Molecular cloning, water channel activity and tissue specific expression of two isoforms of radish vacuolar aquaporin

Keeping time in the dark: Potato diel and circadian rhythmic gene expression reveals tissue-specific circadian clocks

The circadian clock is an internal molecular oscillator and coordinates numerous physiological processes through regulation of molecular pathways. Tissue-specific clocks connected by mobile signals have previously been found to run at different speeds in Arabidopsis thaliana tissues. However, tissue variation in circadian clocks in crop species is unknown. In this study, leaf and tuber global gene expression in cultivated potato under cycling and constant environmental conditions was profiled. In addition, we used a circadian-regulated luciferase reporter construct to study tuber gene expression rhythms. Diel and circadian expression patterns were present among 17.9% and 5.6% of the expressed genes in the tuber. Over 500 genes displayed differential tissue specific diel phases. Intriguingly, few core circadian clock genes had circadian expression patterns, while all such genes were circadian rhythmic in cultivated tomato leaves. Furthermore, robust diel and circadian transcriptional rhythms were observed among detached tubers. Our results suggest alternative regulatory mechanisms and/or clock composition is present in potato, as well as the presence of tissue-specific independent circadian clocks. We have provided the first evidence of a functional circadian clock in below-ground storage organs, holding important implications for other storage root and tuberous crops.

Aquaporin activity of barley tonoplast intrinsic proteins is involved in the delay of the coalescence of protein storage vacuoles in aleurone cells

The coalescence of protein storage vacuoles (PSVs) is one of the most prominent cellular changes occurring in cereal aleurone cells during germination. This structural change is highly coupled with the functional transition of this organelle from a storage compartment to a lytic section. Gibberellic acid (GA) promotes this process, whereas abscisic acid (ABA) prevents it. Previously, we demonstrated that ABA-inducible HvTIP3;1 plays a decisive role in ABA-mediated prevention of PSV fusion. In this follow-up study, we examined whether the aquaporin activity of tonoplast intrinsic protein (TIP) is related to its preventive effect on PSV fusion using various functional mutants. The defective forms of aquaporin (HvTIP3;1m and HvTIP3;1ΔNPA-GFPs for HvTIP3;1, and HvTIP1;2m for HvTIP1;2) were found to be less effective than the usual form in delaying the PSV fusion process occurring in GA-treated cells. In contrast, overexpression of HvTIP3;1m reduced the preventive effect of ABA on PSV fusion. Upon inhibition of aquaporin activity using mercury, PSV fusion occurred to a greater extent in ABA-treated barley protoplasts. These data suggest that the aquaporin activity of TIP is involved in the deterrent effect of TIP on PSV coalescence. TIP3-GFP barley transgenic seeds showed prolonged expression of the TIP3;1 transcript. Moreover, PSV fusion progressed at a much slower rate compared to wild type. Additionally, the degradation of storage proteins was not as efficient, suggesting that a metamorphic transition of PSVs to lytic organelles is closely correlated with the disappearance of HvTIPs and the PSV fusion process.

Evolutionary Understanding of Aquaporin Transport System in the Basal Eudicot Model Species Aquilegia coerulea

Aquaporins (AQPs) play a pivotal role in the cellular transport of water and many other small solutes, influencing many physiological and developmental processes in plants. In the present study, extensive bioinformatics analysis of AQPs was performed in Aquilegia coerulea L., a model species belonging to basal eudicots, with a particular focus on understanding the AQPs role in the developing petal nectar spur. A total of 29 AQPs were identified in Aquilegia, and their phylogenetic analysis performed with previously reported AQPs from rice, poplar and Arabidopsis depicted five distinct subfamilies of AQPs. Interestingly, comparative analysis revealed the loss of an uncharacterized intrinsic protein II (XIP-II) group in Aquilegia. The absence of the entire XIP subfamily has been reported in several previous studies, however, the loss of a single clade within the XIP family has not been characterized. Furthermore, protein structure analysis of AQPs was performed to understand pore diversity, which is helpful for the prediction of solute specificity. Similarly, an AQP AqcNIP2-1 was identified in Aquilegia, predicted as a silicon influx transporter based on the presence of features such as the G-S-G-R aromatic arginine selectivity filter, the spacing between asparagine-proline-alanine (NPA) motifs and pore morphology. RNA-seq analysis showed a high expression of tonoplast intrinsic proteins (TIPs) and plasma membrane intrinsic proteins (PIPs) in the developing petal spur. The results presented here will be helpful in understanding the AQP evolution in Aquilegia and their expression regulation, particularly during floral development.

Genome-Wide Identification and Characterization of Aquaporins and Their Role in the Flower Opening Processes in Carnation (Dianthus caryophyllus)

Aquaporins (AQPs) are associated with the transport of water and other small solutes across biological membranes. Genome-wide identification and characterization will pave the way for further insights into the AQPs’ roles in the commercial carnation (Dianthus caryophyllus). This study focuses on the analysis of AQPs in carnation (DcaAQPs) involved in flower opening processes. Thirty DcaAQPs were identified and grouped to five subfamilies: nine PIPs, 11 TIPs, six NIPs, three SIPs, and one XIP. Subsequently, gene structure, protein motifs, and co-expression network of DcaAQPs were analyzed and substrate specificity of DcaAQPs was predicted. qRT-PCR, RNA-seq, and semi-qRTRCR were used for DcaAQP genes expression analysis. The analysis results indicated that DcaAQPs were relatively conserved in gene structure and protein motifs, that DcaAQPs had significant differences in substrate specificity among different subfamilies, and that DcaAQP genes’ expressions were significantly different in roots, stems, leaves and flowers. Five DcaAQP genes (DcaPIP1;3, DcaPIP2;2, DcaPIP2;5, DcaTIP1;4, and DcaTIP2;2) might play important roles in flower opening process. However, the roles they play are different in flower organs, namely, sepals, petals, stamens, and pistils. Overall, this study provides a theoretical basis for further functional analysis of DcaAQPs.

Cross-talk between ABA and sugar signaling is mediated by the ACGT core and CE1 element reciprocally in OsTIP3;1 promoter

Recently, much effort has been made to determine the molecular links and cross-talk between sugar and abscisic acid (ABA) signaling pathways. ABA-inducible expression of OsTIP3;1, encoding a rice tonoplast intrinsic protein, was enhanced by sugar depletion. Such a stimulatory increase in OsTIP3;1 expression under sugar-starvation is possibly not owing to changes in endogenous ABA content. The transient expression assay indicated that the 5' flanking region of OsTIP3;1 delivered similar collaborative responsiveness to starvation and ABA, suggesting that this gene promoter could be a good molecular probe to examine the interaction between sugar and ABA signaling pathways. Targeted mutagenesis demonstrated that disruption of ACGT cores decreased the induction of OsTIP3;1 promoter activity under either starvation or ABA, whereas mutation of coupling element 1 (CE1), which is an ABI4 binding site, reversely increased it, suggesting that those two distinct cis-regulatory elements reciprocally regulate the responsiveness of this promoter to both sugar and ABA. Consistent with this result, antisense inhibition of ABI4 increased the OsTIP3;1 promoter activity. ABI4 expression was also enhanced by sugars and repressed by ABA, suggesting that reduced ABI4 binding to CE1 in the absence of sugar and presence of ABA could increase ABA-induction of the OsTIP3;1 promoter activity.

