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JXB Advance Access originally published online on January 19, 2007
Journal of Experimental Botany 2007 58(5):947-956; doi:10.1093/jxb/erl255
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© The Author [2007]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

Isolation and functional characterization of PgTIP1, a hormone-autotrophic cells-specific tonoplast aquaporin in ginseng*

Wuling Lin1 {dagger}, Yanhui Peng1,2 {dagger}, Guowei Li1, Rajeev Arora2, Zhangcheng Tang1, Weiai Su1 and Weiming Cai1,{ddagger}

1Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, and Graduate School of the Chinese Academy of Sciences, 300 Fenglin Road, Shanghai 200032, China
2Department of Horticulture, Iowa State University, Ames, IA 50011, USA

{ddagger} To whom correspondence should be addressed. E-mail: wmcai{at}sippe.ac.cn

Received 19 July 2006; Revised 10 October 2006 Accepted 30 October 2006


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Supplementary data
 References
 
The suppression subtractive hybridization technique was used to identify differentially expressed genes between hormone-autotrophic and hormone-dependent Panax ginseng callus lines. A tonoplast intrinsic protein cDNA (PgTIP1) was found to be highly and specifically expressed in hormone-autotrophic ginseng cells, which was slightly up-regulated by cytokinin while significantly down-regulated when treated with auxin. PgTIP1 encodes a polypeptide of 250 amino acids which shows sequence and structure similarity with tonoplast aquaporins in plants. The water channel activity of PgTIP1 was demonstrated by its expression in Xenopus laevis oocytes. When over-expressed in Arabidopsis thaliana, PgTIP1 substantially altered the plant's vegetative and reproductive growth and development. Arabidopsis plants over-expressing PgTIP1 showed significantly enhanced seed size and seed mass plus greatly increased growth rate compared with those of the wild type. Moreover, the seeds from PgTIP1 over-expressing Arabidopsis had 1.85-fold higher fatty acid content than the wild-type control. These results demonstrate a significant function of PgTIP1 in the growth and development of plant cells.

Key words: Arabidopsis, ginseng, habituation PgTIP1, tonoplast intrinsic protein


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Supplementary data
 References
 
The growth and development of plant cells depend on tight regulation of cellular water movement and homeostasis. Aquaporins (AQPs), which facilitate and regulate passive exchange of water across membranes (Agre, 1992; Chrispeels and Agre, 1994; Schäffner, 1998), belong to a highly conserved membrane protein family MIP (major intrinsic protein). In higher plants, AQPs are divided into four subfamilies: PIPs, TIPs, NIPs, and SIPs, mainly based on the membrane location and function (Chaumont et al., 2001; Johanson et al., 2001; Baiges et al., 2002; Quigley et al., 2002). Aquaporin expression can be regulated at both the RNA and protein levels. Over-expression or antisense/knockout reduction of certain AQPs highlights their important roles in numerous physiological processes in plants (Kaldenhoff et al., 1995, 1998; Gerbeau et al., 2002; Javot et al., 2003; Aharon et al., 2003; Uehlein et al., 2003; Hanba et al., 2004; Hachez et al., 2006). Although the discovery of AQPs has resulted in a paradigm shift in the understanding of plant water relations, a comprehensive picture of their physiological role(s) in plant growth and development remains elusive.

It is well established that in vitro plant cell/tissue cultures require exogenous supply of plant hormones (auxins and cytokinins) for their sustained growth (Collin and Edwards, 1998). However, certain cell-lines, although originally grown in hormone-based culture medium, may lose this dependency on one or more externally supplied plant hormones for growth and become hormone-autotrophic or habituated/autonomous (Meins, 1989; Gaspar et al., 2002). Habituation, a stable heritable competency of plant cells to proliferate without hormonal supply, is a distinct response from the one associated with tumour development: tumours are mediated by pathogens or result from genetic transformation (Gaspar, 1998). Despite the reports on habituation in many cell types (Jäger et al., 1997), what controls this phenomenon is not well understood.

