Aquaporins constitute a large and highly divergent protein family in maize

Identification of 33 rice aquaporin genes and analysis of their expression and function

Plant aquaporins form a large protein family including plasma membrane-type (PIPs) and tonoplast-type aquaporins (TIPs), and facilitate osmotic water transport across membranes as a key physiological function. We identified 33 genes for aquaporins in the genome sequence of rice (Oryza sativa L. cv. Nipponbare). We investigated their expression levels in leaf blades, roots and anthers of rice (cv. Akitakomachi) using semi-quantitative reverse transcription-PCR (RT-PCR). At both early tillering (21 d after germination) and panicle formation (56 d) stages, six genes, including OsPIP2;4 and OsPIP2;5, were expressed predominantly in roots, while 14 genes, including OsPIP2;7 and OsTIP1;2, were found in leaf blades. Eight genes, such as OsPIP1;1 and OsTIP4;1, were evenly expressed in leaf blades, roots and anthers. Analysis by stopped-flow spectrophotometry revealed high water channel activity when OsPIP2;4 or OsPIP2;5 were expressed in yeast but not when OsPIP1;1 or OsPIP1;2 were expressed. Furthermore, the mRNA levels of OsPIP2;4 and OsPIP2;5 showed a clear diurnal fluctuation in roots; they showed a peak 3 h after the onset of light and dropped to a minimum 3 h after the onset of darkness. The mRNA levels of 10 genes including OsPIP2;4 and OsPIP2;5 markedly decreased in roots during chilling treatment and recovered after warming. The changes in mRNA levels during and after the chilling treatment were comparable with that of the bleeding sap volume. These results suggested the relationship between the root water uptake and mRNA levels of several aquaporins with high water channel activity, such as OsPIP2;4 and OsPIP2;5.

The 5A structure of heterologously expressed plant aquaporin SoPIP2;1

SoPIP2;1 is one of the major integral proteins in spinach leaf plasma membranes. In the Xenopus oocyte expression system its water channel activity is regulated by phosphorylation at the C terminus and in the first cytosolic loop. To assess its structure, SoPIP2;1 was heterologously expressed in Pichia pastoris as a His-tagged protein and in the non-tagged form. Both forms were reconstituted into 2D crystals in the presence of lipids. Tubular crystals and double-layered crystalline sheets of non-tagged SoPIP2;1 were observed and analyzed by cryo-electron microscopy. Crystalline sheets were highly ordered and diffracted electrons to a resolution of 2.96A. High-resolution projection maps of tilted specimens provided a 3D structure at 5A resolution. Superposition of the SoPIP2;1 potential map with the atomic model of AQP1 demonstrates the generally well conserved overall structure of water channels. Differences concerning the extracellular loop A explain the particular crystal contacts between oppositely oriented membrane sheets of SoPIP2;1 2D crystals, and may have a function in rapid volume changes observed in stomatal guard cells or mesophyll protoplasts. This crystal packing arrangement provides access to the phosphorylated C terminus as well as the loop B phosphorylation site for studies of channel gating.

The yeast osmosensitive mutantfps1Δ transformed by the cauliflower BobTIP1;1 aquaporin withstand a hypo-osmotic shock

Osmoregulation plays an important role in cellular responses to osmotic stress in plants and in yeast. Aquaporins contribute to osmotic adjustment by facilitating transport of water or solutes across membranes. The tonoplastic water channel BobTIP1;1 (original name BobTIP26-1) genes are upregulated during dessication stress in cauliflower meristematic tissue. To investigate the physiological importance of BobTIP1;1, we expressed it in a Saccharomyces cerevisiae osmosensitive mutant fps1Delta. We showed that the defect in the yeast glycerol plasma membrane transporter is complemented by a plant cDNA encoding the aquaporin BobTIP1;1 which is localized in the vacuolar membrane of the complemented yeast cells. To our knowledge, this is the first example of a plant aquaporin for which localization in the vacuolar membrane of yeast cells is related to an osmoresistant phenotype under hypo-osmotic shock.

Aquaporins of the PIP2 class are required for efficient anther dehiscence in tobacco

Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues. Before flower anthesis, anther and pollen dehydration takes place before the release of mature pollen at dehiscence. Aquaporins represent a class of proteins that mediates the movement of water over cellular membranes. Aquaporins of the plasmamembrane PIP2 family are expressed in tobacco (Nicotiana tabacum) anthers and may therefore be involved in the movement of water in this organ. To gain more insight into the role these proteins may play in this process, we have analyzed their localization using immunolocalizations and generated plants displaying RNA interference of PIP2 aquaporins. Our results indicate that PIP2 protein expression is modulated during anther development. Furthermore, in tobacco PIP2 RNA interference plants, anther dehydration was slower, and dehiscence occurred later when compared with control plants. Together, our results suggest that aquaporins of the PIP2 class are required for efficient anther dehydration prior to dehiscence.