Expression of the citrus CsTIP2;1 gene improves tobacco plant growth, antioxidant capacity and physiological adaptation under stress conditions

MAIN CONCLUSION

Overexpression of the citrus CsTIP2;1 improves plant growth and tolerance to salt and drought stresses by enhancing cell expansion, H 2 O 2 detoxification and stomatal conductance. Tonoplast intrinsic proteins (TIPs) are a subfamily of aquaporins, belonging to the major intrinsic protein family. In a previous study, we have shown that a citrus TIP isoform, CsTIP2;1, is highly expressed in leaves and also transcriptionally regulated in leaves and roots by salt and drought stresses and infection by 'Candidatus Liberibacter asiaticus', the causal agent of the Huanglongbing disease, suggesting its involvement in the regulation of the flow of water and nutrients required during both normal growth and stress conditions. Here, we show that the overexpression of CsTIP2;1 in transgenic tobacco increases plant growth under optimal and water- and salt-stress conditions and also significantly improves the leaf water and oxidative status, photosynthetic capacity, transpiration rate and water use efficiency of plants subjected to a progressive soil drying. These results correlated with the enhanced mesophyll cell expansion, midrib aquiferous parenchyma abundance, H2O2 detoxification and stomatal conductance observed in the transgenic plants. Taken together, our results indicate that CsTIP2;1 plays an active role in regulating the water and oxidative status required for plant growth and adaptation to stressful environmental conditions and may be potentially useful for engineering stress tolerance in citrus and other crop plants.

Molecular Characterization of LRB7 Gene and a Water Channel Protein TIP2 in Chorispora bungeana

Background. Water channel proteins, also called aquaporins, are integral membrane proteins from major intrinsic protein (MIP) family and involved in several pathways including not only water transport but also cell signaling, reproduction, and photosynthesis. The full cDNA and protein sequences of aquaporin in Chorispora bungeana Fisch. & C.A. Mey (C. bungeana) are still unknown. Results. In this study, PCR and rapid amplification of cDNA ends approaches were used to clone the full cDNA of LRB7 (GenBank accession number: EU636988) of C. bungeana. Sequence analysis indicated that it was 1235 bp, which had two introns and encoded a protein of 250 amino acids. Structure analysis revealed that the protein had two conserved NPA motifs, one of which is MIP signature sequence (SGxHxNPAVT), six membrane helix regions, and additional membrane-embedded domains. Phylogenetic analysis suggested that the protein was from TIP2 subgroup. Surprisingly, semiquantitative RT-PCR experiment and western blot analysis showed that LRB7 and TIP2 were only detectable in roots, unlike Arabidopsis and Raphanus. Connecting with our previous studies, LRB7 was supported to associate with chilling-tolerance in C. bungeana. Conclusion. This is the first time to characterize the full sequences of LRB7 gene and water channel protein in C. bungeana. Our findings contribute to understanding the water transports in plants under low temperatures.

Plant Aquaporins: Genome-Wide Identification, Transcriptomics, Proteomics, and Advanced Analytical Tools

Aquaporins (AQPs) are channel-forming integral membrane proteins that facilitate the movement of water and many other small molecules. Compared to animals, plants contain a much higher number of AQPs in their genome. Homology-based identification of AQPs in sequenced species is feasible because of the high level of conservation of protein sequences across plant species. Genome-wide characterization of AQPs has highlighted several important aspects such as distribution, genetic organization, evolution and conserved features governing solute specificity. From a functional point of view, the understanding of AQP transport system has expanded rapidly with the help of transcriptomics and proteomics data. The efficient analysis of enormous amounts of data generated through omic scale studies has been facilitated through computational advancements. Prediction of protein tertiary structures, pore architecture, cavities, phosphorylation sites, heterodimerization, and co-expression networks has become more sophisticated and accurate with increasing computational tools and pipelines. However, the effectiveness of computational approaches is based on the understanding of physiological and biochemical properties, transport kinetics, solute specificity, molecular interactions, sequence variations, phylogeny and evolution of aquaporins. For this purpose, tools like Xenopus oocyte assays, yeast expression systems, artificial proteoliposomes, and lipid membranes have been efficiently exploited to study the many facets that influence solute transport by AQPs. In the present review, we discuss genome-wide identification of AQPs in plants in relation with recent advancements in analytical tools, and their availability and technological challenges as they apply to AQPs. An exhaustive review of omics resources available for AQP research is also provided in order to optimize their efficient utilization. Finally, a detailed catalog of computational tools and analytical pipelines is offered as a resource for AQP research.

Abscisic acid prevents the coalescence of protein storage vacuoles by upregulating expression of a tonoplast intrinsic protein gene in barley aleurone

Tonoplast intrinsic proteins (TIPs) are integral membrane proteins that are known to function in plants as aquaporins. Here, we propose another role for TIPs during the fusion of protein storage vacuoles (PSVs) in aleurone cells, a process that is promoted by gibberellic acid (GA) and prevented by abscisic acid (ABA). Studies of the expression of barley (Hordeum vulgare) TIP genes (HvTIP) showed that GA specifically decreased the abundance of HvTIP1;2 and HvTIP3;1 transcripts, while ABA strongly increased expression of HvTIP3;1. Increased or decreased expression of HvTIP3;1 interfered with the hormonal effects on vacuolation in aleurone protoplasts. HvTIP3;1 gain-of-function experiments delayed GA-induced vacuolation, whereas HvTIP3;1 loss-of-function experiments promoted vacuolation in ABA-treated aleurone cells. These results indicate that TIP plays a key role in preventing the coalescence of small PSVs in aleurone cells. Hormonal regulation of the HvTIP3;1 promoter is similar to the regulation of the endogenous gene, indicating that induction of the transcription of HvTIP3;1 by ABA is a critical factor in the prevention of PSV coalescence in response to ABA. Promoter analysis using deletions and site-directed mutagenesis of sequences identified three cis-acting elements that are responsible for ABA responsiveness in the HvTIP3;1 promoter. Promoter analysis also showed that ABA responsiveness of the HvTIP3;1 promoter is likely to occur via a unique regulatory system distinct from that involving the ABA-response promoter complexes.

