Journal of Experimental Botany, Vol. 53, No. 368, pp. 407-413,
March 1, 2002
© 2002 Oxford University Press
Original Papers |
Is a cysteine proteinase inhibitor involved in the regulation of petal wilting in senescing carnation (Dianthus caryophyllus L.) flowers?
Laboratory of Bio-adaptation, Graduate School of Agricultural Sciences, Tohoku University, Tsutsumidori-amamiyamachi 1-1, Aoba-ku, Sendai 981-8555, Japan
Received 27 April 2001; Accepted 27 September 2001
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Abstract |
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Senescence of carnation petals is accompanied by autocatalytic ethylene production and wilting of the petals; the former is caused by the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase genes and the latter is related to the expression of a cysteine proteinase (CPase) gene. CPase is probably responsible for the degradation of proteins, leading to the decomposition of cell components and resultant cell death during the senescence of petals. The carnation plant also has a gene for the CPase inhibitor (DC-CPIn) that is expressed abundantly in petals at the full opening stage of flowers. In the present study, DC-CPIn cDNA was cloned and expressed in E. coli. The recombinant DC-CPIn protein completely inhibited the activities of a proteinase (CPase) extracted from carnation petals and papain. Northern blot analysis showed that the mRNA for CPase (DC-CP1) accumulated in large amounts, whereas that for DC-CPIn disappeared, corresponding to the onset of petal wilting in flowers undergoing natural senescence and exogenous ethylene-induced senescence. Based on these findings, a role of DC-CPIn in the regulation of petal wilting is suggested; DC-CPIn acts as a suppressor of petal wilting, which probably functions to fine-tune petal wilting in contrast to coarse tuning, the up-regulation of CPase activity by gene expression.
Key words: Carnation, cysteine proteinase, cysteine proteinase inhibitor, Dianthus caryophyllus, flower senescence, petal wilting.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Senescence in carnation flowers is characterized by autocatalytic ethylene production, mostly from petals, and subsequent wilting of the petals. The autocatalytic ethylene production is caused by the expression of 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase genes (Jones and Woodson, 1999
; ten Have and Woltering, 1997; Woodson et al., 1992). The wilting of petals, however, is related to the expression of genes for enzymes such as cysteine proteinase (CPase) (Jones et al., 1995) and lipase (Hong et al., 2000), which are probably responsible for the hydrolytic degradation of cell components leading to cell death during the senescence of petals. The CPase gene is expressed in a moderate amount in carnation petals at the time of flower opening and up-regulated during natural, pollination-induced and exogenous ethylene-induced senescence of the petals (Jones et al., 1995). Recently, Kosugi et al. showed that the expression of the CPase gene (DC-CP1), which leads to wilting, is regulated independently of that of ACC synthase and ACC oxidase genes, which leads to ethylene production, in carnation petals (Kosugi et al., 2000).
On the other hand, Kim et al. recently cloned from carnation plants a cDNA encoding a CPase inhibitor (DC-CPIn), and revealed that the amount of mRNA corresponding to the cDNA increased during petal maturation (Kim et al., 1999). Based on these findings, they suggested that DC-CPIn is one of the genes expressed that is specific to flower maturation. These foregoing observations indicated the concomitant presence of DC-CP1 and DC-CPIn in carnation petals. The authors speculated that DC-CPIn could inhibit CPase activity in petals and suppress the onset of petal wilting during the senescence of carnation flowers. The DC-CPIn could play a role in the regulation of petal wilting in senescing carnation flowers and so a detailed study on the role of DC-CPIn in the regulation of petal wilting of senescing carnation flowers was undertaken. In the present work, an examination was made of the action of DC-CPIn, which was synthesized heterologously in E. coli, on the proteinase activity extracted from carnation petals. Furthermore, the changes in the levels of mRNAs for DC-CPIn as well as DC-CP1 in carnation petals during senescence were examined.
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Materials and methods |
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Plant materials
Carnation flowers (Dianthus caryophyllus L. cv. Reiko) were harvested in the morning at the usual commercial stage of flowering at the nursery of a local grower. They were transported to the laboratory on the day of harvest. Stems were trimmed to 30 cm, placed with their cut end in distilled water and held for 1–2 d under white fluorescent light (40 µmol m-2 s-1) at 23 °C. The flowers were used when their outermost petals were at right angles to the stem of the flower.
