS-resistant. Whole caryopses of all varieties showed little ABA-responsiveness during AR; PHS-susceptible varieties were responsive at the beginning of the AR period whereas PHS-resistant showed some responsiveness throughout. Isolated embryos showed relatively little dormancy during grain-filling and most varieties exhibited a window of decreased ABA-responsiveness around the period of maximum dry matter accumulation (physiological maturity). Susceptibility to PHS was assessed by overhead misting of either isolated ears or whole plants during AR; varieties were clearly distinguished using both methods. These analyses allowed an investigation of the interactions between the different components of seed development, compartments, and environment for the six varieties. There was no direct relationship between speed of caryopsis AR and embryo dormancy or ABA-responsiveness during seed maturation. However, the velocity of AR of a variety was closely associated with the degree of susceptibility to PHS during AR suggesting that these characters are developmentally linked. Investigation of genetic components of AR may therefore aid breeding approaches to reduce susceptibility to PHS.

Sulphur flux through the sulphate assimilation pathway is differently controlled by adenosine 5'-phosphosulphate reductase under stress and in transgenic poplar plants overexpressing {gamma}-ECS, SO, or APR

Wed, 18 Nov 2009 06:43:01 PST

Sulphate assimilation provides reduced sulphur for the synthesis of cysteine, methionine, and numerous other essential metabolites and secondary compounds. The key step in the pathway is the reduction of activated sulphate, adenosine 5'-phosphosulphate (APS), to sulphite catalysed by APS reductase (APR). In the present study, [³⁵S]sulphur flux from external sulphate into glutathione (GSH) and proteins was analysed to check whether APR controls the flux through the sulphate assimilation pathway in poplar roots under some stress conditions and in transgenic poplars. (i) O-Acetylserine (OAS) induced APR activity and the sulphur flux into GSH. (ii) The herbicide Acetochlor induced APR activity and results in a decline of GSH. Thereby the sulphur flux into GSH or protein remained unaffected. (iii) Cd treatment increased APR activity without any changes in sulphur flux but lowered sulphate uptake. Several transgenic poplar plants that were manipulated in sulphur metabolism were also analysed. (i) Transgenic poplar plants that overexpressed the -glutamylcysteine synthetase (-ECS) gene, the enzyme catalysing the key step in GSH formation, showed an increase in sulphur flux into GSH and sulphate uptake when -ECS was targeted to the cytosol, while no changes in sulphur flux were observed when -ECS was targeted to plastids. (ii) No effect on sulphur flux was observed when the sulphite oxidase (SO) gene from Arabidopsis thaliana, which catalyses the back reaction of APR, that is the reaction from sulphite to sulphate, was overexpressed. (iii) When Lemna minor APR was overexpressed in poplar, APR activity increased as expected, but no changes in sulphur flux were observed. For all of these experiments the flux control coefficient for APR was calculated. APR as a controlling step in sulphate assimilation seems obvious under OAS treatment, in -ECS and SO overexpressing poplars. A possible loss of control under certain conditions, that is Cd treatment, Acetochlor treatment, and in APR overexpressing poplar, is discussed.

Relationship of source and sink in determining kernel composition of maize

Mon, 16 Nov 2009 05:54:18 PST

The relative role of the maternal source and the filial sink in controlling the composition of maize (Zea mays L.) kernels is unclear and may be influenced by the genotype and the N supply. The objective of this study was to determine the influence of assimilate supply from the vegetative source and utilization of assimilates by the grain sink on the final composition of maize kernels. Intermated B73xMo17 recombinant inbred lines (IBM RILs) which displayed contrasting concentrations of endosperm starch were grown in the field with deficient or sufficient N, and the source supply altered by ear truncation (45% reduction) at 15 d after pollination (DAP). The assimilate supply into the kernels was determined at 19 DAP using the agar trap technique, and the final kernel composition was measured. The influence of N supply and kernel ear position on final kernel composition was also determined for a commercial hybrid. Concentrations of kernel protein and starch could be altered by genotype or the N supply, but remained fairly constant along the length of the ear. Ear truncation also produced a range of variation in endosperm starch and protein concentrations. The C/N ratio of the assimilate supply at 19 DAP was directly related to the final kernel composition, with an inverse relationship between the concentrations of starch and protein in the mature endosperm. The accumulation of kernel starch and protein in maize is uniform along the ear, yet adaptable within genotypic limits, suggesting that kernel composition is source limited in maize.

Response to UV-C radiation in topo I-deficient carrot cells with low ascorbate levels

Mon, 16 Nov 2009 05:54:17 PST

In animal cells, recent studies have emphasized the role played by DNA topoisomerase I (topo I) both as a cofactor of DNA repair complexes and/or as a damage sensor. All these functions are still unexplored in plant cells, where information concerning the relationships between DNA damage, PCD induction, and topo I are also limited. The main goal of this study was to investigate the possible responses activated in topo I-depleted plant cells under oxidative stress conditions which induce DNA damage. The carrot (Daucus carota L.) AT1-β/22 cell line analysed in this study (characterized by an antisense-mediated reduction of top1β gene expression of approximately 46% in association with a low ascorbate content) was more sensitive to UV-C radiation than the control line, showing consistent cell death and high levels of 8-oxo-dG accumulation. The topo I-depleted cells were also highly susceptible to the cross-linking agent mitomycin C. The death response was associated with a lack of oxidative burst and there were no changes in ascorbate metabolism in response to UV-C treatment. Electron and fluorescence microscopy suggested the presence of three forms of cell death in the UV-C-treated AT1-β/22 population: necrosis, apoptotic-like PCD, and autophagy. Taken together, the data reported here support a reduced DNA repair capability in carrot topo I-deficient cells while the putative relationship between topo I-depletion and ascorbate impairment is also discussed.

Differential gene expression analysis provides new insights into the molecular basis of iron deficiency stress response in the citrus rootstock Poncirus trifoliata (L.) Raf.

Fri, 13 Nov 2009 05:19:58 PST

Iron chlorosis is one of the major abiotic stresses affecting fruit trees and other crops in calcareous soils and leads to a reduction in growth and yield. Usual remediation strategies consist of amending iron to soil, which is an expensive practice, or using tolerant cultivars, which are difficult to develop when not available. To understand the mechanisms underlying the associated physiopathy better, and thus develop new strategies to overcome the problems resulting from iron deficiency, the differential gene expression induced by iron deficiency in the susceptible citrus rootstock Poncirus trifoliata (L.) Raf. have been examined. The genes identified are putatively involved in cell wall modification, in determining photosynthesis rate and chlorophyll content, and reducing oxidative stress. Additional studies on cell wall morphology, photosynthesis, and chlorophyll content, as well as peroxidase and catalase activities, support their possible functions in the response to iron deficiency in a susceptible genotype, and the results are discussed.

A different role for hydrogen peroxide and the antioxidative system under short and long salt stress in Brassica oleracea roots

Hernandez, M., Fernandez-Garcia, N., Diaz-Vivancos, P., Olmos, E. — Wed, 11 Nov 2009 05:54:25 PST

Salinity affects normal growth and development of plants depending on their capacity to overcome the induced stress. The present study was focused on the response and regulation of the antioxidant defence system in Brassica oleracea roots under short and long salt treatments. The function and the implications of hydrogen peroxide as a stressor or as a signalling molecule were also studied. Two different zones were analysed—the elongation and differentiation zone and the fully differentiated root zone—in order to broaden the knowledge of the different effects of salt stress in root. In general, an accumulation of hydrogen peroxide was observed in both zones at the highest (80 mM NaCl) concentration. A higher accumulation of hydrogen peroxide was observed in the stele of salt-treated roots. At the subcellular level, mitochondria accumulated hydrogen peroxide in salt-treated roots. The results confirm a drastic decrease in the antioxidant enzymes catalase, ascorbate peroxidase, and peroxidases under short salt treatments. However, catalase and peroxidase activities were recovered under long salt stress treatments. The two antioxidant molecules analysed, ascorbate and glutathione, showed a different trend during salt treatments. Ascorbate was progressively accumulated and its redox state maintained, but glutathione was highly accumulated at 24 h of salt treatment, but then its concentration and redox state progressively decreased. Concomitantly, the antioxidant enzymes involved in ascorbate and glutathione regeneration were modified under salt stress treatments. In conclusion, the increase in ascorbate levels and the maintenance of the redox state seem to be critical for root growth and development under salt stress.