Dissection of autophagy in tobacco BY-2 cells under sucrose starvation conditions using the vacuolar H(+)-ATPase inhibitor concanamycin A and the autophagy-related protein Atg8

Tobacco BY-2 cells undergo autophagy in sucrose-free culture medium, which is the process mostly responsible for intracellular protein degradation under these conditions. Autophagy was inhibited by the vacuolar H(+)-ATPase inhibitors concanamycin A and bafilomycin A1, which caused the accumulation of autophagic bodies in the central vacuoles. Such accumulation did not occur in the presence of the autophagy inhibitor 3-methyladenine, and concanamycin in turn inhibited the accumulation of autolysosomes in the presence of the cysteine protease inhibitor E-64c. Electron microscopy revealed not only that the autophagic bodies were accumulated in the central vacuole, but also that autophagosome-like structures were more frequently observed in the cytoplasm in treatments with concanamycin, suggesting that concanamycin affects the morphology of autophagosomes in addition to raising the pH of the central vacuole. Using BY-2 cells that constitutively express a fusion protein of autophagosome marker protein Atg8 and green fluorescent protein (GFP), we observed the appearance of autophagosomes by fluorescence microscopy, which is a reliable morphological marker of autophagy, and the processing of the fusion protein to GFP, which is a biochemical marker of autophagy. Together, these results suggest the involvement of vacuole type H(+)-ATPase in the maturation step of autophagosomes to autolysosomes in the autophagic process of BY-2 cells. The accumulation of autophagic bodies in the central vacuole by concanamycin is a marker of the occurrence of autophagy; however, it does not necessarily mean that the central vacuole is the site of cytoplasm degradation.

Genome-wide comparative analysis of tonoplast intrinsic protein (TIP) genes in plants

Tonoplast intrinsic proteins (TIPs) play a vital role in water transport across membranes. In the present study, we performed a comparative analysis of TIP genes in ten plant species including both monocots and dicots. A total of 100 TIP aquaporin genes were identified, and their relationships among the plant species were analyzed. Phylogenetic analysis was performed to evaluate the relationship of these genes within the plant species. Based on the phylogenetic analysis results, TIPs were classified into five distinct arbitrary groups (group I to group V), which represented TIP2, TIP5, TIP4, TIP1, and TIP3, respectively. Group I represented the largest arbitrary group, followed by group IV, in the phylogenetic tree. The result clearly indicates that TIP2 and TIP1 are abundant aquaporins and highly related among the species. In the present review, a comparative study of gene structure analysis between dicots and monocots has been performed to analyze their structural variation. Most of the predicted motifs are conserved among the species, signifying an evolutionary relationship. The gene expression analysis indicated that the expression of TIP genes varies during different developmental stages and also during stressed conditions. The results indicated a great degree of evolutionary relationship and variation in the expression levels of TIPs in plants.

Genome-wide identification and characterization of aquaporin genes (AQPs) in Chinese cabbage (Brassica rapa ssp. pekinensis)

Aquaporins (AQPs) are members of a superfamily of integral membrane proteins and play a significant role in the transportation of small molecules across membranes. However, currently little is known about the AQP genes in Chinese cabbage (Brassica rapa ssp. pekinensis). In this study, a genome-wide analysis was carried out to identify the AQP genes in Chinese cabbage. In total, 53 non-redundant AQP genes were identified that were located on all of the 10 chromosomes. The number of AQP genes in Chinese cabbage was greater than in Arabidopsis. They were classified into four subfamilies, including PIP, TIP, NIP, and SIP. Thirty-three groups of AQP orthologous genes were identified between Chinese cabbage and Arabidopsis, but orthologs corresponding to AtNIP1;1 and AtPIP2;8 were not detected. Seventeen groups of paralogous genes were identified in Chinese cabbage. Three-dimensional models of the AQPs of Chinese cabbage were constructed using Phyre2, and ar/R selectivity filters were analyzed comparatively between Chinese cabbage and Arabidopsis. Generally, gene structure was conserved within each subfamily, especially in the SIP subfamily. Intron loss events have occurred during the evolution of the PIP, TIP, and NIP subfamilies. The expression of AQP genes in Chinese cabbage was analyzed in different organs. Most AQP genes were downregulated in response to salt stress. This work shows that the AQP genes of Chinese cabbage have undergone triplication and subsequent biased gene loss.

Optimizing plant transporter expression in Xenopus oocytes

BACKGROUND

Rapid improvements in DNA synthesis technology are revolutionizing gene cloning and the characterization of their encoded proteins. Xenopus laevis oocytes are a commonly used heterologous system for the expression and functional characterization of membrane proteins. For many plant proteins, particularly transporters, low levels of expression can limit functional activity in these cells making it difficult to characterize the protein. Improvements in synthetic DNA technology now make it quick, easy and relatively cheap to optimize the codon usage of plant cDNAs for Xenopus. We have tested if this optimization process can improve the functional activity of a two-component plant nitrate transporter assayed in oocytes.

RESULTS

We used the generally available software (http://www.kazusa.or.jp/codon/; http://genomes.urv.es/OPTIMIZER/) to predict a DNA sequence for the plant gene that is better suited for Xenopus laevis. Rice OsNAR2.1 and OsNRT2.3a DNA optimized sequences were commercially synthesized for Xenopus expression. The template DNA was used to synthesize cRNA using a commercially available kit. Oocytes were injected with cRNA mixture of optimized and original OsNAR2.1 and OsNRT2.3a. Oocytes injected with cRNA obtained from using the optimized DNA template could accumulate significantly more NO3- than the original genes after 16 h incubation in 0.5 mM Na15NO3. Two-electrode voltage clamp analysis of the oocytes confirmed that the codon optimized template resulted in significantly larger currents when compared with the original rice cDNA.

CONCLUSION

The functional activity of a rice high affinity nitrate transporter in oocytes was improved by DNA codon optimization of the genes. This methodology offers the prospect for improved expression and better subsequent functional characterization of plant proteins in the Xenopus oocyte system.

Expression profiles of aquaporin homologues and petal movement during petal development in Tulipa gesneriana

Previously, we have characterized two tonoplast intrinsic proteins (TIPs) and four plasma membrane intrinsic proteins (PIPs) from the 2-day-old petals of tulip (Tulipa gesneriana). In this study, we analyzed the development of tulip petals and stems, temperature-dependent petal movement, the amount of ³H₂O transported into petals and stems during petal movement, and the transcript levels of two TIP (TgTIP1;1 and TgTIP1;2) and four TgPIP genes in petals and stems, from the first day of petal opening to day 12. The development of the petals and stems was completed by days 6 and 9, respectively, after the first day of petal opening. Temperature-dependent petal movement and the amount of ³H₂O that was transported into petals could be detected at significant levels up to day 6 with petal movement reaching a peak at day 3. Real-time reverse transcription-polymerase chain reaction analysis revealed that TgTIP1;1 and TgTIP1;2 were expressed ubiquitously in petals, stems, leaves, bulbs and roots. However, the expression level of TgTIP1;2 was very low in bulbs. The expression of both TgTIP1 genes was upregulated in close association with the development of petals but not with that of the stem. The four TgPIP genes were expressed at almost the same level during the development of the petals and the stem. However, the levels of the TgTIP1 and TgPIP transcripts in petals decreased during the course of petal wilting from day 9 onwards. These results suggest that TgTIP1;1 and TgTIP1;2 may contribute to petal development.