Treatment of flowers
Flowers at their full opening stage (day 0) were trimmed to 5 cm in stem length and placed with their stem end in 20 ml distilled water in 50 ml glass vials (one flower per vial). Usually five flowers were used per treatment. For natural senescence, the flowers were left under the conditions described above, and ethylene production was measured every day.
For the treatment with ethylene, the flowers on day 0 were incubated in a 53.2 l glass chamber with ethylene at 10 µl l-1 for 0–24 h under the same conditions as described above. At 4 h intervals, samples of three flowers were taken from the chamber and held in open air for 1 h to diffuse out the accumulated ethylene from flower tissues prior to the measurement of ethylene production. Then ethylene production from the flowers was assayed.
Petals, ovaries and styles were detached from flowers after the assay for ethylene production, immediately frozen in liquid N2 and stored at -80 °C until isolation of RNA.
Assay for ethylene production
Ethylene production by the whole flower was measured by enclosing the flowers in 350 ml glass containers (one flower per container) for 1 h at 23 °C (Kosugi et al., 1997). A 1 ml gas sample was taken into a hypodermic syringe from the inside of the container through a rubber septum of a sampling port on the lid of container, and analysed for ethylene with a gas chromatograph (Model 263-30, Hitachi) equipped with an alumina column and a flame ionization detector.
Isolation and molecular analysis of DC-CPIn cDNA
A cDNA encoding DC-CPIn was amplified by RT-PCR with total RNAs obtained from carnation petals and primers derived from the sequence of DC-CPIn cDNA (Kim et al., 1999). The sequence of primers were 5'-CCCGGATCCGTCTAATAAAAAATGGCAAC-3' for the upstream primer and 5'-CCCGAGCTCCAACCATAAGTTTCTGTC-3' for the downstream primer. RT-PCR was performed according to the standard procedure with necessary optimization. The PCR product was cloned into pT7Blue T-Vector (Novagen) for sequencing. Sequence data were analysed using DNASIS software (Hitachi Software Engineering).
Expression of DC-CPIn in E. coli
For the construction of an expression plasmid for DC-CPIn, the entire coding region was amplified from DC-CPIn cDNA by PCR with the upstream and downstream primers, 5'-CCCCATATGGCAACAGTTGGTGGAA-3' and 5'-CCCGGATCCGTACATGAACATCTCTCC-3', respectively. The amplified product was digested with NdeI and BamHI, and the resultant fragment was inserted into the corresponding site of pET15b (Novagen).
The constructed plasmid was introduced into E. coli BL21. The E. coli cells were cultured in LB medium (10 g of polypeptone, 5 g of yeast extract and 10 g of NaCl in 1.0 l, pH 7) at 37 °C for 7 h until the A600 reached about 0.6. Thereafter isopropylthio-ß-D-galactoside (IPTG) was added to the culture at 1 mM. The culture was further incubated for 3 h at 37 °C, and cells were collected. The soluble extract from cells was subjected to purification with a nickel chelate matrix (HisTrap Kit, Amersham Pharmacia Biotech) according to the manufacturer's instructions. The obtained DC-CPIn was checked for purity by SDS–PAGE and detection with dye-staining as well as immunodetection with an antibody to His-Tag® sequence.
Preparation and assay of proteinase and inhibition
Proteinase was prepared from carnation petals at the full-opening stage according to the method used for the preparation from daylily tepal tissue (Guerrero et al., 1998). The macromolecular fraction after dialysis was used as an enzyme extract. Proteinase activity was measured (according to the method of Irie et al., 1996). In brief, reaction mixtures contained a sample of enzyme extract (1 µg protein), 1 mM EDTA, 1 mM DTT, and 100 mM Na-phosphate, pH 6.0, in a total volume of 2 ml. Each reaction mixture was preincubated at 37 °C for 10 min, and Z-Phe-Arg-MCA (Peptide Institute) as a fluorescent substrate was added to the mixture at a final concentration of 2 µM. The reaction mixture was further incubated at 37 °C for 1 min, during which period the increase in fluorescence of 7-amino-4-methylcoumarin (AMC) liberated from the substrate was monitored at 370 nm (excitation) and 460 nm (emission). Proteinase activity was defined as the amount of AMC (pmol) released min-1. For comparison, the enzyme reaction was also conducted with papain, a typical CPase (P3125, Sigma). To assay the inhibitory action of DC-CPIn against the carnation proteinase and papain, given amounts (0–0.05 µg ml-1 at the final concentration) of DC-CPIn were added to the reaction mixtures. Protein contents were determined using bovine serum albumin as the standard (Bradford, 1976).