Silencing Sl-EBF1 and Sl-EBF2 expression causes constitutive ethylene response phenotype, accelerated plant senescence, and fruit ripening in tomato

Yang, Y., Wu, Y., Pirrello, J., Regad, F., Bouzayen, M., Deng, W., Li, Z. — Tue, 10 Nov 2009 05:26:12 PST

The hormone ethylene regulates a wide range of plant developmental processes and EBF (EIN3-binding F-box) proteins were shown to negatively regulate the ethylene signalling pathway via mediating the degradation of EIN3/EIL proteins. The present study reports on the identification of two tomato F-box genes, Sl-EBF1 and Sl-EBF2 from the EBF subfamily. The two genes display contrasting expression patterns in reproductive and vegetative tissues and in response to ethylene and auxin treatment. Sl-EBF1 and Sl-EBF2 genes are actively regulated at crucial stages in the development of the reproductive organs. Their dynamic expression in flowers during bud-to-anthesis and anthesis-to-post-anthesis transitions, and at the onset of fruit ripening, suggests their role in situations where ethylene is required for stimulating flower opening and triggering fruit ripening. VIGS-mediated silencing of a single tomato EBF gene uncovered a compensation mechanism that tends to maintain a threshold level of Sl-EBF expression via enhancing the expression of the second Sl-EBF gene. In line with this compensation, tomato plants silenced for either of the Sl-EBF genes were indistinguishable from control plants, indicating functional redundancy among Sl-EBF genes. By contrast, co-silencing of both Sl-EBFs resulted in ethylene-associated phenotypes. While reports on EBF genes to date have focused on their role in modulating ethylene responses in Arabidopsis, the present study uncovered their role in regulating crucial stages of flower and fruit development in tomato. The data support the hypothesis that protein degradation via the ubiquitin/26S proteasome pathway is a control point of fruit ripening and open new leads for engineering fruit quality.

Ectopic expression of PtaRHE1, encoding a poplar RING-H2 protein with E3 ligase activity, alters plant development and induces defence-related responses

Thu, 05 Nov 2009 02:18:30 PST

RING (really interesting new gene)-H2 domain-containing proteins are widely represented in plants and play important roles in the regulation of many developmental processes as well as in plant–environment interactions. In the present report, experiments were performed to unravel the role of the poplar gene PtaRHE1, coding for a RING-H2 protein. In vitro ubiquitination assays indicate a functional E3 ligase activity for PtaRHE1 with the specific E2 ubiquitin-conjugating enzyme UbcH5a. The overexpression of PtaRHE1 in tobacco resulted in a pleiotropic phenotype characterized by a curling of the leaves, the formation of necrotic lesions on leaf blades, growth retardation, and a delay in floral transition. The plant gene expression response to PtaRHE1 overexpression provided evidence for the up-regulation of defence- and/or programmed cell death-related genes. Moreover, genes coding for WRKY transcription factors as well as for mitogen-activated protein kinases, such as wound-induced protein kinase (WIPK), were also found to be induced in the transgenic lines as compared with the wild type. In addition, histochemical β-glucuronidase staining showed that the PtaRHE1 promoter is induced by plant pathogens and by elicitors such as salicylic acid and cellulase. Taken together, these results suggest that the E3 ligase PtaRHE1 plays a role in the ubiquitination-mediated regulation of defence response, possibly by acting upstream of WIPK and/or in the activation of WRKY factors.

The constitutive expression of Arabidopsis plasmodesmal-associated class 1 reversibly glycosylated polypeptide impairs plant development and virus spread

Zavaliev, R., Sagi, G., Gera, A., Epel, B. L. — Tue, 03 Nov 2009 03:52:28 PST

Arabidopsis class 1 reversibly glycosylated polypeptides (^C1RGPs) were shown to be plasmodesmal-associated proteins. Transgenic tobacco (Nicotiana tabacum) plants constitutively expressing GFP tagged AtRGP2 under the control of the CaMV 35S promoter are stunted, have a rosette-like growth pattern, and in source leaves exhibit strong chlorosis, increased photoassimilate retention and starch accumulation that results in elevated leaf specific fresh and dry weights. Basal callose levels around plasmodesmata (Pd) of leaf epidermal cells in transgenic plants are higher than in WT. Such a phenotype is characteristic of virus-infected plants and some transgenic plants expressing Pd-associated viral movement proteins (MP). The local spread of Tobacco mosaic virus (TMV) is inhibited in AtRGP2:GFP transgenics compared to WT. Taken together these observations suggest that overexpression of the AtRGP2:GFP leads to a reduction in Pd permeability to photoassimilate, thus lowering the normal rate of translocation from source leaves to sink organs. Such a reduction may also inhibit the local cell-to-cell spread of viruses in transgenic plants. The observed reduction in Pd permeability could be due to a partial Pd occlusion caused either by the accumulation of AtRGP2:GFP fusion in Pd, and/or by constriction of Pd by the excessive callose accumulation.

A nematode effector protein similar to annexins in host plants

Tue, 03 Nov 2009 03:52:27 PST

Nematode parasitism genes encode secreted effector proteins that play a role in host infection. A homologue of the expressed Hg4F01 gene of the root-parasitic soybean cyst nematode, Heterodera glycines, encoding an annexin-like effector, was isolated in the related Heterodera schachtii to facilitate use of Arabidopsis thaliana as a model host. Hs4F01 and its protein product were exclusively expressed within the dorsal oesophageal gland secretory cell in the parasitic stages of H. schachtii. Hs4F01 had a 41% predicted amino acid sequence identity to the nex-1 annexin of C. elegans and 33% identity to annexin-1 (annAt1) of Arabidopsis, it contained four conserved domains typical of the annexin family of calcium and phospholipid binding proteins, and it had a predicted signal peptide for secretion that was present in nematode annexins of only Heterodera spp. Constitutive expression of Hs4F01 in wild-type Arabidopsis promoted hyper-susceptibility to H. schachtii infection. Complementation of an AnnAt1 mutant by constitutive expression of Hs4F01 reverted mutant sensitivity to 75mM NaCl, suggesting a similar function of the Hs4F01 annexin-like effector in the stress response by plant cells. Yeast two-hybrid assays confirmed a specific interaction between Hs4F01 and an Arabidopsis oxidoreductase member of the 2OG-Fe(II) oxygenase family, a type of plant enzyme demonstrated to promote susceptibility to oomycete pathogens. RNA interference assays that expressed double-stranded RNA complementary to Hs4F01 in transgenic Arabidopsis specifically decreased parasitic nematode Hs4F01 transcript levels and significantly reduced nematode infection levels. The combined data suggest that nematode secretion of an Hs4F01 annexin-like effector into host root cells may mimic plant annexin function during the parasitic interaction.

The AMI1 gene family: indole-3-acetamide hydrolase functions in auxin biosynthesis in plants

Mano, Y., Nemoto, K., Suzuki, M., Seki, H., Fujii, I., Muranaka, T. — Tue, 03 Nov 2009 03:52:26 PST

Novel genes that function in the conversion of indole-3-acetamide (IAM) into indole-3-acetic acid (IAA), which were previously thought to exist only in the bacterial genome, have been isolated from plants. The finding of the AtAMI1 gene in Arabidopsis thaliana and the NtAMI1 gene in Nicotiana tabacum, which encode indole-3-acetamide hydrolase, indicates the existence of a new pathway for auxin biosynthesis in plants. This review summarizes the characteristics of these genes involved in auxin biosynthesis and discusses the possibility of the AMI1 gene family being widely distributed in the plant kingdom. Its evolutionary relationship to bacterial indole-3-acetamide hydrolase, based on phylogenetic analyses, is also discussed.