Correlation of aquaporins and transmembrane solute transporters revealed by genome-wide analysis in developing maize leaf

Aquaporins are multifunctional membrane channels that facilitate the transmembrane transport of water and solutes. When transmembrane mineral nutrient transporters exhibit the same expression patterns as aquaporins under diverse temporal and physiological conditions, there is a greater probability that they interact. In this study, genome-wide temporal profiling of transcripts analysis and coexpression network-based approaches are used to examine the significant specificity correlation of aquaporins and transmembrane solute transporters in developing maize leaf. The results indicate that specific maize aquaporins are related to specific transmembrane solute transporters. The analysis demonstrates a systems-level correlation between aquaporins, nutrient transporters, and the homeostasis of mineral nutrients in developing maize leaf. Our results provide a resource for further studies into the physiological function of these aquaporins.

Tonoplast vesicles of Beta vulgaris storage root show functional aquaporins regulated by protons

BACKGROUND INFORMATION

Water is crucial for plant development and growth, and its transport pathways inside a plant are an ongoing topic for study. Plants express a large number of membrane intrinsic proteins whose role is now being re-evaluated by considering not only the control of the overall plant water balance but also in adaptation to environmental challenges that may affect their physiology. In particular, we focused our work on water movements across the root cell TP (tonoplast), the delimiting membrane of the vacuole. This major organelle plays a central role in osmoregulation.

RESULTS

An enriched fraction of TP vesicles from Beta vulgaris (red beet) storage roots obtained by a conventional method was used to characterize its water permeability properties by means of the stopped-flow technique. The preparation showed high water permeability (485 microm x s(-1)), consistent with values reported in the literature. The water permeability was strongly blocked by HgCl(2) (reduced to 16%) and its energy activation was low. These observations allow us to postulate the presence of functional water channels in this preparation. Moreover, Western-blot analysis demonstrated the presence of a tonoplast intrinsic protein. With the purpose of studying the regulation of water channels, TP vesicles were exposed to different acidic pH media. When the pH of a medium was low (pH 5.6), the water permeability exhibited a 42% inhibition.

CONCLUSIONS

Our findings prove that although almost all water channels present in the TP vesicles of B. vulgaris root are sensitive to HgCl(2), not all are inhibited by pH. This interesting selectivity to acidification of the medium could play a role in adapting the water balance in the cell-to-cell pathway.

Keep an eye on PPi: the vacuolar-type H+-pyrophosphatase regulates postgerminative development in Arabidopsis

Postgerminative growth of seed plants requires specialized metabolism, such as gluconeogenesis, to support heterotrophic growth of seedlings until the functional photosynthetic apparatus is established. Here, we show that the Arabidopsis thaliana fugu5 mutant, which we show to be defective in AVP1 (vacuolar H(+)-pyrophosphatase), failed to support heterotrophic growth after germination. We found that exogenous supplementation of Suc or the specific removal of the cytosolic pyrophosphate (PPi) by the heterologous expression of the cytosolic inorganic pyrophosphatase1 (IPP1) gene from budding yeast (Saccharomyces cerevisiae) rescued fugu5 phenotypes. Furthermore, compared with the wild-type and AVP1(Pro):IPP1 transgenic lines, hypocotyl elongation in the fugu5 mutant was severely compromised in the dark but recovered upon exogenous supply of Suc to the growth media. Measurements revealed that the peroxisomal β-oxidation activity, dry seed contents of storage lipids, and their mobilization were unaffected in fugu5. By contrast, fugu5 mutants contained ~2.5-fold higher PPi and ~50% less Suc than the wild type. Together, these results provide clear evidence that gluconeogenesis is inhibited due to the elevated levels of cytosolic PPi. This study demonstrates that the hydrolysis of cytosolic PPi, rather than vacuolar acidification, is the major function of AVP1/FUGU5 in planta. Plant cells optimize their metabolic function by eliminating PPi in the cytosol for efficient postembryonic heterotrophic growth.

Vacuolar proton pumps and aquaporins involved in rapid internode elongation of deepwater rice

Rapid growth of the submerged shoots of deepwater rice is essential for survival during the rainy season. We investigated changes in the expression of vacuolar H(+)-ATPase (V-ATPase), H(+)-pyrophosphatase (V-PPase), and aquaporins under submerged conditions. The amounts of vacuolar proton pumps, which support the active transport of ions into the vacuoles, were maintained on a membrane protein basis in the developing vacuoles. Among the six isogenes of V-PPase, OsVHP1;3 was markedly enhanced by submersion. The gene expression of efficient water channels, OsTIP1;1, OsTIP2;2, OsPIP1;1, OsPIP2;1, and OsPIP2;2, was markedly enhanced by submersion. The increase in aquaporin expression might support quick elongation of internodes. The mRNA levels of OsNIP2;2 and OsNIP3;1, which transport silicic and boric acids respectively, clearly decreased. The present study indicates that internodes of deepwater rice upregulate vacuolar proton pumps and water channel aquaporins and downregulate aquaporins that allow permeation of the substrates that suppress internode growth.

A look inside: localization patterns and functions of intracellular plant aquaporins

Aquaporins form a superfamily of intrinsic channel proteins in the plasma and intracellular membranes of plant cells. While a lot of research effort has substantiated the importance of plasma membrane aquaporins for the regulation of plant water homeostasis, comparably little is known about the function of intracellular aquaporins. Yet, various low-molecular-weight compounds, in addition to water, were recently shown to permeate some of these aquaporins. In this review, we examine the diversity of transport properties and localization patterns of intracellular aquaporins. The discussed profiles include, for example, water and ammonia transport across the tonoplast or CO2 transport through the chloroplast envelope. Furthermore, we try to assess to what extent the diverse aquaporin distribution patterns, in relation to the high degree of compartmentation of plant cells, can be linked to a wide range of cellular functions.

Improved accumulation of ajmalicine and tetrahydroalstonine in Catharanthus cells expressing an ABC transporter

The biosynthetic pathway of monoterpenoid indole alkaloids in Catharanthus roseus is located throughout various membranes at both the cellular and intercellular levels. ATP-binding cassette (ABC) transporters are known to export vincristine and vinblastine from human cancer cells. It has recently been shown that ABC transporters are also involved in the transport of various monoterpenoid alkaloids in Catharanthus roseus cells. Over-expression of an ABC transporter in this plant might therefore affect the regulation of the alkaloid biosynthetic pathway. CjMDR1, an ABC transporter gene originally isolated from Coptis japonica, was expressed in Catharanthus roseus cell cultures. Cells showing a positive PCR signal of the transgene in both cDNA and genomic DNA samples were subject to transport studies using selected substrates. Unexpectedly, transport of the isoquinoline alkaloid berberine, the main substrate of CjMDR1 transporter in Coptis japonica, was not affected as compared with control and wild-type Catharanthus cells. On the other hand, the endogenous alkaloids ajmalicine and tetrahydroalstonine were accumulated significantly more in Catharanthus roseus cells expressing CjMDR1 in comparison with control lines after feeding these alkaloids.