Although increasing the levels of auxins and cytokinins would result in cell expansion, the physiological factor that directly drives plant cell expansion is turgor pressure, which is mainly generated by a rapid influx of water into the cells. The uptake of water by expanding plant cells may well involve AQPs. Evidence showed that some AQPs were up-regulated by exogenous auxin during cell growth (Werner et al., 2001; Ozga et al., 2002). We have on hand habitutated callus line from Panax ginseng that exhibits autotrophy for both auxin and cytokinin. No significant difference between these two callus lines was found with respect to the level of active free auxin and cytokinin concentration by ELISA measurement (data not shown). In order to explore the molecular differences between habituated and non-habituated ginseng calli, the differentially expressed genes were screened using the suppression subtractive hybridization (SSH) method. Besides the down-regulation of some cDNAs in habituated cells, for example, early auxin responsive gene GH3, periodic tryptophan protein gene PWP, aconitase gene ACO, retrotransposon-like gene, and many other genes with so far unknown functions, an aquaporin gene (PgTIP1) was detected, which belongs to plant TIPs subfamily, and was specifically and highly expressed in hormone-autotrophic ginseng cells. The results from our study, which demonstrated that PgTIP1 significantly altered the growth and developmental attributes of plants when over-expressed in Arabidopsis thaliana, are reported here.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Supplementary data
 References
 
Plant cell cultures
Hormone-autotrophic (H; habituated) and hormone-dependent (NH; non-habituated) Panax ginseng calli were cultured on hormone-free 67V medium and 67V medium, respectively (Veliky and Martin, 1970); the latter supplemented with 1.5 mg·l–1 dichlorophenoxyacetic acid (2,4-D), 1 mg l–1 indole-3-acetic acid (IAA), 0.1 mg l–1 naphthalene acetic acid (NAA), and 0.25 mg l–1 kinetin (KT), pH 5.8. Both callus lines were maintained at 24 °C in the dark. Suspension cultures of the ginseng calli were maintained at 24 °C in 250 ml flasks with 50 ml medium; cultures were aerated by shaking at 0.5 g on a rotary shaker in the dark, and were subcultured every 28 d.

Suppression subtractive PCR and northern blot analysis
PolyA+ RNA were isolated from hormone-autotrophic and hormone-dependent ginseng cells using mRNA purification kit (Qiagen). The polyA+ samples were used to construct driver- and tester-cDNAs following the protocol of the PCR-Select cDNA subtraction kit provided by the manufacturer (Clontech). Selectively amplified products were inserted into pMD18-T vector using a T/A cloning kit (Takara). Northern blot analysis was performed using the digoxigenin-dUTP system (Roche) and the cDNA fragment screened from SSH library was labelled as a probe for hybridization.

Full-length cDNA cloning of PgTIP1 and bioinformatic analysis
A cDNA library of hormone-autotrophic ginseng cells was constructed and screened according to the instructions in ZAP Express Predigested Vector Kit and ZAP Express Predigested Gigapack Cloning Kit (Stratagene). Nucleotide and deduced amino acid sequences were analysed with Bioedit software. Homology searches were made in all major databases. Alignments of amino acid sequences were generated and edited with DNAStar software. Predicted stereo structure of PgTIP1 and AQP2 and ar/R region simulation were performed at http://swissmodel.expasy.org (Schwede et al., 2003).

Hormone treatments and analysis of PgTIP1 expression
The phytohormones KT and 2, 4-D or both were added to ginseng suspension cultures at the final concentration of 0.25 mg l–1 and 1.5 mg·l–1, respectively. To determine the transcript levels in different hormone-treated samples, the real-time quantification of RNA target was performed in the Rotor-Gene 3000 real-time thermal cycling system using SYBR Green RT-PCR kit (Toyobo). The PCR primers for Act were 5'-GTGTTGCCCCAGAAGAGC-3' in sense and 5'-CAGAATCCAGCACAATACCT-3' in antisense orientation, and those for PgTIP1 were 5'-CTCAGGCTTGGCATTTAG-3' and 5'-CCCAGTTCTCCCTTCTTT-3', respectively. The reaction mixture (25 µl) contained 200 ng of total RNA, 0.5 µM of each primer, and appropriate amounts of enzymes and fluorescent dyes as recommended by the manufacturer. The Rotor-Gene 3000 cycler was programmed as follows: 2 min at 95 °C for pre-denature; 40 cycles of 15 s at 94 °C, 15 s at 55 °C, 20 s at 72 °C for Act and 30 s at 94 °C, 30 s at 55 °C, 30 s at 72 °C for PgTIP1. The data were collected during the extension step. No detectable fluorescence signal was detected in control samples where H2O was added to the reaction mixture instead of RNAs. A possible contamination by genomic DNA of the RNA sample was carefully monitored and avoided.