Isolation and functional characterization of a lycopene {beta}-cyclase gene that controls fruit colour of papaya (Carica papaya L.)

Devitt, L. C., Fanning, K., Dietzgen, R. G., Holton, T. A. — Tue, 03 Nov 2009 03:52:25 PST

The colour of papaya fruit flesh is determined largely by the presence of carotenoid pigments. Red-fleshed papaya fruit contain lycopene, whilst this pigment is absent from yellow-fleshed fruit. The conversion of lycopene (red) to β-carotene (yellow) is catalysed by lycopene β-cyclase. This present study describes the cloning and functional characterization of two different genes encoding lycopene β-cyclases (lcy-β1 and lcy-β2) from red (Tainung) and yellow (Hybrid 1B) papaya cultivars. A mutation in the lcy-β2 gene, which inactivates enzyme activity, controls lycopene production in fruit and is responsible for the difference in carotenoid production between red and yellow-fleshed papaya fruit. The expression level of both lcy-β1 and lcy-β2 genes is similar and low in leaves, but lcy-β2 expression increases markedly in ripe fruit. Isolation of the lcy-β2 gene from papaya, that is preferentially expressed in fruit and is correlated with fruit colour, will facilitate marker-assisted breeding for fruit colour in papaya and should create possibilities for metabolic engineering of carotenoid production in papaya fruit to alter both colour and nutritional properties.

Mild salinity stimulates a stress-induced morphogenic response in Arabidopsis thaliana roots

Zolla, G., Heimer, Y. M., Barak, S. — Mon, 02 Nov 2009 09:57:38 PST

Plant roots exhibit remarkable developmental plasticity in response to local soil conditions. It is shown here that mild salt stress stimulates a stress-induced morphogenic response (SIMR) in Arabidopsis thaliana roots characteristic of several other abiotic stresses: the proliferation of lateral roots (LRs) with a concomitant reduction in LR and primary root length. The LR proliferation component of the salt SIMR is dramatically enhanced by the transfer of seedlings from a low to a high NO₃^– medium, thereby compensating for the decreased LR length and maintaining overall LR surface area. Increased LR proliferation is specific to salt stress (osmotic stress alone has no stimulatory effect) and is due to the progression of more LR primordia from the pre-emergence to the emergence stage, in salt-stressed plants. In salt-stressed seedlings, greater numbers of LR primordia exhibit expression of a reporter gene driven by the auxin-sensitive DR5 promoter than in unstressed seedlings. Moreover, in the auxin transporter mutant aux1-7, the LR proliferation component of the salt SIMR is completely abrogated. The results suggest that salt stress promotes auxin accumulation in developing primordia thereby preventing their developmental arrest at the pre-emergence stage. Examination of ABA and ethylene mutants revealed that ABA synthesis and a factor involved in the ethylene signalling network also regulate the LR proliferation component of the salt SIMR.

Light can rescue auxin-dependent synchrony of cell division in a tobacco cell line

Qiao, F., Petrasek, J., Nick, P. — Fri, 30 Oct 2009 10:49:07 PDT

Pattern formation in plants has to cope with ambient variability and therefore must integrate environmental cues such as light. Synchrony of cell divisions was previously observed in cell files of tobacco suspension cultures, which represents a simple case of pattern formation. To develop cellular approaches for light-dependent patterning, light-responsive tobacco cell lines were screened from the cell line Nicotiana tabacum L. cv. Virginia Bright Italia 0 (VBI-0). The light responsive and auxin-autonomous cell line VBI-3 was isolated. As in the progenitor line VBI-0, cell divisions are synchronized in VBI-3 during exponential growth phase. This synchrony can be inhibited by 1-N-naphthylphthalamic acid, an auxin transport inhibitor, and this process was accompanied by the disassembly of actin filaments. However, the synchrony could be rescued when the cells were cultured under white light or with exogenous indolyl-3-acetic acid. The rescue was most efficient for continuous far-red light followed by continuous blue light, whereas continuous red light was least effective. These findings are discussed in the context of phytochrome-induced auxin biosynthesis and auxin-dependent synchrony of cell division.

Peroxidases identified in a subtractive cDNA library approach show tissue-specific transcript abundance and enzyme activity during seed germination of Lepidium sativum

Linkies, A., Schuster-Sherpa, U., Tintelnot, S., Leubner-Metzger, G., Muller, K. — Fri, 30 Oct 2009 10:49:06 PDT

The micropylar endosperm is a major regulator of seed germination in endospermic species, to which the close Brassicaceae relatives Arabidopsis thaliana and Lepidium sativum (cress) belong. Cress seeds are about 20 times larger than the seeds of Arabidopsis. This advantage was used to construct a tissue-specific subtractive cDNA library of transcripts that are up-regulated late in the germination process specifically in the micropylar endosperm of cress seeds. The library showed that a number of transcripts known to be up-regulated late during germination are up-regulated in the micropylar endosperm cap. Detailed germination kinetics of SALK lines carrying insertions in genes present in our library showed that the identified transcripts do indeed play roles during germination. Three peroxidases were present in the library. These peroxidases were identified as orthologues of Arabidopsis AtAPX01, AtPrx16, and AtPrxIIE. The corresponding SALK lines displayed significant germination phenotypes. Their transcripts were quantified in specific cress seed tissues during germination in the presence and absence of ABA and they were found to be regulated in a tissue-specific manner. Peroxidase activity, and particularly its regulation by ABA, also differed between radicles and micropylar endosperm caps. Possible implications of this tissue-specificity are discussed.

Ethylene signal transduction elements involved in chilling injury in non-climacteric loquat fruit

Wang, P., Zhang, B., Li, X., Xu, C., Yin, X., Shan, L., Ferguson, I., Chen, K. — Fri, 30 Oct 2009 10:49:05 PDT

Loquat (Eriobotrya japonica Lindl.) is a subtropical fruit, with some cultivars such as ‘Luoyangqing’ (LYQ) susceptible to chilling injury (CI), while others such as ‘Baisha’ (BS) are resistant. Although loquats are non-climacteric, modulation of ethylene has an effect on ripening-related post-harvest CI. Therefore the role of ethylene signalling in the development of CI was investigated in fruit of both the LYQ and BS cultivars. Three ethylene receptor genes, one CTR1-like gene, and one EIN3-like gene were isolated and characterized in ripening fruit. All of these genes were expressed differentially within and between fruit of the two cultivars. Transcripts either declined over fruit development (EjERS1a in both cultivars and EjEIL1 in LYQ) or showed an increase in the middle stages of fruit development before declining (EjETR1, EjERS1b, and EjCTR1 in both cultivars and EjEIL1 in BS). The main cultivar differences were in levels rather than in patterns of expression during post-harvest storage. EjETR1, EjCTR1, and EjEIL1 genes showed increased expression in response to low temperature and this was particularly notable for EjETR1, and EjEIL1 during CI development in LYQ fruit. The genes were also differentially responsive to ethylene treatment, 1-methycyclopropene (1-MCP) and low temperature conditioning, confirming a role for ethylene in regulation of CI in loquat fruit.

1-Aminocyclopropane-1-carboxylic acid (ACC) concentration and ACC synthase expression in soybean roots, root tips, and soybean cyst nematode (Heterodera glycines)-infected roots

Tucker, M. L., Xue, P., Yang, R. — Tue, 27 Oct 2009 12:31:22 PDT

Colonization of plant roots by root knot and cyst nematodes requires a functional ethylene response pathway. However, ethylene plays many roles in root development and whether its role in nematode colonization is direct or indirect, for example lateral root initiation or root hair growth, is not known. The temporal requirement for ethylene and localized synthesis of ethylene during the life span of soybean cyst nematode (SCN) on soybean roots was further investigated. Although a significant increase in ethylene evolution was not detected from SCN-colonized roots, the concentration of the immediate precursor to ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), was higher in SCN-colonized root pieces and root tips than in other parts of the root. Moreover, expression analysis of 17 ACC synthase (ACS) genes indicated that a select set of ACS genes is expressed in SCN-colonized root pieces that is clearly different from the set of genes expressed in non-colonized roots or root tips. Semi-quantitative real-time PCR indicated that ACS transcript accumulation correlates with the high concentration of ACC in root tips. In addition, an ACS-like sequence was found in the public SCN nucleotide database. Acquisition of a full-length sequence for this mRNA (accession GQ389647) and alignment with transcripts for other well-characterized ACS proteins indicated that the nematode sequence is missing a key element required for ACS activity and therefore probably is not a functional ACS. Moreover, no significant amount of ACC was found in any growth stage of SCN that was tested.