Roles of aquaporin-3 water channels in volume-regulatory water flow in a human epithelial cell line

Membrane water transport is an essential event not only in the osmotic cell volume change but also in the subsequent cell volume regulation. Here we investigated the route of water transport involved in the regulatory volume decrease (RVD) that occurs after osmotic swelling in human epithelial Intestine 407 cells. The diffusion water permeability coefficient (Pd) measured by NMR under isotonic conditions was much smaller than the osmotic water permeability coefficient (Pf) measured under an osmotic gradient. Temperature dependence of Pf showed the Arrhenius activation energy (Ea) of a low value (1.6 kcal/mol). These results indicate an involvement of a facilitated diffusion mechanism in osmotic water transport. A mercurial water channel blocker (HgCl(2)) diminished the Pf value. A non-mercurial sulfhydryl reagent (MMTS) was also effective. These blockers of water channels suppressed the RVD. RT-PCR and immunocytochemistry demonstrated predominant expression of AQP3 water channel in this cell line. Downregulation of AQP3 expression induced by treatment with antisense oligodeoxynucleotides was found to suppress the RVD response. Thus, it is concluded that AQP3 water channels serve as an essential pathway for volume-regulatory water transport in, human epithelial cells.

Role of aquaporins in leaf physiology

Playing a key role in plant growth and development, leaves need to be continuously supplied with water and carbon dioxide to fulfil their photosynthetic function. On its way through the leaf from the xylem to the stomata, water can either move through cell walls or pass from cell to cell to cross the different tissues. Although both pathways are probably used to some degree, evidence is accumulating that living cells contribute substantially to the overall leaf hydraulic conductance (K(leaf)). Transcellular water flow is facilitated and regulated by water channels in the membranes, named aquaporins (AQPs). This review addresses how AQP expression and activity effectively regulate the leaf water balance in normal conditions and modify the cell membrane water permeability in response to different environmental factors, such as irradiance, temperature, and water supply. The role of AQPs in leaf growth and movement, and in CO(2) transport is also discussed.

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant-water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry.

Na+/H+-transporter, H+-pumps and an aquaporin in light and heavy tonoplast membranes from organic acid and NaCl accumulating vacuoles of the annual facultative CAM plant and halophyte Mesembryanthemum crystallinum L

Crassulacean acid metabolism (CAM) was induced in Mesembryanthemum crystallinum L. by either NaCl- or high light (HL)- stress. This generated in mesophyll cells predominantly of NaCl-stressed plants two different types of vacuoles: the generic acidic vacuoles for malic acid accumulation and additionally less acidic ("neutral") vacuoles for NaCl sequestration. To examine differences in the tonoplast properties of the two types of vacuoles, we separated microsomal membranes of HL- and NaCl-stressed M. crystallinum plants by centrifugation in sucrose density gradients. Positive immunoreactions of a set of antibodies directed against tonoplast specific proteins and tonoplast specific ATP- and PPi-hydrolytic activity were used as markers for vacuolar membranes. With these criteria tonoplast membranes were detected in both HL- and NaCl-stressed plants in association with the characteristic low sucrose density but also at an unusual high sucrose density. In HL-stressed plants most of the ATP- and PPi-hydrolytic activity and cross reactivity with antibodies including that directed against the Na+/H+-antiporter from Arabidopsis thaliana was detected with light sucrose density. This relationship was inverted in NaCl-stressed plants; they exhibited most pump activity and immunoreactivity in the heavy fraction. The relative abundance of the heavy membrane fraction reflects the relative occurrence of "neutral" vacuoles in either HL- or NaCl-stressed plants. This suggests that tonoplasts of the "neutral" vacuoles sediment at high sucrose densities. This is consistent with the view that this type of vacuoles serves for Na+ sequestration and is accordingly equipped with a high capacity of proton pumping and Na+ uptake via the Na+/H+-antiporter.

Immunochemical analysis of aquaporin isoforms in Arabidopsis suspension-cultured cells

Aquaporins mediate the movement of water across biomembranes. Arabidopsis thaliana contains 35 aquaporins that belong to four subfamilies (PIP, TIP, SIP, and NIP). We investigated their expression profiles immunochemically in suspension-cultured Arabidopsis thaliana cells during growth and in response to salt and osmotic stresses. Protein amounts of all aquaporins were much lower in cultured cells than in the plant tissues. This is consistent with the low water permeability of protoplasts from cultured cells. After treatment with NaCl, the protein amounts of PIP2;1, PIP2;2, and PIP2;3 in the cells increased several-fold, and those of TIP1;1 and TIP1;2, 15- and 3-fold respectively. PIP1 did not change under the stress. Cell death began after 19 d in culture, accompanied by marked accumulation of PIPs and TIPs and a gradual decrease in SIPs. Our results suggest the followings: (i) Accumulation of aquaporin isoforms was individually regulated at low levels in single cells. (ii) At least PIP2;2, PIP2;3, TIP1;1, and TIP1;2 are stress-responsive aquaporins in suspension cells. (iii) A sudden increment of several members of PIP2 and TIP1 subfamilies might be related to cell death.

Early effects of salinity on water transport in Arabidopsis roots. Molecular and cellular features of aquaporin expression

Aquaporins facilitate the uptake of soil water and mediate the regulation of root hydraulic conductivity (Lp(r)) in response to a large variety of environmental stresses. Here, we use Arabidopsis (Arabidopsis thaliana) plants to dissect the effects of salt on both Lp(r) and aquaporin expression and investigate possible molecular and cellular mechanisms of aquaporin regulation in plant roots under stress. Treatment of plants by 100 mm NaCl was perceived as an osmotic stimulus and induced a rapid (half-time, 45 min) and significant (70%) decrease in Lp(r), which was maintained for at least 24 h. Macroarray experiments with gene-specific tags were performed to investigate the expression of all 35 genes of the Arabidopsis aquaporin family. Transcripts from 20 individual aquaporin genes, most of which encoded members of the plasma membrane intrinsic protein (PIP) and tonoplast intrinsic protein (TIP) subfamilies, were detected in nontreated roots. All PIP and TIP aquaporin transcripts with a strong expression signal showed a 60% to 75% decrease in their abundance between 2 and 4 h following exposure to salt. The use of antipeptide antibodies that cross-reacted with isoforms of specific aquaporin subclasses revealed that the abundance of PIP1s decreased by 40% as early as 30 min after salt exposure, whereas PIP2 and TIP1 homologs showed a 20% to 40% decrease in abundance after 6 h of treatment. Expression in transgenic plants of aquaporins fused to the green fluorescent protein revealed that the subcellular localization of TIP2;1 and PIP1 and PIP2 homologs was unchanged after 45 min of exposure to salt, whereas a TIP1;1-green fluorescent protein fusion was relocalized into intracellular spherical structures tentatively identified as intravacuolar invaginations. The appearance of intracellular structures containing PIP1 and PIP2 homologs was occasionally observed after 2 h of salt treatment. In conclusion, this work shows that exposure of roots to salt induces changes in aquaporin expression at multiple levels. These changes include a coordinated transcriptional down-regulation and subcellular relocalization of both PIPs and TIPs. These mechanisms may act in concert to regulate root water transport, mostly in the long term (> or =6 h).