Xenopus oocyte expression
The coding region of PgTIP1 and AQP2 (positive control) were cloned into pXBG-ev1 vector (Li et al., 2000) using a Bgl II restriction site. After digestion and linearization of the plasmid, the complementary RNAs (cRNA) were synthesized in vitro using the mMESSAGE mMACHINE High Yield Capped RNA Transcription Kit (Ambion). Oocyte preparation, injection, and expression were performed as described by Daniels et al. (1996). Osmotic water permeability of oocytes was determined essentially as described by Weig et al. (1997).

Generation of PgTIP1-overexpressing Arabidopsis plants and their phenotypic analysis
The ORF of PgTIP1 was cloned into pHB vector (Mao et al., 2005) using a HindIII and a XbaI restriction site to generate double 35S:PgTIP1 transgene. Arabidopsis plants (ecotype Columbia-0) were transformed with this transgene using the floral-dipping method (Clough and Bent, 1998). Independent hygromycin-resistant lines (T0) were isolated and amplified. Experiments were conducted with homozygote T2 plants.

The root length measurements were made on 1-week-old plants cultured with MS medium on a vertical plate. Leaf histological analysis (fifth-leaf samples, 2-week-old) was performed according to Hu et al. (2003).

Average mass of the seeds produced by PgTIP1 over-expressing Arabidopsis plants was determined by weighing mature dry seeds in batches of 1000; at least three sample batches (of 1000 each) were weighed for a given data point and average value was from three independent transgenic lines. Size distributions of WT and transgenic seed populations were analysed by a scanning electron microscope equipped with ‘smileview’ software (JEDL JSM-6360LV, Japan).

Fatty acid extraction was performed as described by Fiehn et al. (2000) using 0.3 g dry seeds. Nonadecanoic acid methyl ester stock solution (2 mg ml–1 CHCL3) and ribitol stock solution (0.2 mg ml–1 H2O) were used as internal standards for the lipid phase and the polar phase, respectively. Selected subsamples were injected in a GC-mass spectrometer (6890N GC System/5973 MS Selective Detector) and resultant electron ionization mass spectra were used to identify and quantify individual fatty acid species. The quantity of each fatty acid was determined by comparison with the internal standard and average value of total fatty acid content was from three independent transgenic lines as example. Seeds were collected from Arabidopsis thaliana plants that were grown at 22 °C with a 16/8 h day/night cycle in a greenhouse.

Leaf net photosynthetic rates, stomatal conductance, intercellular CO2 concentration, and transpiration rates were measured by a portable gas analysis system, Li-Cor 6400 with a light-emitting diode light source (Li-Cor Inc. Lincoln, Nebraska, USA).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Supplementary data
 References
 
PgTIP1 belongs to the MIP super gene family
SSH employing the mRNAs from autonomous and hormone-dependent ginseng cells allowed the isolation of a cDNA for tonoplast intrinsic protein that is referred to here as PgTIP1. The full-length cDNA of PgTIP1 includes a 750 bp open reading frame (GenBank accession number DQ237285), and encodes a protein of 250 amino acids (Fig. 1). BLASTX and ClustalX analyses (Thompson et al., 1997) of this gene indicated high similarities to putative plant aquaporins: PgTIP1 deduced protein is most similar to NtAQP1 (80.8% identity), a tonoplast aquaporin, and has high homology to AtTIP1;1, OsTIP1;1, and ZmTIP1;2 at 78.8%, 77.2%, and 68.4%, respectively. The hydrophobicity profiles of PgTIP1, as determined by using BioEdit and TMHMM (http://www.cbs.dtu.dk/services/TMHMM), indicated six highly hydrophobic regions (A–F) corresponding to membrane-spanning putative {alpha}-helices that are characteristic of AQPs. It also has two NPA domains, which form the water pore within the membrane lipid bilayer. Secondary and stereo structural analyses (see Fig. 1 in the supplementary data at JXB online) revealed the similarity of three-dimensional structure of PgTIP1 to that of aquaporin 2 (AQP2). The conserved narrow selectivity filter region (the aromatic/Arg [ar/R] filter) of PgTIP1 is formed by H-65(H2), I-186(H5), A-195(LE1), and V-201(LE2) (see Fig. 2 in the supplementary data at JXB online). This ar/R tetrad was identical to that of the Group I TIP in Arabidopsis (AtTIP1;1) (Wallace and Roberts, 2004), which has already been shown to have high water channel activity.