Aquaporins of the PIP2 class are required for efficient anther dehiscence in tobacco

Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues. Before flower anthesis, anther and pollen dehydration takes place before the release of mature pollen at dehiscence. Aquaporins represent a class of proteins that mediates the movement of water over cellular membranes. Aquaporins of the plasmamembrane PIP2 family are expressed in tobacco (Nicotiana tabacum) anthers and may therefore be involved in the movement of water in this organ. To gain more insight into the role these proteins may play in this process, we have analyzed their localization using immunolocalizations and generated plants displaying RNA interference of PIP2 aquaporins. Our results indicate that PIP2 protein expression is modulated during anther development. Furthermore, in tobacco PIP2 RNA interference plants, anther dehydration was slower, and dehiscence occurred later when compared with control plants. Together, our results suggest that aquaporins of the PIP2 class are required for efficient anther dehydration prior to dehiscence.

The role of aquaporins and membrane damage in chilling and hydrogen peroxide induced changes in the hydraulic conductance of maize roots

When chilling-sensitive plants are chilled, root hydraulic conductance (L(o)) declines precipitously; L(o) also declines in chilling-tolerant plants, but it subsequently recovers, whereas in chilling-sensitive plants it does not. As a result, the chilling-sensitive plants dry out and may die. Using a chilling-sensitive and a chilling-tolerant maize genotype we investigated the effect of chilling on L(o), and its relationship to osmotic water permeability of isolated root cortex protoplasts, aquaporin gene expression, aquaporin abundance, and aquaporin phosphorylation, hydrogen peroxide (H(2)O(2)) accumulation in the roots and electrolyte leakage from the roots. Because chilling can cause H(2)O(2) accumulation we also determined the effects of a short H(2)O(2) treatment of the roots and examined the same parameters. We conclude from these studies that the recovery of L(o) during chilling in the chilling-tolerant genotype is made possible by avoiding or repairing membrane damage and by a greater abundance and/or activity of aquaporins. The same changes in aquaporins take place in the chilling-sensitive genotype, but we postulate that membrane damage prevents the L(o) recovery. It appears that the aquaporin response is necessary but not sufficient to respond to chilling injury. The plant must also be able to avoid the oxidative damage that accompanies chilling.

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.

What are aquaporins for?

The prime function of aquaporins (AQPs) is generally believed to be that of increasing water flow rates across membranes by raising their osmotic or hydraulic permeability. In addition, this applies to other small solutes of physiological importance. Notable applications of this 'simple permeability hypothesis' (SPH) have been epithelial fluid transport in animals, water exchanges associated with transpiration, growth and stress in plants, and osmoregulation in microbes. We first analyze the need for such increased permeabilities and conclude that in a range of situations at the cellular, subcellular and tissue levels the SPH cannot satisfactorily account for the presence of AQPs. The analysis includes an examination of the effects of the genetic elimination or reduction of AQPs (knockouts, antisense transgenics and null mutants). These either have no effect, or a partial effect that is difficult to explain, and we argue that they do not support the hypothesis beyond showing that AQPs are involved in the process under examination. We assume that since AQPs are ubiquitous, they must have an important function and suggest that this is the detection of osmotic and turgor pressure gradients. A mechanistic model is proposed--in terms of monomer structure and changes in the tetrameric configuration of AQPs in the membrane--for how AQPs might function as sensors. Sensors then signal within the cell to control diverse processes, probably as part of feedback loops. Finally, we examine how AQPs as sensors may serve animal, plant and microbial cells and show that this sensor hypothesis can provide an explanation of many basic processes in which AQPs are already implicated. Aquaporins are molecules in search of a function; osmotic and turgor sensors are functions in search of a molecule.

Functional identification of the glycerol transport activity of Chlamydomonas reinhardtii CrMIP1

By searching a Chlamydomonas expressed sequence tag database and by comparing the retrieved data with homologous sequences from a DNA database, we identified an expressed Chlamydomonas reinhardtii putative major intrinsic protein (MIP) gene. The nucleotide sequence, consisting of 1,631 bp, contains an open reading frame coding for a 300-amino-acid protein named CrMIP1. It possesses conserved NPA motifs, but is not highly homologous to known aquaporins. CrMIP1 was expressed in Saccharomyces cerevisiae and assayed for water and glycerol transport activity. By the stopped-flow spectrophotometric assay, CrMIP1 did not enhance the osmotic water permeability of membrane vesicles of the yeast transformant. However, the transformant cells showed glycerol transport activity in the in vivo assay using [14C]glycerol. This is the first report on the isolation and functional identification of a MIP member from algae.