Golgi membrane dynamics after induction of a dominant-negative mutant Sar1 GTPase in tobacco

Osterrieder, A., Hummel, E., Carvalho, C. M., Hawes, C. — Tue, 27 Oct 2009 12:31:19 PDT

An inducible system has been established in Nicotiana tabacum plants allowing controlled expression of Sar1-GTP and thus the investigation of protein dynamics after inhibition of endoplasmic reticulum (ER) to Golgi transport. Complete Golgi disassembly and redistribution of Golgi markers into the ER was observed within 18–24h after induction. At the ultrastructural level Sar1-GTP expression led to a decrease in Golgi stack size followed by Golgi fragmentation and accumulation of vesicle remnants. Induction of Sar1-GTP resulted in redistribution of the green fluorescent protein (GFP)-tagged Arabidopsis golgins AtCASP and GC1 (golgin candidate 1, an Arabidopsis golgin 84 isoform) into the ER or cytoplasm, respectively. Additionally, both fusion proteins were observed in punctate structures, which co-located with a yellow fluorescent protein (YFP)-tagged version of Sar1-GTP. The Sar1-GTP-inducible system is compared with constitutive Sar1-GTP expression and brefeldin A treatment, and its potential for the study of the composition of ER exit sites and early cis-Golgi structures is discussed.

Constitutive water-conserving mechanisms are correlated with the terminal drought tolerance of pearl millet [Pennisetum glaucum (L.) R. Br.]

Kholova, J., Hash, C. T., Kakkera, A., Kocova, M., Vadez, V. — Tue, 27 Oct 2009 12:31:16 PDT

Pearl millet, a key staple crop of the semi-arid tropics, is mostly grown in water-limited conditions, and improving its performance depends on how genotypes manage limited water resources. This study investigates whether the control of water loss under non-limiting water conditions is involved in the terminal drought tolerance of pearl millet. Two pairs of tolerantxsensitive pearl millet genotypes, PRLT 2/89-33–H77/833-2 and 863B-P2–ICMB 841-P3, and near-isogenic lines (NILs), introgressed with a terminal drought tolerance quantitative trait locus (QTL) from the donor parent PRLT 2/89-33 into H77/833-2 (NILs-QTL), were tested. Upon exposure to water deficit, transpiration began to decline at lower fractions of transpirable soil water (FTSW) in tolerant than in sensitive genotypes, and NILs-QTL followed the pattern of the tolerant parents. The transpiration rate (Tr, in g water loss cm^–2 d^–1) under well-watered conditions was lower in tolerant than in sensitive parental genotypes, and the Tr of NILs-QTL followed the pattern of the tolerant parents. In addition, Tr measured in detached leaves (g water loss cm^–2 h^–1) from field-grown plants of the parental lines showed lower Tr values in tolerant parents. Defoliation led to an increase in Tr that was higher in sensitive than in tolerant genotypes. The differences in Tr between genotypes was not related to the stomatal density. These results demonstrate that constitutive traits controlling leaf water loss under well-watered conditions correlate with the terminal drought tolerance of pearl millet. Such traits may lead to more water being available for grain filling under terminal drought.

CELL WALL INVERTASE 4 is required for nectar production in Arabidopsis

Ruhlmann, J. M., Kram, B. W., Carter, C. J. — Tue, 27 Oct 2009 12:31:13 PDT

To date, no genes have been reported to directly affect the de novo production of floral nectar. In an effort to identify genes involved in nectar production, the Affymetrix^® ATH1 GeneChip was previously used to examine global gene expression profiles in Arabidopsis thaliana nectaries. One of the genes displaying highly enriched expression in nectaries was CELL WALL INVERTASE 4 (AtCWINV4, At2g36190), which encodes an enzyme that putatively catalyses the hydrolysis of sucrose into glucose and fructose. RT-PCR was used to confirm the nectary-enriched expression of AtCWINV4, as well as an orthologue from Brassica rapa. To probe biological function, two independent Arabidopsis cwinv4 T-DNA mutants were isolated. Unlike wild-type plants, cwinv4 lines did not produce nectar. While overall nectary morphology appeared to be normal, cwinv4 flowers accumulated higher than normal levels of starch in the receptacle, but not within the nectaries themselves. Conversely, wild-type, but not cwinv4, nectarial stomata stained intensely for starch. Cell wall extracts prepared from mutant flowers displayed greatly reduced invertase activity when compared with wild-type plants, and cwinv4 flowers also accumulated significantly lower levels of total soluble sugar. Cumulatively, these results implicate CWINV4 as an absolutely required factor for nectar production in the Brassicaceae, specifically by maintaining constant sink status within nectaries, thus allowing them to accumulate the sugars necessary for nectar production. In addition, CWINV4 is probably responsible for the hexose-rich composition observed for many Brassicaceae nectars.

K+ deprivation induces xylem water and K+ transport in sunflower: evidence for a co-ordinated control

Benlloch-Gonzalez, M., Fournier, J. M., Benlloch, M. — Tue, 27 Oct 2009 12:31:09 PDT

The effect of K⁺ deprivation on water and K⁺ transport in roots was studied in sunflower plants. Deprivation was achieved in two different ways: by removing K⁺ from the growth medium for varying intervals; and by growing plants permanently in a low-K⁺ medium. Removal of K⁺ from the growth medium for a few hours prompted a significant increase in xylem sap exudation, associated with an increase in root hydraulic conductivity; however, it did not give rise to any significant change in plant K⁺ content, nor did it favour root K⁺ exudation. By contrast, prolonged K⁺ deprivation led to a decline in the internal K⁺ content and stimulated water and K⁺ transport in roots. Leaf application of K⁺ (Rb⁺) in plants grown permanently in a low-K⁺ medium inhibited the effect of K⁺ deprivation on root water and K⁺ transport, without significantly modifying the internal K⁺ content of the plants. This treatment had no effect on normal-K⁺ plants. These results suggest the existence of mechanisms enabling perception of plant K⁺ status and/or K⁺ availability in the medium, which trigger transduction processes governing the transport of water and K⁺ from the root to the shoot.

Three cotton genes preferentially expressed in flower tissues encode actin-depolymerizing factors which are involved in F-actin dynamics in cells

Tue, 27 Oct 2009 12:31:05 PDT

To investigate whether the high expression levels of actin-depolymerizing factor genes are related to pollen development, three GhADF genes (cDNAs) were isolated and characterized in cotton. Among them, GhADF6 and GhADF8 were preferentially expressed in petals, whereas GhADF7 displayed the highest level of expression in anthers, revealing its anther specificity. The GhADF7 transcripts in anthers reached its peak value at flowering, suggesting that its expression is developmentally-regulated in anthers. The GhADF7 gene including the promoter region was isolated from the cotton genome. To demonstrate the specificity of the GhADF7 promoter, the 5'-flanking region, including the promoter and 5'-untranslated region, was fused with the GUS gene. Histochemical assays demonstrated that the GhADF7:GUS gene was specifically expressed in pollen grains. When pollen grains germinated, very strong GUS staining was detected in the elongating pollen tube. Furthermore, overexpression of GhADF7 gene in Arabidopsis thaliana reduced the viable pollen grains and, consequently, transgenic plants were partially male-sterile. Overexpression of GhADF7 in fission yeast (Schizosaccharomyces pombe) altered the balance of actin depolymerization and polymerization, leading to the defective cytokinesis and multinucleate formation in the cells. Given all the above results together, it is proposed that the GhADF7 gene may play an important role in pollen development and germination.