Figure 1
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  		Fig. 1. The sequence alignment of PgTIP1 and other plant TIPs.

 
PgTIP1 is a novel gene that is specifically expressed in habituated ginseng cells and inhibited by auxin
PgTIP1 fragment was strongly expressed in autonomous ginseng cells but was not detected in hormone-dependent cells as indicated by northern blot analysis (Fig. 2A). This is a first report on an aquaporin expression response in context with the phenomenon of autonomous growth. Using the real-time reverse transcriptase (RT)-PCR method, the expression of PgTIP1 under different hormone treatments was studied in suspension-cultured hormone-autotrophic ginseng cells. Data indicated that the PgTIP1 was up-regulated by kinetin (KT, 0.25 mg l–1) treatment while significantly down-regulated by 1.5 mg l–1 2, 4-dichlorophenoxyacetic acid (2, 4-D); its expression was also down-regulated when the cells were exposed to both the hormones together (Fig. 2C).


Figure 2
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  		Fig. 2. PgTIP1 expression in habituated and non-habituated ginseng cells: effect of hormones. (A) Northern blot analysis of PgTIP1 in habituated (H) and non-habituated (NH) ginseng cells. (B) Habituated ginseng calli (H) and non-habituated ginseng calli (NH). (C) PgTIP1 transcription in hormone-autotrophic suspension ginseng cells when treated with 0.25 mg l–1 KT and/or 1.5 mg l–1 2,4-D for 6, 12, 24, 48, 96 h, respectively; the data are given as the mean ±SE (n=3).

 
Water-channel activity of PgTIP1
To find out whether PgTIP1 is a functional aquaporin, water-channel activity of PgTIP1 was assayed in the Xenopus oocytes system; AQP2 was included as a positive control in these assays. Three days after cRNA or water injection, the rate of cell volume change (Fig. 3A) and the osmotic Pf values (Fig. 3B) were calculated in the presence of an osmotic gradient. The Pf of PgTIP1-expressing, AQP2-expressing, and water-injected oocytes was 3.19 x 10–2 cm s–1, 2.36x10–2 cm s–1, and 0.14x10–2 cm s–1, respectively. Oocytes expressing PgTIP1 yielded 23-fold and 1.35-fold higher Pf than that of the water-injected oocytes and the positive control, respectively, suggesting that PgTIP1 is, indeed, a functional aquaporin with high water-channel activity.


Figure 3
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  		Fig. 3. Water-channel activity PgTIP1. (A) Initial swelling rates of Xenopus laevis oocytes injected with cRNA encoding PgTIP1, mammalian AQP2 (as positive control) or water (as negative control). The rate of oocyte swelling upon immersion in hypo-osmotic medium is plotted as V/V0 versus time, where V is the volume at a given time point and V0 is the initial volume. (B) Osmotic water permeability coefficient (Pf) of oocytes injected with cRNA encoding PgTIP1, AQP2, or water. The Pf values were calculated from the initial rate of oocyte swelling. Data are given as the mean ±SE (n=30).

 
Increased growth rate and enhanced seed size, mass, and fatty acid content in Arabidopsis plants over-expressing PgTIP1
Specific expression of PgTIP1 in hormone-autotrophic ginseng cells suggested its potential involvement in cell division and/or growth. A PgTIP1 over-expression construct was generated in Arabidopsis to explore its physiological function, if any. The transcription of PgTIP1 was observed in roots, stems, leaves, flowers, and siliques of Arabidopsis transformants by real-time RT-PCR analysis (data not shown). Among 24 independent transformed lines, 22 (approximately 92%) PgTIP1 over-expressers exhibited faster growth rate than the wild-type (WT) control as evidenced by the root elongation, leaf expansion, and weight increase of the aerial parts (Fig. 4). Root length of the one-week-old transgenic Arabidopsis seedlings was significantly greater than that of the WT control (Fig. 4A, B). The overall leaf-size was bigger in transgenic plants as indicated by the length and width of the leaf-blade and the petiole length of 3-week-old; fifth leaves (Fig. 4C, E, F). Histological observations using 2-week-old seedlings also indicated the size of mesophyll cells in transgenic plants to be bigger than that of the WT leaves (Fig. 4D). The increase in the weight of the aerial parts over time (days after germination) for the WT and PgTIP1 over-expressors demonstrated faster and stronger growth of transgenic plants (Fig. 4G). PgTIP1 over-expression also resulted in precocious flowering in Arabidopsis plants; transgenic plants flowered at least 3 d earlier than WT (Fig. 4H). The most intriguing phenotype of Arabidopsis over-expressing PgTIP1 was that of the size and mass of mature seeds in that both attributes were significantly higher in transgenic plants compared to the WT (Fig. 4I, J; Table 1).