The distribution of aquaporin subtypes (PIP1, PIP2 and gamma-TIP) is tissue dependent in soybean (Glycine max) root nodules

BACKGROUND AND AIMS The inner cortical cells (IC-cells) of legume root nodules have been previously shown to regulate the resistance to nodule O2 diffusion by a rapid contraction/expansion mechanism, which controls the volume of intercellular spaces and their occlusion by a liquid phase. The expression of aquaporins in IC-cells was also found to be involved in this nodule O2 diffusion mechanism. The aim of this study was to compare the expression of plasma membrane intrinsic proteins (PIP) aquaporin isoforms with tonoplast intrinsic protein (gamma-TIP) in both IC-cells and adjacent cell types.

METHODS

Using immunogold labelling in ultra-thin sections of Glycine max nodules, the expression of two PIP isoforms was observed and compared with the gamma-TIP pattern.

KEY RESULTS

The plasma membrane aquaporins PIP1 and PIP2 were expressed more in IC-cells and endodermis than in pericycle and infected cells. The tonoplast aquaporin gamma-TIP has shown a distribution pattern similar to that of the PIPs.

CONCLUSIONS

PIPs and gamma-TIP aquaporins are highly expressed in both plasmalemma and tonoplast of nodule IC-cells. This distribution is consistent with the putative role of water fluxes associated with the regulation of nodule conductance to O2 diffusion and the subsequent ATP-dependent nitrogenase activity. In the endodermis, these aquaporins might also be involved in nutrient transport between the infected zone and vascular traces.

Isolation and Expression of Two Aquaporin-Encoding Genes from the Marine Phanerogam Posidonia oceanica

Seagrasses such as Posidonia oceanica (L.) Delile are marine phanerogams, widespread in various seas, where they form large prairies representing dynamic substrates exceeding the area of the sediment surface several times over and allowing settlement of epiphyte organisms. Studying mechanisms involved in water transport in marine plants, we isolated two aquaporin-encoding genes, PoPIP1;1 and PoTIP1;1, showing high similarity to plasma membrane- and tonoplast-intrinsic protein-encoding genes, respectively. PoPIP1;1 is unique in the genome of P. oceanica, while PoTIP1;1 belongs to an aquaporin subfamily of at least four members. PoPIP1;1 and PoTIP1;1 encode functional proteins, as indicated by expression experiments in Xenopus oocytes. Both genes are constitutively expressed in the leaves, with higher levels of transcripts in young than in differentiated leaf tissues. Variations of salt concentration in aquarium determined different PoPIP1;1 and PoTIP1;1 transcript accumulation, indicating the existence of adaptation mechanisms related to gene expression also in marine plants, i.e. adapted to very high salt concentrations. Hyposalinity induced lower levels of PIP1 transcripts, while hypersalinity determined more PIP1 transcripts than normal salinity. TIP1 transcripts increased in response to both hypo- and hypersalinity after 2 days of treatment and went back to control levels after 5 d.

Loss of TIP1;1 aquaporin in Arabidopsis leads to cell and plant death

Arabidopsis TIP1;1 (gammaTIP) is a member of the tonoplast family of aquaporins (AQP). Using RNA interference (RNAi) we reduced TIP1;1 to different extent in various lines. When most severely affected, miniature plants died, a phenotype partially complemented by the TIP1;1 homolog McMIP-F. Less severely affected lines produced small plants, early senescence, and showed lesion formation. The relative water content in TIP1;1 RNAi plants was not significantly affected. Global expression profiling suggested a disturbance in carbon metabolism in RNAi lines with upregulated transcripts for functions in carbon acquisition and respiration, vesicle transport, signaling and transcription, and radical oxygen stress. Metabolite profiles showed low glucose, fructose, inositol, and threonic, succinic, fumaric, and malic acids, but sucrose levels were similar to WT. Increased amounts were found for raffinose and several unknown compounds. TIP1;1 RNAi plants also contained high starch and apoplastic carbohydrate increased. A GFP-TIP1;1 fusion protein indicated tonoplast location in spongy mesophyll cells, and high signal intensity in palisade mesophyll associated with vesicles near plastids. Signals in vascular tissues were strongest not only in vesicle-like structures but also outlined large vacuoles. Compromised routing of carbohydrate and lack of sucrose provision for cell-autonomous functions seems to characterize this RNAi phenotype. We suggest a function for TIP1;1 in vesicle-based metabolite routing through or between pre-vacuolar compartments and the central vacuole. Phenotype and expression characteristics support a view of TIP1;1 functioning as a marker for vesicles that are targeted to the central vacuole.

Contribution of the plasma membrane and central vacuole in the formation of autolysosomes in cultured tobacco cells

Autolysosomes accumulate in tobacco cells cultured under sucrose starvation conditions in the presence of a cysteine protease inhibitor. We characterized these plant autolysosomes using fluorescent dyes and green fluorescent protein (GFP). Observation using the endocytosis markers, FM4-64 and Lucifer Yellow CH, suggested that there is a membrane flow from the plasma membrane to autolysosomes. Using these dyes as well as GFP-AtVam3p, sporamin-GFP and gamma-VM23-GFP fusion proteins as markers of the central vacuole, we found transport of components of the central vacuole to autolysosomes. Thus endocytosis and the supply from the central vacuole may contribute to the formation of autolysosomes.

Water channel activity of radish plasma membrane aquaporins heterologously expressed in yeast and their modification by site-directed mutagenesis

Plants contain a number of aquaporin isoforms. We developed a method for determining the water channel activity of individual isoforms of aquaporin. Six plasma membrane aquaporins (RsPIPs) and two vacuolar membrane aquaporins (RsTIPs) of radish (Raphanus sativus) were expressed heterologously in Saccharomyces cerevisiae BJ5458, which is deficient in endogenous functional aquaporin. Aquaporins were detected by immunoblot analysis with corresponding antibodies. Water permeability of membranes from yeast transformants was assayed by stopped-flow spectrophotometry. The water channel activity of members of the RsPIP2 and RsTIP subfamilies was about 10 times and 5 times greater, respectively, than that of the control; however, RsPIP1s had little (RsPIP1-2 and RsPIP1-3) or no activity (RsPIP1-1). Site-directed mutation of several residues conserved in RsPIP1s or RsPIP2s markedly altered the water transport activity. Exchange of Ile244 of RsPIP1-3 with valine increased the activity to 250% of the wild type RsPIP1-3. On the other hand, exchange of Val235 of RsPIP2-2, which corresponds to RsPIP1-3 Ile244, with isoleucine caused a marked inactivation to 45% of the original RsPIP2-2. Mutation at possible phosphorylation sites at the N- and C-terminal tails also altered the activity. These results suggest that these residues in the half-helix loop E and the tails are involved in the water transport and the functional regulation of RsPIP1 and RsPIP2.