Aluminium-induced inhibition of root elongation in Arabidopsis is mediated by ethylene and auxin

Sun, P., Tian, Q.-Y., Chen, J., Zhang, W.-H. — Sun, 25 Oct 2009 23:16:25 PDT

Aluminium (Al) is phytotoxic when solubilized into Al³⁺ in acidic soils. One of the earliest and distinct symptoms of Al³⁺ toxicity is inhibition of root elongation. To decipher the mechanism by which Al³⁺ inhibits root elongation, the role of ethylene and auxin in Al³⁺-induced inhibition of root elongation in Arabidopsis thaliana was investigated using the wild type and mutants defective in ethylene signalling (etr1-3 and ein2-1) and auxin polar transport (aux1-7 and pin2). Exposure of wild-type Arabidopsis to AlCl₃ led to a marked inhibition of root elongation, and elicited a rapid ethylene evolution and enhanced activity of the ethylene reporter EBS:GUS in root apices. Root elongation in etr1-3 and ein2-1 mutants was less inhibited by Al³⁺ than that in wild-type plants. Ethylene synthesis inhibitors, Co²⁺ and aminoethoxyvinylglycine (AVG), and an antagonist of ethylene perception (Ag⁺) abolished the Al³⁺-induced inhibition of root elongation. There was less inhibition of root elongation by Al³⁺ in aux1-7 and pin2 mutants than in the wild type. The auxin polar transport inhibitor, naphthylphthalamic acid (NPA), substantially alleviated the Al³⁺-induced inhibition of root elongation. The Al³⁺ and ethylene synthesis precursor aminocyclopropane carboxylic acid (ACC) increased auxin reporter DR5:GUS activity in roots. The Al³⁺-induced increase in DR5:GUS activity was reduced by AVG, while the Al³⁺-induced increase in EBS:GUS activity was not altered by NPA. Al³⁺ and ACC increased transcripts of AUX1 and PIN2, and this effect was no longer observed in the presence of AVG and Co²⁺. These findings indicate that Al³⁺-induced ethylene production is likely to act as a signal to alter auxin distribution in roots by disrupting AUX1- and PIN2-mediated auxin polar transport, leading to arrest of root elongation.

Physiological and proteomic approaches to address heat tolerance during anthesis in rice (Oryza sativa L.)

Sun, 25 Oct 2009 23:16:24 PDT

Episodes of high temperature at anthesis, which in rice is the most sensitive stage to temperature, are expected to occur more frequently in future climates. The morphology of the reproductive organs and pollen number, and changes in anther protein expression, were studied in response to high temperature at anthesis in three rice (Oryza sativa L.) genotypes. Plants were exposed to 6 h of high (38 °C) and control (29 °C) temperature at anthesis and spikelets collected for morphological and proteomic analysis. Moroberekan was the most heat-sensitive genotype (18% spikelet fertility at 38 °C), while IR64 (48%) and N22 (71%) were moderately and highly heat tolerant, respectively. There were significant differences among the genotypes in anther length and width, apical and basal pore lengths, apical pore area, and stigma and pistil length. Temperature also affected some of these traits, increasing anther pore size and reducing stigma length. Nonetheless, variation in the number of pollen on the stigma could not be related to measured morphological traits. Variation in spikelet fertility was highly correlated (r=0.97, n=6) with the proportion of spikelets with ≥20 germinated pollen grains on the stigma. A 2D-gel electrophoresis showed 46 protein spots changing in abundance, of which 13 differentially expressed protein spots were analysed by MS/MALDI-TOF. A cold and a heat shock protein were found significantly up-regulated in N22, and this may have contributed to the greater heat tolerance of N22. The role of differentially expressed proteins and morphology during anther dehiscence and pollination in shaping heat tolerance and susceptibility is discussed.

Developmental and heat stress-regulated expression of HsfA2 and small heat shock proteins in tomato anthers

Fri, 23 Oct 2009 03:42:10 PDT

The high sensitivity of male reproductive cells to high temperatures may be due to an inadequate heat stress response. The results of a comprehensive expression analysis of HsfA2 and Hsp17-CII, two important members of the heat stress system, in the developing anthers of a heat-tolerant tomato genotype are reported here. A transcriptional analysis at different developmental anther/pollen stages was performed using semi-quantitative and real-time PCR. The messengers were localized using in situ RNA hybridization, and protein accumulation was monitored using immunoblot analysis. Based on the analysis of the gene and protein expression profiles, HsfA2 and Hsp17-CII are finely regulated during anther development and are further induced under both short and prolonged heat stress conditions. These data suggest that HsfA2 may be directly involved in the activation of protection mechanisms in the tomato anther during heat stress and, thereby, may contribute to tomato fruit set under adverse temperatures.

Flower numbers, pod production, pollen viability, and pistil function are reduced and flower and pod abortion increased in chickpea (Cicer arietinum L.) under terminal drought

Fang, X., Turner, N. C., Yan, G., Li, F., Siddique, K. H.M. — Fri, 23 Oct 2009 03:42:05 PDT

Terminal drought during the reproductive stage is a major constraint to yield of chickpea in many regions of the world. Termination of watering (WS) during podding in a small-seeded desi chickpea (Cicer arietinum L.) cultivar, Rupali, and a large-seeded kabuli chickpea cultivar, Almaz, induced a decrease in predawn leaf water potential (LWP), in the rate of photosynthesis, and in stomatal conductance. Compared to well-watered (WW) controls, the WS treatment reduced flower production by about two-thirds. In the WW treatment, about 15% of the flowers aborted and 42% (Rupali) and 67% (Almaz) of the pods aborted, whereas in the WS treatment 37% and 56% of the flowers aborted and 54% and 73% of the pods aborted, resulting in seed yields of 33% and 15% of the yields in WW plants in Rupali and Almaz, respectively. In vitro pollen viability and germination in Rupali decreased by 50% and 89% in the WS treatment, and pollen germination decreased by 80% in vivo when pollen from a WS plant was placed on a stigma of a WW plant. While about 37% of the germinated pollen tubes from WW plants and 22% from the WS plants reached the ovary in the WW plants, less than 3% of pollen grains reached the ovary when pollen from either WS or WW plants was placed on a stigma of a WS plant. It is concluded that, in addition to pod abortion, flower abortion is an important factor limiting yield in chickpea exposed to terminal drought and that water deficit impaired the function of the pistil/style more than the pollen.

Overexpression of the CBF2 transcriptional activator in Arabidopsis delays leaf senescence and extends plant longevity

Sharabi-Schwager, M., Lers, A., Samach, A., Guy, C. L., Porat, R. — Fri, 23 Oct 2009 03:42:00 PDT

Leaf senescence is a programmed developmental process governed by various endogenous and exogenous factors, such as the plant developmental stage, leaf age, phytohormone levels, darkness, and exposure to stresses. It was found that, in addition to its well-documented role in the enhancement of plant frost tolerance, overexpression of the C-repeat/dehydration responsive element binding factor 2 (CBF2) gene in Arabidopsis delayed the onset of leaf senescence and extended the life span of the plants by approximately 2 weeks. This phenomenon was exhibited both during developmental leaf senescence and during senescence of detached leaves artificially induced by either darkness or phytohormones. Transcriptome analysis using the Affymetrix ATH1 genome array revealed that overexpression of CBF2 significantly influenced the expression of 286 genes in mature leaf tissue. In addition to 30 stress-related genes, overexpression of CBF2 also affected the expression of 24 transcription factor (TF) genes, and 20 genes involved in protein metabolism, degradation, and post-translational modification. These results indicate that overexpression of CBF2 not only increases frost tolerance, but also affects other developmental processes, most likely through interactions with additional TFs and protein modification genes. The present findings shed new light on the crucial relationship between plant stress tolerance and longevity, as reported for other eukaryotic organisms.