Figure 4
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  		Fig. 4. Phenotypic and molecular characterization of PgTIP1 over-expressing Arabidopsis plants. (A) One-week-old seedlings of WT and transgenic Arabidopsis cultured with MS medium on a vertical glass plate. (B) Root length of 1-week-old seedlings from WT and transgenic Arabidopsis. Data are given as the mean ±SE (n=50). (C) Three-week-old seedlings of WT and PgTIP1 over-expressing Arabidopsis plants. (D) Transverse sections of the fifth leaves (2-week-old) of WT control and PgTIP1 over-expressing plants. The palisade (p) and spongy mesophyll (s) cells are indicated. Bars=100 µm. (E) Morphology of 3-week-old fifth leaves. (F) Dimensions of 3-week-old fifth leaves as shown in (E). Data are given as the mean ±SE (n=15). (G) The aerial parts weight over time (days after germination) for the WT and PgTIP1 over-expressing Arabidopsis plants. (a) Fresh weight increases during one month of growth after germination. (b) Dry weight increases during one month of growth after germination. Data are given as the mean ±SE (n=15). (H) Four-week-old WT and PgTIP1 over-expressing Arabidopsis plants, indicating precocious flowering in transgenic plants. (I) Mature dried seeds from WT and PgTIP1 over-expressing Arabidopsis plants. (J) Scanning electron micrographs of mature Arabidopsis seeds from WT control and PgTIP1 over-expressing Arabidopsis plants. Bars=100 µm.

 

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  		Table 1. Morphometric measurements of dimensions and mass of seeds from the WT and PgTIP1 over-expressing Arabidopsis plants: data are given as means ±SE

 
Differential expression of aquaporin genes is essential to plant growth and stress tolerance but their effect on seed characteristics has never been reported. In order to explore this intriguing transgenic effect further, the protein, sugar and fatty acid content of the seeds from transgenic and WT plants was analysed. Total protein and sugar contents of the seeds showed no significant difference between the transgenic and WT plants (data not shown), while the total fatty acid content per unit weight of the seeds from transgenic plants was ~1.85-fold of that from the WT. Furthermore, compositional analysis of fatty acids showed some differences between the two seed types; with the relative proportion of 18:1 increased and 18:3 decreased in the PgTIP1 over-expressors (Table 2).


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  		Table 2. Fatty acid content and composition of seeds from the WT and PgTIP1 over-expressing Arabidopsis plants: the data are given as the mean ±SE (n=3)

 
Transpiration and photosynthetic behaviour of Arabidopsis plants over-expressing PgTIP1
Leaf transpiration rate (Tr), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and leaf net photosynthetic rates (Pn) in WT and PgTIP1 over-expressing plants were measured when leaves were fully expanded. Results indicated that Arabidopsis plants over-expressing PgTIP1 had higher Gs and Tr than the WT plants (Fig. 5A, C), indicating a stronger water absorption and transpiration ability. A higher Ci concentration was also detected in transgenic Arabidopsis (Fig. 5B), but the level of Pn in PgTIP1 over-expressing Arabidopsis had no significant change compared to the WT plants (Fig. 5D); higher Ci in PgTIP1 over-expressing Arabidopsis may be a consequence of greater Gs (potentially resulting in greater CO2 influx) but similar Pn rates of WT and transgenic plants. The explanation for no apparent difference in the Pn rates for the two genotypes, despite the difference in their Ci levels, could not be determined in this study. It is noteworthy, however, that stomatal conductance (Gs) is just one of the factors that would control the CO2 acquiring capacity and Pn level in transgenic Arabidopsis. We still postulate that the transgenic Arabidopsis plants may accumulate more assimilation product due to their larger leaf size.