Novel regulation of aquaporins during osmotic stress

Aquaporin protein regulation and redistribution in response to osmotic stress was investigated. Ice plant (Mesembryanthemum crystallinum) McTIP1;2 (McMIPF) mediated water flux when expressed in Xenopus leavis oocytes. Mannitol-induced water imbalance resulted in increased protein amounts in tonoplast fractions and a shift in protein distribution to other membrane fractions, suggesting aquaporin relocalization. Indirect immunofluorescence labeling also supports a change in membrane distribution for McTIP1;2 and the appearance of a unique compartment where McTIP1;2 is expressed. Mannitol-induced redistribution of McTIP1;2 was arrested by pretreatment with brefeldin A, wortmannin, and cytochalasin D, inhibitors of vesicle trafficking-related processes. Evidence suggests a role for glycosylation and involvement of a cAMP-dependent signaling pathway in McTIP1;2 redistribution. McTIP1;2 redistribution to endosomal compartments may be part of a homeostatic process to restore and maintain cellular osmolarity under osmotic-stress conditions.

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.

Homology modeling of representative subfamilies of Arabidopsis major intrinsic proteins. Classification based on the aromatic/arginine selectivity filter

Major intrinsic proteins (MIPs) are a family of membrane channels that facilitate the bidirectional transport of water and small uncharged solutes such as glycerol. The 35 full-length members of the MIP family in Arabidopsis are segregated into four structurally homologous subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like intrinsic membrane proteins (NIPs), and small basic intrinsic proteins (SIPs). Computational methods were used to construct structural models of the putative pore regions of various plant MIPs based on homology modeling with the atomic resolution crystal structures of mammalian aquaporin 1 and the bacterial glycerol permease GlpF. Based on comparisons of the narrow selectivity filter regions (the aromatic/Arg [ar/R] filter), the members of the four phylogenetic subfamilies of Arabidopsis MIPs can be classified into eight groups. PIPs possess a uniform ar/R signature characteristic of high water transport aquaporins, whereas TIPs are highly diverse with three separate conserved ar/R regions. NIPs possess two separate conserved ar/R regions, one that is similar to the archetype, soybean (Glycine max) nodulin 26, and another that is characteristic of Arabidopsis NIP6;1. The SIP subfamily possesses two ar/R subgroups, characteristic of either SIP1 or SIP2. Both SIP ar/R residues are divergent from all other MIPs in plants and other kingdoms. Overall, these findings suggest that higher plant MIPs have a common fold but show distinct differences in proposed pore apertures, potential to form hydrogen bonds with transported molecules, and amphiphilicity that likely results in divergent transport selectivities.

An expression analysis of a gene family encoding plasma membrane aquaporins in response to abiotic stresses in Arabidopsis thaliana

Aquaporin belongs to a highly conserved group of membrane proteins called major intrinsic proteins that facilitate water transport across biological membranes. The genome of Arabidopsis encodes 35 aquaporin genes with 13 homologs in the plasma membrane intrinsic protein (PIP) subgroup. However, the function of each individual aquaporin isoform and the integrated function of plant aquaporins under various physiological conditions remain unclear. As a step toward understanding the aquaporin function in plants under various environmental stimuli, the expressions of a gene family encoding 13 PIPs in Arabidopsis thaliana under various abiotic stress conditions including drought, cold, and high salinity, or abscisic acid (ABA) treatment were investigated by a quantitative real-time reverse transcription-PCR analysis. Several PIP genes were predominantly expressed either in the roots or in the flowers. The expressions of both the highly expressed aquaporins including PIP1;1, PIP1;2, and PIP2;7 and the weakly expressed aquaporins such as PIP1;4, PIP2;1, PIP2;4, and PIP2;5 were modulated by external stimuli. The analyses of our data revealed that only the PIP2;5 was up-regulated by cold treatment, and most of the PIP genes were down-regulated by cold stress. Marked up- or down-regulation in PIP expression was observed by drought stress, whereas PIP genes were less-severely modulated by high salinity. The responsiveness of each aquaporin to ABA were different, implying that the regulation of aquaporin expression involves both ABA-dependent and ABA-independent signaling pathways. Together, our comprehensive expression profile of the 13 members of the PIP gene family provides novel basis to allocate the stress-related biological function to each PIP gene.