The effects of sap ionic composition on xylem vulnerability to cavitation

Cochard, H., Herbette, S., Hernandez, E., Holtta, T., Mencuccini, M. — Mon, 19 Oct 2009 08:22:58 PDT

Recent evidence of ion-mediated changes in pit membrane porosity suggests that plants may modulate the hydraulic conductance of their xylem conduits. Under the current paradigm, membrane porosity also determines conduit vulnerability to water stress-induced cavitation. Therefore, the hypothesis of an ion-mediated regulation of xylem vulnerability to cavitation in trees was tested. Segments of five Angiosperm and two Gymnosperm species were infiltrated with ultra-pure deionized water as a reference fluid or with a 50 mM KCl solution. KCl had a strong impact on segment conductance with either a positive or a negative effect across species. When 1 mM CaCl₂ was added to the reference solution, the effect of KCl was minimized for most species. By contrast, segment vulnerability to cavitation was only slightly influenced by the presence of KCl in the solution. From this it was concluded that the mechanisms controlling pit membrane permeability to water flow and its resistance to the penetration of air bubbles are largely uncoupled, which suggests that the hypothesis of a porous structure of pit membranes should be revisited.

Gene expression analysis in cadmium-stressed roots of a low cadmium-accumulating solanaceous plant, Solanum torvum

Fri, 16 Oct 2009 04:45:15 PDT

Solanum torvum Sw. cv. Torubamubiga (TB) is a low cadmium (Cd)-accumulating plant. To elucidate the molecular mechanisms of the Cd acclimation process in TB roots, transcriptional regulation was analysed in response to mild Cd treatment: 0.1 µM CdCl₂ in hydroponic solution. A unigene set consisting of 6296 unigene sequences was constructed from 18 816 TB cDNAs. The distribution of functional categories was similar to tomato, while 330 unigenes were suggested to be TB specific. For expression profiling, the SuperSAGE method was adapted for use with Illumina sequencing technology. Expression tag libraries were constructed from Cd-treated (for 3 h, 1 d, and 3 d) and untreated roots, and 34 269 species of independent tags were collected. Moreover, 6237 tags were ascribed to the TB or eggplant (aubergine) unigene sequences. Time-course changes were examined, and 2049 up- and 2022 down-regulated tags were identified. Although no tags annotated to metal transporter genes were significantly regulated, a tag annotated to AtFRD3, a xylem-loading citrate transporter, was down-regulated. In addition to induction of heavy metal chaperone proteins, antioxidative and sulphur-assimilating enzymes were induced, confirming that oxidative stress developed even using a mild Cd concentration. Rapid repression of dehydration-related transcription factors and aquaporin isoforms suggests that dehydration stress is a potential constituent of Cd-induced biochemical impediments. These transcriptional changes were also confirmed by real-time reverse transcription-PCR. Further additions of TB unigene sequences and functional analysis of the regulated tags will reveal the molecular basis of the Cd acclimation process, including the low Cd-accumulating characteristics of TB.

Function of antioxidant enzymes and metabolites during maturation of pea fruits

Matamoros, M. A., Loscos, J., Dietz, K.-J., Aparicio-Tejo, P. M., Becana, M. — Sun, 11 Oct 2009 21:11:05 PDT

In plant cells, antioxidants keep reactive oxygen species at low concentrations, avoiding oxidative damage while allowing them to play crucial functions in signal transduction. However, little is known about the role of antioxidants during fruit maturation, especially in legumes. Snap pea (Pisum sativum) plants, which have edible fruits, were grown under nodulating and non-nodulating conditions. Fruits were classified in three maturity stages and antioxidants were determined in the seeds and seedless pods. Maturation or prolonged storage of fruits at 25 °C led to a decline in antioxidant activities and metabolites and in -glutamylcysteine synthetase protein. Notable exceptions were superoxide dismutase activity and glutathione peroxidase protein, which increased in one or both of these processes. During maturation, cytosolic peroxiredoxin decreased in seeds but increased in pods, and ascorbate oxidase activity was largely reduced in seeds. In stored fruits, ascorbate oxidase activity was nearly abolished in seeds but doubled in pods. It is concluded that symbiotic nitrogen fixation is as effective as nitrogen fertilization in maintaining the antioxidant capacity of pea fruits and that, contrary to climacteric fruits, a general decrease in antioxidants during maturation does not involve oxidative stress. Results underscore the importance of the antioxidant system in reproductive organs and point to ascorbate–glutathione metabolism and cytosolic peroxiredoxin as key players in pea fruit development.

Physiological and molecular changes in Oryza meridionalis Ng., a heat-tolerant species of wild rice

Scafaro, A. P., Haynes, P. A., Atwell, B. J. — Fri, 09 Oct 2009 04:39:53 PDT

Oryza meridionalis Ng. is a wild relative of Oryza sativa L. found throughout northern Australia where temperatures regularly exceed 35 °C in the monsoon growing season. Heat tolerance in O. meridionalis was established by comparing leaf elongation and photosynthetic rates at 45 °C with plants maintained at 27 °C. By comparison with O. sativa ssp. japonica cv. Amaroo, O. meridionalis was heat tolerant. Elongation rates of the third leaf of O. meridionalis declined by 47% over 24 h at 45 °C compared with a 91% decrease for O. sativa. Net photosynthesis was significantly higher in O. sativa at 27 °C whereas the two species had the same assimilation rates at 45 °C. The leaf proteome and expression levels of individual heat-responsive genes provided insight into the heat response of O. meridionalis. After 24 h of heat exposure, many enzymes involved in the Calvin Cycle were more abundant, while mRNA of their genes generally decreased. Ferredoxin-NADP(H) oxidoreductase, a key enzyme in photosynthetic electron transport had both reduced abundance and gene expression, suggesting light reactions were highly susceptible to heat stress. Rubisco activase was strongly up-regulated after 24 h of heat, with the large isoform having the largest relative increase in protein abundance and a significant increase in gene expression. The protective proteins Cpn60, Hsp90, and Hsp70 all increased in both protein abundance and gene expression. A thiamine biosynthesis protein (THI1), previously shown to act protectively against stress, increased in abundance during heat, even as thiamine levels fell in O. meridionalis.

Phytochrome functions in Arabidopsis development

Franklin, K. A., Quail, P. H. — Thu, 08 Oct 2009 10:56:01 PDT

Light signals are fundamental to the growth and development of plants. Red and far-red light are sensed using the phytochrome family of plant photoreceptors. Individual phytochromes display both unique and overlapping roles throughout the life cycle of plants, regulating a range of developmental processes from seed germination to the timing of reproductive development. The evolution of multiple phytochrome photoreceptors has enhanced plant sensitivity to fluctuating light environments, diversifying phytochrome function, and facilitating conditional cross-talk with other signalling systems. The isolation of null mutants, deficient in all individual phytochromes, has greatly advanced understanding of phytochrome functions in the model species, Arabidopsis thaliana. The creation of mutants null for multiple phytochrome combinations has enabled the dissection of redundant interactions between family members, revealing novel regulatory roles for this important photoreceptor family. In this review, current knowledge of phytochrome functions in the light-regulated development of Arabidopsis is summarised.

Developmental and molecular physiological evidence for the role of phosphoenolpyruvate carboxylase in rapid cotton fibre elongation

Li, X.-R., Wang, L., Ruan, Y.-L. — Thu, 08 Oct 2009 10:56:00 PDT

Cotton fibres are hair-like single-cells that elongate to several centimetres long after their initiation from the ovule epidermis at anthesis. The accumulation of malate, along with K⁺ and sugars, is thought to play an important role in fibre elongation through osmotic regulation and charge balance. However, there is a lack of evidence for or against such an hypothesis. Phosphoenolpyruvate carboxylase (PEPC) is a key enzyme responsible for the synthesis of malate. The potential role of PEPC in cotton fibre elongation is examined here. Developmentally, PEPC activity was higher at the rapid elongation phase than that at the slow elongation stage. Genotypically, PEPC activity correlated positively with the rate of fibre elongation and the final fibre length attained. Importantly, suppression of PEPC activity by LiCl that reduces its phosphorylation status decreased fibre length. To examine the molecular basis underlying PEPC activity, two cDNAs encoding PEPC, GhPEPC1 and 2, were cloned, which represents the major PEPC genes expressed in cotton fibre. RT-PCR analyses revealed that GhPEPC1 and 2 were highly expressed at the rapid elongation phase but weakly at the slow-to-terminal elongation period. In situ hybridization detected mRNA of GhPEPC1 and 2 in 1 d young fibres but not in the ovule epidermis prior to fibre initiation. Collectively, the data indicate that cotton fibre elongation requires high activity of PEPC, probably through the expression of the GhPEPC1 and 2 genes.