Figure 5
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  		Fig. 5. Leaf stomatal conductance (Gs) (A), intercellular CO2 concentration (Ci) (B), transpiration rate (Tr) (C), and leaf net photosynthesis rates (Pn) (D) in WT control and PgTIP1 over-expressing Arabidopsis. Values indicate a mean of three measurements with standard deviations, each with a sample size of eight leaves. One of the triplicate trials is shown. Regression analysis confirmed that the Gs, Ci, and Tr values of PgTIP1 over-expressing plants differ significantly from those of the WT (asterisk, P≤0.01).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Supplementary data
 References
 
Habituation refers to a naturally occurring phenomenon whereby callus cultures, upon continued subculture, lose their requirement for auxin, cytokinin, or both and it is considered to be an in vitro epigenetic switch to autotrophy (Meins, 1983; Syono and Fujita, 1994). The physiological basis of habituation is still unknown. General opinion is that habituation results from an enhanced accumulation by cells of the hormone for which they are habituated. But there are reports that auxin and cytokinin are present in both cell types at roughly the same concentration (Meins, 1989). It is noteworthy that no significant difference with respect to the endogenous levels of auxin or cytokinin was observed between the habituated and non-habituated ginseng cells in this study. Based on the latest research on a cytokinin-habituated callus line in Arabidopsis (Pischke et al., 2006), it also seems less likely that habituation is caused by an over-production of endogenous hormones, and more likely that it may be caused by altered expression of one or more other genes, for example, cytokinin-signalling genes. Cytokinin sensitivity may be modulated through regulation of cytokinin-receptor production. And epigenetic changes, instead of increases in hormone concentration, contribute to the acquisition of auxin/cytokinin-habituation. All these indicate that the endogenous hormone level might not be the key factor in the habituation course and habituation may arise from processes downstream from perception of hormone stimuli, which control cell division and expansion, as in animal cancer cells where activation and expression of genes bypass the requirement for specific growth factors (Hagège et al., 1994). Furthermore, over-expression of a specific gene has been shown artificially to confer habituation in callus tissues (Kakimoto, 1996; Hwang and Sheen, 2001; Sakai et al., 2001; Osakabe et al., 2002). It is speculated that altered hormone signalling routes and hormone sensitivity might lead to this complicated phenomenon. In fact, the proliferation of habituated ginseng callus tissues was inhibited by exogenously applied auxin (the same concentration as in the media for non-habituated cell lines) in our study, which suggested that the sensitivity to auxin in a habituated ginseng cell line might be enhanced during the habituation course. The down-regulation of PgTIP1 by exogenous auxin might be a side-off effect rather than the result of a directly negative regulation. High level and specific expression of PgTIP1 in the habituated cell line, as observed in the present study, should result from an acclimation to the environment (exogenous hormones subtracted from the media).

Plant vacuole is a multifunctional organelle with important roles in space filling, osmotic adjustment, storage, and digestion. Vacuole biogenesis and enlargement require transport of osmotically active substances across the tonoplast, followed by a rapid influx of water into the vacuole. This influx generates the turgor pressure that drives cell expansion and maintains the cell shape. Rapid cell expansion may require a high hydraulic permeability of the tonoplast to support water entry into the vacuole. Although vacuole volume increase can never be triggered by the water channel function of aquaporins for the passive process of water transport via them; the uptake of water by expanding vacuoles may well involve tonoplast aquaporins (Chaumont et al., 1998; Javot and Maurel, 2002). Reisen et al. (2003) reported that the heterologous expression of a cauliflower tonoplast aquaporin (BobTIP26;1, orthologous to AtTIP1;1) in tobacco suspension cells had no effect on the growth rate, but the cells were larger than in the wild-type. In present study, it is shown that PgTIP1, which is highly and specifically expressed in hormone-autotrophic ginseng cells, has a high water-channel activity. In the roots and leaves of two detected transgenic lines, PgTIP1 has a ~20-fold higher transcription than endogenous AtTIP1;1 and AtTIP1;2 (data not shown), both known to be highly expressed in Arabidopsis plants. Transgenic Arabidopsis plants over-expressing PgTIP1 had faster growth rate, longer roots, and bigger leaves and leaf cells; these results support the interpretation that TIPs might be involved in cell enlargement by modulating the permeability of the tonoplast. The volume increase of leaf cells might be triggered by enlargement of the vacuolar compartment facilitated by accumulating osmotically active substances, and following water influx into the vacuole, which was fine regulated by TIPs.