Northern blot analysis
Total RNA was isolated by an SDS–phenol method (Palmiter, 1974) from carnation flower tissues. poly(A)+ RNA was obtained from total RNA using Oligotex-dT30 (Takara) according to the manufacturer's instructions. The probe for DC-CPIn mRNA was constructed from the almost entire region of DC-CPIn cDNA (417 bp) by labelling with 32P. The construction of the DNA probe for DC-CP1 mRNA was as described previously (Kosugi et al., 2000), but labelled with 32P in the present experiment. Also, the construction of the 32P-labelled DNA probe for mRNA of carnation actin (DC-ACT1) was as described previously (Waki et al., 2001). Northern blot analysis was conducted as described previously (Shibuya et al., 2000). Blots were used for multiple hybridization after stripping in boiling 0.1% (w/v) SDS. Hybridization signals for DC-CP1 and DC-CPIn mRNAs were normalized using the NIH image software against the signals of actin mRNA, and data obtained were shown as the relative levels of mRNAs.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Inhibition by DC-CPIn of proteinase activity extracted from carnation petals
A cDNA encoding DC-CPIn was amplified by PCR from total RNA obtained from carnation (Dianthus caryophyllus L. cv. Reiko) petals at the full opening stage with the primers derived from the previously reported sequence of the cDNA for DC-CPIn in carnation cv. Degio (Kim et al., 1999
). The cDNA was 481 bp long and contained a 294 bp long open reading frame, 12 bp long 5'-flanking sequence and 175 bp long 3'-flanking sequence. The predicted protein consisted of 98 amino acids and had a calculated molecular mass of 10.8 kDa. The predicted amino acid sequence had an identity of 93.8% with that of the previously reported DC-CPIn (Kim et al., 1999). A marked difference was found in the amino acid sequence at the COOH terminus; the predicted protein had Pro80-Trp81 instead of Pro80-Gly81 in the previously reported DC-CPIn (Kim et al., 1999). The Pro-Trp sequence near the COOH terminus is commonly present in plant CPase inhibitors (phytocystatin) as well as family II and III CPase inhibitors (cystatin) and is shown to be one of the motifs necessary for binding to CPase protein (Abe, 1997; Kondo et al., 1990; Machleidt et al., 1989). Other amino acid differences probably result from genotypic differences between ‘Reiko’ and ‘Degio’ cultivars, which were used in the present and previous studies, respectively. In spite of these differences, the overall high identity between the present and previous cDNAs for DC-CPIn indicates that they originated from the corresponding genes in the two cultivars of carnation plants. The present cDNA was regarded to have originated from the gene DC-CPIn, and deposited in the GenBank under Accession No. AY028994. The DC-CPIn cDNA was expressed in E. coli as a fusion protein with His-Tag® and purified by affinity chromatography with a nickel chelate matrix. The expressed product gave a single band after resolution by SDS–PAGE and detection with dye-staining of protein as well as immunodetection with antibody against the His-Tag® (data not shown). The fusion protein was expected to be comprised of 118 amino acid residues. The NH2-terminal of 20 amino acid residues originated from the pET15b vector and contained His-Tag® sequence, and the following 98 amino acid residues from DC-CPIn.
The recombinant DC-CPIn (rDC-CPIn) was used for measurement of its inhibitory activity against a crude proteinase extracted from carnation petals and papain as a CPase reference (Fig. 1). rDC-CPIn inhibited both the carnation petal proteinase and papain with similar dose-inhibition profiles; 50% inhibition was attained at around 0.005 µg ml-1 and complete inhibition at 0.05 µg ml-1 for both carnation proteinase and papain. These findings indicated that the crude proteinase fraction from carnation petals contained a papain-type CPase, probably DC-CP1, whose action was inhibited by rDC-CPIn. However, the present observation did not always rule out the presence of more than one papain-type CPases in the crude proteinase fraction. Therefore, further characterization of rDC-CPIn activity was not conducted with the crude proteinase fraction. Precise characterization of rDC-CPIn, such as determination of Ki against carnation CPase, using recombinant carnation CPase remains a future project.