Hypoxia-responsive microRNAs and trans-acting small interfering RNAs in Arabidopsis

Moldovan, D., Spriggs, A., Yang, J., Pogson, B. J., Dennis, E. S., Wilson, I. W. — Thu, 08 Oct 2009 10:55:58 PDT

Low-oxygen (hypoxia) stress associated with natural phenomena such as waterlogging, results in widespread transcriptome changes and a metabolic switch from aerobic respiration to anaerobic fermentation. High-throughput sequencing of small RNA libraries obtained from hypoxia-treated and control root tissue identified a total of 65 unique microRNA (miRNA) sequences from 46 families, and 14 trans-acting small interfering RNA (tasiRNA) from three families. Hypoxia resulted in changes to the abundance of 46 miRNAs from 19 families, and all three tasiRNA families. Chemical inhibition of mitochondrial respiration caused similar changes in expression in a majority of the hypoxia-responsive small RNAs analysed. Our data indicate that miRNAs and tasiRNAs play a role in gene regulation and possibly developmental responses to hypoxia, and that a major signal for these responses is likely to be dependent on mitochondrial function.

Elevated CO2 concentration around alfalfa nodules increases N2 fixation

Fischinger, S. A., Hristozkova, M., Mainassara, Z.-A., Schulze, J. — Thu, 08 Oct 2009 10:55:57 PDT

Nodule CO₂ fixation via PEPC provides malate for bacteroids and oxaloacetate for N assimilation. The process is therefore of central importance for efficient nitrogen fixation. Nodule CO₂ fixation is known to depend on external CO₂ concentration. The hypothesis of the present paper was that nitrogen fixation in alfalfa plants is enhanced when the nodules are exposed to elevated CO₂ concentrations. Therefore nodulated plants of alfalfa were grown in a hydroponic system that allowed separate aeration of the root/nodule compartment that avoided any gas leakage to the shoots. The root/nodule compartments were aerated either with a 2500 µl l^–1 (+CO₂) or zero µl l^–1 (–CO₂) CO₂-containing N₂/O₂ gas flow (80/20, v/v). Nodule CO₂ fixation, nitrogen fixation, and growth were strongly increased in the +CO₂ treatment in a 3-week experimental period. More intensive CO₂ and nitrogen fixation coincided with higher per plant amounts of amino acids and organic acids in the nodules. Moreover, the concentration of asparagine was increased in both the nodules and the xylem sap. Plants in the +CO₂ treatment tended to develop nodules with higher %N concentration and individual activity. In a parallel experiment on plants with inefficient nodules (fix^–) the +CO₂ treatment remained without effect. Our data support the thesis that nodule CO₂ fixation is pivotal for efficient nitrogen fixation. It is concluded that strategies which enhance nodule CO₂ fixation will improve nitrogen fixation and nodule formation. Moreover, sufficient CO₂ application to roots and nodules is necessary for growth and efficient nitrogen fixation in hydroponic and aeroponic growth systems.

Pseudomonas spp.-induced systemic resistance to Botrytis cinerea is associated with induction and priming of defence responses in grapevine

Verhagen, B. W. M., Trotel-Aziz, P., Couderchet, M., Hofte, M., Aziz, A. — Wed, 07 Oct 2009 08:24:37 PDT

Non-pathogenic rhizobacteria Pseudomonas spp. can reduce disease in plant tissues through induction of a defence state known as induced systemic resistance (ISR). This resistance is based on multiple bacterial determinants, but nothing is known about the mechanisms underlying rhizobacteria-induced resistance in grapevine. In this study, the ability of Pseudomonas fluorescens CHA0 and Pseudomonas aeruginosa 7NSK2 to induce resistance in grapevine against Botrytis cinerea is demonstrated. Both strains also triggered an oxidative burst and phytoalexin (i.e. resveratrol and viniferin) accumulation in grape cells and primed leaves for accelerated phytoalexin production upon challenge with B. cinerea. Treatment of cell cultures with crude cell extracts of bacteria strongly enhanced oxidative burst, but resulted in comparable amounts of phytoalexins and resistance to B. cinerea to those induced by living bacteria. This suggests the production of bacterial compounds serving as inducers of disease resistance. Using other strains with different characteristics, it is shown that P. fluorescens WCS417 (Pch-deficient), P. putida WCS358 (Pch- and SA-deficient) and P. fluorescens Q2-87 (a DAPG producer) were all capable of inducing resistance to an extent similar to that induced by CHA0. However, in response to WCS417 (Pch-negative) the amount of H₂O₂ induced is less than for the CHA0. WCS417 induced low phytoalexin levels in cells and lost the capacity to prime for phytoalexins in the leaves. This suggests that, depending on the strain, SA, pyochelin, and DAPG are potentially effective in inducing or priming defence responses. The 7NSK2 mutants, KMPCH (Pch- and Pvd-negative) and KMPCH-567 (Pch-, Pvd-, and SA-negative) induced only partial resistance to B. cinerea. However, the amount of H₂O₂ triggered by KMPCH and KMPCH-567 was similar to that induced by 7NSK2. Both mutants also led to a low level of phytoalexins in grapevine cells, while KMPCH slightly primed grapevine leaves for enhanced phytoalexins. This highlights the importance of SA, pyochelin, and/or pyoverdin in priming phytoalexin responses and induced grapevine resistance by 7NSK2 against B. cinerea.

The expression of a chromoplast-specific lycopene beta cyclase gene is involved in the high production of saffron's apocarotenoid precursors

Ahrazem, O., Rubio-Moraga, A., Lopez, R. C., Gomez-Gomez, L. — Fri, 18 Sep 2009 00:28:01 PDT

Crocus sativus is a triploid sterile plant characterized by its long red stigmas, which produce and store significant quantities of carotenoid derivatives formed from the oxidative cleavage of β-carotene and zeaxanthin. The present study reports on the genomic structures of two lycopene-β-cyclase genes, CstLcyB1 and CstLcyB2a, and on their transcription patterns in different C. sativus tissues. Phylogenetic analysis showed that both proteins are located in different groups: CstLcyB2a encodes chromoplast-specific lycopene cyclases, with an expression analysis showing strongly in flower stigmas where it activates and boosts β-carotene accumulation. The CstLcyB1 transcript, however, was present in leaves, tepals, and stigmas at lower levels. In vivo assays in transgenic Arabidopsis demonstrated lycopene β-cyclase activity of CstLcyB2a. CstLcyB2a is a CstLcyB1 paralogue derived through a gene duplication event, while promoter analysis showed that both genes have diverged in their regulatory sequences after duplication. Furthermore, it was found that the CstLcyB2a gene was absent from Crocus kotschyanus and, although present in C. goulimyi and C. cancellatus, the absence of transcripts suggests that transcriptional regulation of CstLcyB2a is responsible for the low apocarotenoid content in these species.

Functional characterization of B class MADS-box transcription factors in Gerbera hybrida

Fri, 18 Sep 2009 05:54:10 PDT

According to the classical ABC model, B-function genes are involved in determining petal and stamen development. Most core eudicot species have B class genes belonging to three different lineages: the PI, euAP3, and TM6 lineages, although both Arabidopsis and Antirrhinum appear to have lost their TM6-like gene. Functional studies were performed for three gerbera (Gerbera hybrida) B class MADS-box genes—PI/GLO-like GGLO1, euAP3 class GDEF2, and TM6-like GDEF1—and data are shown for a second euAP3-like gene, GDEF3. In phylogenetic analysis, GDEF3 is a closely related paralogue of GDEF2, and apparently stems from a duplication common to all Asteraceae. Expression analysis and transgenic phenotypes confirm that GGLO1 and GDEF2 mediate the classical B-function since they determine petal and stamen identities. However, based on assays in yeast, three B class heterodimer combinations are possible in gerbera. In addition to the interaction of GGLO1 and GDEF2 proteins, GGLO1 also pairs with GDEF1 and GDEF3. This analysis of GDEF1 represents the first functional characterization of a TM6-like gene in a core eudicot species outside Solanaceae. Similarly to its relatives in petunia and tomato, the expression pattern and transgenic phenotypes indicate that GDEF1 is not involved in determination of petal identity, but has a redundant role in regulating stamen development.