Whereas, the principal function of vacuoles is the maintenance of cell turgor, they can also accumulate macromolecules and secondary metabolites (Marty, 1999). Plant cells have different types of vacuoles that can coexist in the same cell and the different TIP isoforms may have entirely different or similar functions. While {alpha}-TIP alone is a marker for autophagic vacuole, it, coupled with {delta}- or {gamma}-TIP, is involved in the protein storage function. By contrast, {gamma}-TIP alone marked the lytic vacuoles and it, combined with {delta}-TIP, has a role in the storage of vegetative storage proteins and pigments (Jauh et al., 1999). Takahashi et al. (2004) isolated three novel {gamma}-TIP cDNAs in rice, OsTIP1, OsTIP2, and OsTIP3, and demonstrated their specialized function. By contrast with OsTIP2 and OsTIP3, which are expressed specifically in roots and seeds, respectively, OsTIP1 was expressed in mature seed embryos and during early seed germination. These observations indicate that different TIP isoforms, alone or in combination with each other, play complicated physiological functions in plants. Storage vacuole is one of the most important vacuole types in plants, especially in seed cells. A greatly enhanced seed size and mass, and the significantly increased seed fatty acid content of Arabidopsis plants over-expressing PgTIP1 indicate that PgTIP1 might have a role in substance storage and metabolism, in addition to involving in cell expansion. From sequence homology, protein structure to water channel activity, PgTIP1 shares characteristics with Arabidopsis tonoplast TIP1;1, which has been classified as a {gamma}-TIP. The physiological role of AtTIP1;1 was investigated in plants using RNA interference (Ma et al., 2004). Data indicated that a strong down-regulation of AtTIP1;1 led to plant death and suggested an essential physiological role of this otherwise highly expressed isoform. Transcript, metabolite profiling, and the cellular localization data suggested a role for AtTIP1;1 in carbon distribution, possibly by regulation of vesicle trafficking towards the central vacuole. We postulate that, similar to AtTIP1;1, PgTIP1 might be involved in substance storage and metabolism in the seeds of Arabidopsis plants over-expressing this gene.

As mentioned earlier, AQPs might have physiological functions other than facilitating water movement across cellular membranes. AQP over-expression highlights those physiological processes in which AQPs act as bottlenecks. Transgenic tobacco over-expression of AtPIP1;2 significantly increased plant growth and transpiration rate, stomatal density, and photosynthetic activity (Aharon et al., 2003). Similarly, transgenic rice over-expression of HvPIP2;1 also resulted in faster growth rate and higher internal CO2 (by 40%), stomatal conductance (by 27%), and CO2 assimilation (by 14%) than the wild-type plants (Hanba et al., 2004). These studies indicated PIPs may play an important role in regulating plant vigour. However, the mechanism of the involvement of PgTIP1 in this process might be different from that of PIPs, since the photosynthetic activity of transgenic Arabidopsis plants over-expressing PgTIP1 was found to be similar to that of wild-type plants. However, it has been shown here that PgTIP1 might be involved in cell enlargement via transport of water across tonoplast, which often represents a limiting factor that regulates plant vigour. Further understanding of the physiological roles of PgTIP1 in plant growth and development should be helpful in understanding AQP functions in plants more comprehensively.


   Supplementary data
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Supplementary data
References
 
Supplementary data are available at JXB online. Supplementary Figure 1 shows the predicted stereostructure of PgTIP1 (right panel) and AQP2 (left panel) and Supplementary Fig. 2 shows the ar/R selectivity filter of PgTIP1. Space-filling side-chain residues are also shown.


   Acknowledgements
 
We thank Dr HongQuan Yang for providing the pHB vector. We are grateful to Dr DaQuan Xu for his technical suggestions and assistance. This work was supported by the Chinese Academy of Sciences (Grant No.KSCX2-SW-329), Institute of Plant Physiology and Ecology and National Natural Science Foundation of China (Grant No. 30570157), and by Hatch Act and State of Iowa funds.


   Footnotes
 
* Data deposition footnote: The GenBank accession numbers for NtAQP1, AtTIP1;1, OsTIP1;1, and ZmTIP1;2 are Y08161, X72581, XM_470213, and AF326500, respectively. Back

{dagger} These authors have contributed equally to this work. Back


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Supplementary data
 References
 
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