Differences in aquaporin levels among cell types of radish and measurement of osmotic water permeability of individual protoplasts

We investigated tissue- and cell-specific accumulation of radish aquaporin isoforms by immunocytochemical analysis. In taproots, the plasma membrane aquaporins (RsPIP1 and RsPIP2) were accumulated at high levels in the cambium, while the tonoplast aquaporin (RsTIP) was distributed in all tissues. The three isoforms were highly accumulated in the central cylinder of seedling roots and hypocotyls, and rich in the vascular tissue of the petiole of mature plants. The results suggest that RsPIP1 and RsPIP2 are abundant in the cells surrounding the sieve tube of the radish plant. The swelling rate of protoplasts in a hypotonic solution was determined individually by a newly established method to compare the osmotic water permeability of different cell types. All cells of the cortex and endodermis in seedlings showed a high water permeability of more than 300 microm s(-1). There was no marked difference in the values between the root endodermis and cortex protoplasts, although the RsPIP level was lower in the cortex than in the endodermis. This inconsistency suggests two possibilities: (1) a low level of aquaporin is enough for high water permeability and (2) the water channel activity of aquaporin in the tissues is regulated individually. The uneven distribution of aquaporins in tissues is discussed along with their physiological roles.

Molecular physiology of aquaporins in plants

In plants, membrane channels of the major intrinsic protein (MIP) super-family exhibit a high diversity with, for instance, 35 homologues in the model species Arabidopsis thaliana. As has been found in other organisms, plant MIPs function as membrane channels permeable to water (aquaporins) and in some cases to small nonelectrolytes. The aim of the present article is to integrate into plant physiology what has been recently learned about the molecular and functional properties of aquaporins in plants. Exhaustive compilation of data in the literature shows that the numerous aquaporin isoforms of plants have specific expression patterns throughout plant development and in response to environmental stimuli. The diversity of aquaporin homologues in plants can also be explained in part by their presence in multiple subcellular compartments. In recent years, there have been numerous reports that describe the activity of water channels in purified membrane vesicles, in isolated organelles or protoplasts, and in intact plant cells or even tissues. Altogether, these data suggest that the transport of water and solutes across plant membranes concerns many facets of plant physiology. Because of the high degree of compartmentation of plant cells, aquaporins may play a critical role in cell osmoregulation. Water uptake in roots represents a typical process in which to investigate the role of aquaporins in transcellular water transport, and the mechanisms and regulations involved are discussed.

Aquaporin isoforms responsive to salt and water stresses and phytohormones in radish seedlings

Aquaporins in the plasma and vacuolar membranes play a key role in the intercellular and intracellular water transport in plants. First, we quantitated the absolute amounts for mRNAs of eight aquaporin isoforms in hypocotyls of radish seedlings. Then, we investigated the effects of salt and water stresses (150 mM NaCl, 300 mM mannitol and 20% polyethylene glycol) and phytohormones (gibberellic acid, abscisic acid and brassinolide) on the mRNA and protein levels of aquaporins in the plasma membrane (RsPIP1-1, 1-2, 1-3, 2-1, 2-2 and 2-3) and vacuolar membrane (RsTIP1-1 and 2-1). The mRNA and protein levels of RsTIP1-1, RsTIP2-1, RsPIP1-1, RsPIP1-2 and RsPIP1-3 were comparatively constant. In contrast, mannitol treatment altered the mRNA levels of RsPIP2-1, RsPIP2-2 and RsPIP2-3 in roots. Immunoblot analysis showed that the RsPIP2-1 protein level was increased by NaCl treatment and decreased by treatment with mannitol and polyethylene glycol. Gibberellic acid and abscisic acid suppressed the levels of mRNAs of RsPIP2-1, RsPIP2-2 and RsPIP2-3 and the protein level of RsPIP2-1 in roots. On the other hand, the protein levels of RsPIP1-group members and RsTIPs were scarcely changed by these phytohormones. In the case of hypocotyls and cotyledons, the mRNA and protein levels of eight isoforms were not markedly affected by any treatment. These results indicate that aquaporins in the root, especially the RsPIP2 group, may be a stress responsive type of aquaporin at least in the protein level.

Membrane anchors for vacuolar targeting: application in plant bioreactors

Transgenic plants are attractive expression systems for producing recombinant proteins. Plant cells compartmentalize and store metabolites and proteins in vacuoles, but foreign proteins need to be targeted to the correct compartments if they are to accumulate in a stable fashion. Here we present a general strategy in which unique transmembrane and cytoplasmic tail sequences are used as anchors for delivering recombinant proteins via distinct vesicular transport pathways to specific vacuolar compartments where stable accumulation can occur.

The protein storage vacuole: a unique compound organelle

Storage proteins are deposited into protein storage vacuoles (PSVs) during plant seed development and maturation and stably accumulate to high levels; subsequently, during germination the storage proteins are rapidly degraded to provide nutrients for use by the embryo. Here, we show that a PSV has within it a membrane-bound compartment containing crystals of phytic acid and proteins that are characteristic of a lytic vacuole. This compound organization, a vacuole within a vacuole whereby storage functions are separated from lytic functions, has not been described previously for organelles within the secretory pathway of eukaryotic cells. The partitioning of storage and lytic functions within the same vacuole may reflect the need to keep the functions separate during seed development and maturation and yet provide a ready source of digestive enzymes to initiate degradative processes early in germination.

Low aquaporin content and low osmotic water permeability of the plasma and vacuolar membranes of a CAM plant Graptopetalum paraguayense: comparison with radish

Aquaporin facilitates the osmotic water transport across biomembranes and is involved in the transcellular and intracellular water flow in plants. We immunochemically quantified the aquaporin level in leaf plasma membranes (PM) and tonoplast of Graptopetalum paraguayense, a Crassulacean acid metabolism (CAM) plant. The aquaporin content in the Graptopetalum tonoplast was approximately 1% of that of radish. The content was calculated to be about 3 microg mg(-1) of tonoplast protein. The level of PM aquaporin in Graptopetalum was determined to be less than 20% of that of radish, in which an aquaporin was a major protein of the PM. The PM aquaporin was detected in the mesophyll tissue of Graptopetalum leaf by tissue print immunoblotting. The osmotic water permeability of PM and tonoplast vesicles prepared from both plants was determined with a stopped-flow spectrophotometer. The water permeability of PM was lower than that of the tonoplast in both plants. The Graptopetalum PM vesicles hardly showed water permeability, although the tonoplast showed a relatively high permeability. The water permeability changed depending on the assay temperature and was also partially inhibited by a sulfhydryl reagent. Furthermore, measurement of the rate of swelling and shrinking in different mannitol concentrations revealed that the protoplasts of Graptopetalum showed low water permeability. These results suggest that the low content of aquaporins in PM and tonoplast is one of the causes of the low water permeability of GRAPTOPETALUM: The relationship between the water-storage function of succulent leaves of CAM plants and the low aquaporin level is also discussed.