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) Carnation petal proteinase; (•) papain.
Changes in mRNA levels of DC-CPIn and DC-CP1 during natural senescence of carnation flowers
Changes in the levels of mRNAs for DC-CPIn and DC-CP1 were monitored by Northern blot analysis with probes for DC-CPIn and DC-CP1 mRNAs (Fig. 2). mRNAs for analysis were isolated from petals, ovaries and styles of carnation flowers, which underwent natural senescence. The DC-CPIn probe detected one mRNA of about 0.6 kbp, and the DC-CP1 probe one mRNA of 1.9 kbp. However, this did not always rule out possible hybridization with mRNAs for unidentified CPase and CPase inhibitors, if any, other than DC-CPIn and DC-CP1 in carnation plants since the probes were derived from the coding region of DC-CPIn and DC-CP1 cDNAs.
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) DC-CP1 mRNA; (
) DC-CPIn mRNA.During natural senescence of the flower, ethylene production was detected on day 4 after full opening (day 0) of the flowers, attaining the maximum rate on day 6 (Fig. 2A
). Petal wilting became visible on day 6 and developed markedly on day 7. In agreement with the increase in ethylene production, mRNAs for ACC synthase and ACC oxidase in the petals and ovaries accumulated (data not shown).
The mRNAs for DC-CP1 and DC-CPIn were present in significant amounts in petals, ovaries and styles of the flowers at the full opening stage. In petals, the amount of mRNA for DC-CP1 remained almost constant until day 4, increased abundantly on day 5, corresponding to the onset of petal wilting, and declined on day 6. By contrast, DC-CPIn mRNA decreased and disappeared during the petal-wilting phase of flower senescence; the mRNA level remained unchanged until day 4, decreased on day 5 and no mRNA was detected on day 6. In ovaries, the level of DC-CP1 mRNA remained unchanged until day 2 and increased up to day 6, but in styles it remained almost the same for 6 d. In ovaries and styles, the mRNA level for DC-CPIn did not decrease during senescence, rather it increased slightly in the former at the latest stage of senescence. Changes in mRNA level for DC-CP1 were basically in agreement with those found in the previous study (Jones et al., 1995).
Increase in CPase activity in petals undergoing wilting
The data presented above suggested an increase in CPase activity due to up-regulation of DC-CP1 and down-regulation of DC-CPIn in petals at the later stage of senescence, corresponding to the onset of wilting. Thus, the activity of the proteinase extracted from senescing petals as well as ethylene production from whole flowers were monitored during the senescence of carnation flowers (Fig. 3). In this experiment, the onset of ethylene production was delayed by 2 d, as compared to that shown in Fig. 2, probably because of the difference in carnation samples that were harvested at different seasons. The proteinase activity remained unchanged until day 4 and increased substantially on day 6 through day 8.
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Effects of exogenous ethylene on the levels of mRNA for DC-CPIn and DC-CP1 in petals
In carnation flowers treated with exogenous ethylene on day 0, autocatalytic ethylene production started 4 h after the application of ethylene, and increased thereafter up to 20 h (Fig. 4A). The in-rolling of petal margins of flowers became visible 4 h after the start of ethylene treatment, advanced thereafter and the flowers wilted completely at 24 h.
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Ethylene treatment led to inverse changes in mRNA levels for DC-CP1 and DC-CPIn; an increase in the former versus a decrease in the latter (Fig. 4B
). The DC-CP1 mRNA level increased by more than 2-fold at 4 h and remained elevated until 20 h of exposure. By contrast, the decrease in DC-CPIn mRNA level was noticeable after 8 h of ethylene exposure and the levels at 16–24 h were below 50% of that at 0 h. These findings indicated a role of ethylene in the down-regulation of the level of mRNA for DC-CPIn in addition to the up-regulation of that of DC-CP1 (Jones et al., 1995; Kosugi et al., 2000).