Utricularia carnivory revisited: plants supply photosynthetic carbon to traps

Sirova, D., Borovec, J., Santruckova, H., Santrucek, J., Vrba, J., Adamec, L. — Tue, 15 Sep 2009 08:36:32 PDT

The rootless, aquatic Utricularia species belong to the largest and most cosmopolitan carnivorous plant genus. Populations of Utricularia plants are an important component of many standing, nutrient-poor, and humic waters. Carbon (C) allocation is an aspect of Utricularia’s ecophysiology that has not been studied previously and there is considerable uncertainty about the functional and ecological benefit of the trap-associated microbial community and the potential role played by C exudation in enhancing plant–microbe interactions. A ¹³C-labelling experiment was conducted in greenhouse conditions to determine the C allocation between plant tissues of increasing age and trap fluid in two Utricularia species. Both species allocated a majority of the newly fixed C into the fast growing shoot apex (46.1±8.6% in U. vulgaris and 56.1% in U. australis). Carbon allocation rapidly decreased with increasing age of the shoot, constituting only 8.0±4.0% and 6.7% of the total newly fixed C in the oldest analysed segments in U. vulgaris and U. australis, respectively. In the trap-bearing shoot segments, the ratio of C exuded into the trap fluid to that in plant tissues increased markedly with age—in the oldest analysed segments twice as much newly fixed C was allocated into the trap fluid than the plant tissue. Overall, a significant amount of the newly fixed C, approximately 25% (U. vulgaris) and 20% (U. australis), was allocated to the trap fluid. The importance of C exudation for the development of the microbial community associated with the traps as well as for the growth and ecology of aquatic Utricularia is discussed.

A mutation in amino acid permease AAP6 reduces the amino acid content of the Arabidopsis sieve elements but leaves aphid herbivores unaffected

Tue, 15 Sep 2009 08:36:34 PDT

The aim of this study was to investigate the role of the amino acid permease gene AAP6 in regulating phloem amino acid composition and then to determine the effects of this altered diet on aphid performance. A genotype of Arabidopsis thaliana (L.) was produced in which the function of the amino acid permease gene AAP6 (At5g49630) was abolished. Plants homozygous for the insertionally inactivated AAP6 gene had a significantly larger mean rosette width than the wild type and a greater number of cauline leaves. Seeds from the aap6 mutant were also significantly larger than those from the wild-type plants. Sieve element (SE) sap was collected by aphid stylectomy and the amino acids derivatized, separated, and quantified using Capillary Electrophoresis with Laser Induced Fluorescence (CE-LIF). In spite of the large variation across samples, the total amino acid concentration of SE sap of the aap6 mutant plants was significantly lower than that of the wild-type plants. The concentrations of lysine, phenylalanine, leucine, and aspartic acid were all significantly lower in concentration in the aap6 mutant plants compared with wild-type plants. This is the first direct demonstration of a physiological role for an amino acid transporter in regulating SE composition in vivo. The amino acid availability in sieve element sap is thought to be the major limiting factor for aphid growth and reproduction. Despite the changes in their diet, the aphid Myzus persicae (Sulzer) displayed only small changes in feeding behaviour on mutant plants when measured using the Electronic Penetration Graph (EPG) technique. Salivation by the aphid into the SE (E1 phase) was increased on mutant plants but there was no significant effect on other feeding EPG behaviours, or in the rate of honeydew production. Consistent with the small effect on aphid feeding behaviour, there was only a small effect of reduced sieve element amino acid concentration on aphid reproduction. The data are discussed in relation to the regulation of phloem composition and the role of phloem amino acids in regulating aphid performance.

Putative Vitis vinifera Rop- and Rab-GAP-, GEF-, and GDI-interacting proteins uncovered with novel methods for public genomic and EST database analysis

Abbal, P., Tesniere, C. — Mon, 07 Sep 2009 00:07:17 PDT

To understand how grapevine Rop and Rab proteins achieve their functional versatility in signalling, identification of the putative VvRop- and VvRab-interacting proteins was performed using newly designed tools. In this study, sequences encoding eight full-length proteins for VvRop GTPase-activating proteins (GAPs), five for VvRabGAPs, six for VvRop guanine nucleotide exchange factors (GEFs), one for VvRabGEF, five for VvRop GDP dissociation inhibitors (GDIs), and three for VvRabGDIs were identified. These proteins had a CRIB motif or PH domain, a TBC domain, a PRONE domain, a DENN domain, or GDI signatures, respectively. By bootstrap analysis, an unrooted consensus phylogenetic tree was constructed which indicated that VvRopGDIs and VvRopGEFs—but not VvRopGAP—belonged to the same clade, and that VvRabGEF1 protein was more closely related to VvRopGAPs than to the other putative VvRab-interacting proteins. Twenty-two genes out of 28 encoding putative VvRop- and VvRab-interacting proteins could be located on identified grapevine chromosomes. Generally one gene was anchored on one chromosome, but in some cases up to four genes were located on the same chromosome. Expression patterns of the genes encoding putative VvRop- and VvRab-interacting proteins were also examined using a newly developed tool based on public expressed sequence tag (EST) database analysis. Expression patterns were sometimes found to be specific to an organ or a developmental stage. Although some limitations exist, the use of EST database analysis is stressed, in particular in the case of species where expression data are obtained at high costs in terms of time and effort.

Flowering Newsletter bibliography for 2008

Compiled by, Tooke, F., Chiurugwi, T., Battey, N. — Tue, 23 Jun 2009 06:17:59 PDT

My favourite flowering image

Glover, B. J. — Tue, 03 Mar 2009 07:29:13 PST

Choosing a favourite image is very difficult to do, not least because different images are important to us for different reasons. I have decided to focus here on an image that is not only intrinsically beautiful, but that also emphasizes the importance of looking and seeing properly when trying to understand the world around us. For me, this image, the adaxial surface of the petal of Veronica caucasia, exemplifies how looking at things in different ways can provide unexpected insights into the way nature works.

Flowering Newsletter bibliography for 2007

Compiled by, Tooke, F., Chiurugwi, T., Battey, N. — Fri, 18 Jul 2008 02:28:24 PDT

Preface

Battey, N. — Thu, 17 Jul 2008 22:09:14 PDT

My favourite flowering image

Endress, P. K. — Wed, 27 Feb 2008 23:03:01 PST

The heuristic value of drawings in research is emphasized, based on a drawing of flowers of Eupomatia bennettii.

Flowering Newsletter bibliography for 2006

Fri, 20 Apr 2007 08:51:43 PDT

The colour of creation: Gertrude Jekyll and the art of flowers

Bisgrove, R. — Wed, 18 Apr 2007 16:07:05 PDT

Flowers were central to the life and work of Gertrude Jekyll (1843–1932), one of the 20th century's most influential garden designers. Born of parents with interests across a broad range of the art–science spectrum, Miss Jekyll developed an early interest in many arts and crafts, including painting and gardening in particular. During her course at the Central School of Design in Kensington she studied closely the work of JMW Turner. Many of the compositional elements of Turner's paintings, especially his use of colour, can be seen in Miss Jekyll's subsequent designs for c. 250 gardens. The use of blue and yellow flowers to create a sense of light, and the contrast of cool blue flowers and grey foliage with vivid reds and oranges are recurrent themes in her planting schemes, but many other aspects of her designs also reflect her broad interest in the art, craft, and science of plant cultivation. She encouraged others to seek the satisfaction offered by gardening as an art, convinced that a life spent seeking perfection would gradually yield ‘the power of intelligent combination, the nearest thing we can know to the mighty force of creation’.