MIP genes are down-regulated under drought stress in Nicotiana glauca

Water flux across cell membranes has been shown to occur not only through the lipid bilayer, but also through aquaporins, which are members of the major intrinsic protein (MIP) super-family of channel proteins. Aquaporins greatly increase the membrane permeability for water, but may also be regulated, allowing cellular control over the rate of water influx/efflux. Water flux is crucial for stomatal opening and closing, but little is known about the role that aquaporins play in stomatal physiology. Our initial goal was to isolate and characterize the MIP genes expressed in guard cells of the model plant, Nicotiana glauca. Degenerate oligonucleotides corresponding to amino acid sequences conserved in tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic proteins (PIPs) were used to amplify portions of MIP genes by RT-PCR. These PCR products were used as probes in screening a N. glauca guard cell cDNA library. We isolated three clones (NgMIP1, NgMIP2 and NgMIP3) homologous to TIPs and two clones (NgMIP4 and NgMIP5) homologous to PIPs. All of the MIP genes we characterized displayed highest levels of mRNA accumulation in roots or stems, with lower levels of expression in mesophyll cells and whole leaves, and lowest transcript accumulation in guard cell RNA. Interestingly, the accumulation of transcripts arising from NgMIP2, NgMIP3 and NgMIP4 diminished dramatically in drought-stressed plants. This down-regulation of MIP gene expression may result in reduced membrane water permeability and may encourage cellular water conservation during periods of dehydration stress.

TONOPLAST TRANSPORTERS: Organization and Function

Regulation of the contents and volume of vacuoles in plant cells depends on the coordinated activities of transporters and channels located in the tonoplast (vacuolar membrane). The three major components of the tonoplast are two proton pumps, the vacuolar H+-ATPase (V-ATPase) and H+-pyrophosphatase (V-PPase), and aquaporins. The tertiary structure of the V-ATPase complex and properties of its subunits have been characterized by biochemical and genetic techniques. These studies and a comparison with the F-type ATPase have enabled estimation of the dynamics of V-ATPase activity during catalysis. V-PPase, a simple proton pump, has been identified and cloned from various plant species and other organisms, such as algae and phototrophic bacteria, and functional motifs of the enzyme have been determined. Aquaporin, serving as the water channel, is the most abundant protein in the tonoplast in most plants. A common molecular architecture of aquaporins in mammals and plants has been determined by two-dimensional crystallographic analysis. Furthermore, recent molecular biological studies have revealed several other types of tonoplast transporters, such as the Ca2+-ATPase, Ca2+/H+ antiporter and Na+/H+ antiporter. Many other transporters and channels in the tonoplast remain to be identified; their activities have already been detected. This review presents an overview of the field and discusses recent findings on the tonoplast protein components that have been identified and their physiological consequences.

Xenopus oocytes as an expression system for plant transporters

The Xenopus oocyte provides a powerful system for the expression and characterisation of plant membrane proteins. Many different types of plant membrane proteins have been expressed and characterised using this system. As there are already several general reviews on the methodology for oocyte expression of channel proteins, we have summarised the particular advantages and disadvantages of using the system for the characterisation of plant cotransporter proteins. As an example of how the system can be used to identify transporters, we describe evidence for a low affinity nitrate transporter in oocytes injected with poly(A) RNA extracted from nitrate-induced barley roots. Furthermore, we describe evidence that the expression of some transporters in oocytes can modify the properties of endogenous membrane proteins. We conclude that although care must be taken in the interpretation of results and in choosing appropriate controls for experiments, oocyte expression is an excellent tool which will have an important role in characterising plant membrane proteins.

The role of aquaporins in cellular and whole plant water balance

Aquaporins are water channel proteins belonging to the major intrinsic protein (MIP) superfamily of membrane proteins. More than 150 MIPs have been identified in organisms ranging from bacteria to animals and plants. In plants, aquaporins are present in the plasma membrane and in the vacuolar membrane where they are abundant constituents. Functional studies of aquaporins have hitherto mainly been performed by heterologous expression in Xenopus oocytes. A main issue is now to understand their role in the plant, where they are likely to be important both at the cellular and at the whole plant level. Plants contain a large number of aquaporin isoforms with distinct cell type- and tissue-specific expression patterns. Some of these are constitutively expressed, whereas the expression of others is regulated in response to environmental factors, such as drought and salinity. At the protein level, regulation of water transport activity by phosphorylation has been reported for some aquaporins.

An abundant TIP expressed in mature highly vacuolated cells

Aquaporins are water channel proteins found in vacuolar membranes and plasma membranes, and belong to the major intrinsic protein (MIP) family of proteins. In the present study, we purified a 75 kDa MIP protein from a crude fraction of spinach leaf intracellular membranes. Upon urea/SDS-PAGE, the 75 kDa protein appeared as a 21 kDa polypeptide, and the 75 kDa species therefore probably represents a tetramer. The corresponding cDNA was obtained by PCR cloning and had an open reading frame encoding a 25.1 kDa protein. The protein, So-deltaTIP, was most homologous to the tonoplast intrinsic protein (TIP) subfamily of plant MIPs. Using affinity-purified So-deltaTIP-specific peptide antibodies, we investigated the subcellular and tissue distribution of So-deltaTIP. So-deltaTIP was specifically located in the vacuolar membrane. It was abundant in most vacuolated cells in all vegetative organs, but was excluded from the leaf epidermis as well as from the root phloem parenchyma and meristem. In spite of the high sequence homology between delta-TIPs of spinach, Arabidopsis, sunflower and radish, their expression patterns were totally different. However, a comparison of the expression pattern of So-deltaTIP with that of more distantly related TIPs showed similarities with Arabidopsis gamma-TIP, which is expressed in zones of cell elongation/differentiation but excluded from meristematic tissues. Meristematic cells are characterized by many small vacuoles as opposed to elongating and mature cells, which generally harbour a single, large vacuole. Our results indicate that the expression of So-deltaTIP may be induced when the large vacuole is formed.

Sorting of proteins to vacuoles in plant cells

An individual plant cell may contain at least two functionally and structurally distinct types of vacuoles: protein storage vacuoles and lytic vacuoles. Presumably a cell that stores proteins in vacuoles must maintain these separate compartments to prevent exposure of the storage proteins to an acidified environment with active hydrolytic enzymes where they would be degraded. Thus, the organization of the secretory pathway in plant cells, which includes the vacuoles, has a fascinating complexity not anticipated from the extensive genetic and biochemical studies of the secretory pathway in yeast. Plant cells must generate the membranes to form two separate types of tonoplast, maintain them as separate organelles, and direct soluble proteins from the secretory flow specifically to one or the other via separate vesicular pathways. Individual soluble and membrane proteins must be recognized and sorted into one or the other pathway by distinct, specific mechanisms. Here we review the emerging picture of how separate plant vacuoles are organized structurally and how proteins are recognized and sorted to each type.