Comparison among tissues in the levels of DC-CPIn mRNA
The DC-CPIn mRNA was present in leaves and stems in addition to flower tissues of carnation plants at the full-opening stage (Fig. 5). The mRNA level was higher in the petal and style tissues than in the other tissues. These findings were different from the previous observation (Kim et al., 1999), which showed that the expression of DC-CPIn was restricted to petals and styles within carnation flowers. The difference between the present results and the previous ones was not investigated further.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Senescence of carnation petals is accompanied by autocatalytic ethylene production and wilting of the petals. The latter is triggered by the action of hydrolytic enzymes, which decompose cell constituents, such as protein and lipids, resulting in disintegration of the cell structure and cell death during the senescence of petals. DC-CP1 is probably responsible for the degradation of proteins, since its gene is greatly up-regulated at a later stage of senescence of carnation petals (Jones et al., 1995
). However, DC-CP1 is also expressed in a significant amount at the time of flower opening in carnation petals (Jones et al., 1995). Thus, it is possible that there is some unknown mechanism to suppress the action of CPase in petal cells, which functions at the stage of flower opening and becomes out of function at a later stage of flower senescence.
Carnation CPase inhibitor was obtained by expressing DC-CPIn cDNA in E. coli and the inhibitory effect of the recombinant inhibitor (rDC-CPIn) was examined against the proteinase extracted from carnation petals (Fig. 1). rDC-CPIn actually inhibited the action of the proteinase of carnation petals; 50% inhibition was attained at around 0.005 µg ml-1 and complete inhibition at 0.05 µg ml-1. Since the inhibitor acted with a similar mode on papain, a CPase, the proteinase extracted from carnation petals was thought to be a papain-type CPase. Judging from the presence of DC-CP1 mRNA in carnation petals (Fig. 2; Jones et al., 1995), it was highly probable that the proteinase was DC-CP1, although the contribution of papain-type cysteine proteinases other than DC-CP1 was not ruled out. Based on the present findings and considerations, it is suggested that DC-CPIn would inhibit the action of DC-CP1 in carnation petals.
The amounts of mRNAs for DC-CP1 and DC-CPIn in petals of flowers undergoing natural senescence inversely changed at the late stage of petal senescence (days 5 and 6) corresponding to the onset of petal wilting; the DC-CP1 mRNA accumulated in a large amounts, whereas the DC-CPIn mRNA disappeared (Fig. 2). In addition, exogenously applied ethylene accelerated the inverse changes in mRNA levels between DC-CP1 and DC-CPIn and petal wilting in the treated flowers (Fig. 4). Simultaneously with the inverse changes in both mRNAs, the proteinase activity in petals increased substantially during natural senescence of carnation flowers (Fig. 3). Superficially these findings show the increase in DC-CP1 protein and the decrease in DC-CPIn protein in the senescing petals. The DC-CP1 activity, however, can increase by the decrease in the amount of DC-CPIn protein without the increase in the DC-CP1 protein. Also it is likely that even if the transcription of the DC-CPIn mRNA stops there is still plenty of previously transcribed DC-CPIn protein in petals. To address these questions, further studies are needed to determine, by using specific antibodies, changes in the contents and localization of DC-CPIn, as well as those of DC-CP1, in the petal cells of carnation flowers.
The present findings, as a whole, implied that the inhibition, if any, by an inhibitor of CPase activity in petals decreased along with the progress of senescence and disappeared at the latest stage of the senescence of carnation flowers. Thus, DC-CPIn is thought to play a role in the regulation of petal senescence of carnation flowers. In other words, DC-CPIn would act as a suppressor of petal wilting, which probably functions to fine-tune petal wilting in contrast to coarse tuning, the up-regulation of CPase activity by gene expression.
In ovaries, the level of DC-CPIn mRNA, along with that of DC-CP1 mRNA, increased toward the late stage of natural senescence, whereas in styles the levels of both mRNAs remained almost unchanged during the senescence period. These results implied that proteinase activities in both tissues did not change significantly during the senescence period. The increased or unchanged levels of DC-CPIn mRNA, against those of DC-CP1 mRNA, may play a role in protection of the tissues from deleterious effects of CPase for the sound growth of the tissue, especially ovary tissue, during and after senescence of the carnation flowers.
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Notes |
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1 To whom correspondence should be addressed. Fax: +81227178834. E-mail: ssatoh{at}bios.tohoku.ac.jp
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