Characterization in maize of ZmTIP2-3, a root-specific tonoplast intrinsic protein exhibiting aquaporin activity

The characterization of the tonoplast intrinsic protein ZmTIP2-3 cDNA isolated from maize roots is reported. ZmTIP2-3 belongs to the TIP2 group according to the present nomenclature. The aquaporin function of ZmTIP2-3 protein was demonstrated using expression in X. laevis oocytes. Northern blot analyses revealed that ZmTIP2-3 was specifically expressed in roots. Salt and water stresses induced the accumulation of ZmTIP2-3 transcripts. By contrast, no effect of ABA was observed. An oscillation of ZmTIP2-3 transcript amount during the day-night cycle was observed with some typical features of genes regulated by a circadian mechanism.

Functional properties of soybean nodulin 26 from a comparative three-dimensional model

A model of the nodulin 26 channel protein has been constructed based on comparative modeling and molecular dynamics simulations. Structural features of the protein indicate a selectivity filter that differs from those of the known structures of Escherichia coli glycerol facilitator and mammalian aquaporin 1. The model structure also reveals important roles of Ser207 and Phe96 in ligand binding and transport.

Interactions between plasma membrane aquaporins modulate their water channel activity

Plant plasma membrane intrinsic proteins (PIPs) cluster in two evolutionary subgroups, PIP1 and PIP2, with different aquaporin activities when expressed in Xenopus oocytes. Maize ZmPIP1;1 and ZmPIP1;2 do not increase the osmotic water permeability coefficient (Pf), whereas ZmPIP2;1, ZmPIP2;4, and ZmPIP2;5 do. Here, we show that coexpression of the nonfunctional ZmPIP1;2 and the functional ZmPIP2;1, ZmPIP2;4, or ZmPIP2;5 resulted in an increase in Pf that was dependent on the amount of injected ZmPIP1;2 complementary RNA. Confocal analysis of oocytes expressing ZmPIP1;2-green fluorescent protein (GFP) alone or ZmPIP1;2-GFP plus ZmPIP2;5 showed that the amount of ZmPIP1;2-GFP present in the plasma membrane was significantly greater in coexpressing cells. Nickel affinity chromatography purification of ZmPIP2;1 fused to a His tag coeluted with ZmPIP1;2-GFP demonstrated physical interaction and heteromerization of both isoforms. Interestingly, coexpression of ZmPIP1;1 and ZmPIP2;5 did not result in a greater increase in Pf than did the expression of ZmPIP2;5 alone, but coexpression of the ZmPIP1;1 and ZmPIP1;2 isoforms induced a Pf increase, indicating that PIP1 isoform heteromerization is required for both of them to act as functional water channels. Mutational analysis demonstrated the important role of the C-terminal part of loop E in PIP interaction and water channel activity induction. This study has revealed a new mechanism of plant aquaporin regulation that might be important in plant water relations.

Diurnal Regulation of Water Transport and Aquaporin Gene Expression in Maize Roots: Contribution of PIP2 Proteins

In maize (Zea mays) roots, xylem water transfer supported by root pressure occurs during the day and is less important at night. Diurnal modifications of osmotic pressure gradient between medium and xylem could not explain the oscillation of water flux in young maize roots during the day-night cycle. We observed a high turgor pressure of root cortical cells associated with a high flux. In maize roots, ZmPIP transcripts oscillate during the day-night cycle exhibiting some characteristics of genes regulated by a circadian mechanism. The PIP protein level profile is different from the mRNA pattern. Moreover, ZmPIP1 and ZmPIP2 protein levels are differentially regulated during the light and dark period and in response to continuous darkness suggesting different roles for both classes of PIP. Finally, our results suggest that aquaporins from ZmPIP2 subgroup may contribute to root water transfer by cellular pathway that occurs during the light and the dark period of the day-night cycle.

Members of the aquaporin family in the developing pea seed coat include representatives of the PIP, TIP, and NIP subfamilies

Water and nutrients required by developing seeds are mainly supplied by the phloem and have to be released from a maternal parenchyma tissue before being utilized by the filial tissues of embryo and endosperm. To identify aquaporins that could be involved in this process four full-length cDNAs were cloned and sequenced from a cDNA library of developing seed coats of pea (Pisum sativum L.). The cDNA of PsPIP1-1 appeared to be identical to that of clone 7a/TRG-31, a turgor-responsive gene cloned previously from pea roots. PsPIP1-1, PsPIP2-1, and PsTIP1-1, or their possible close homologues, were also expressed in cotyledons of developing and germinating seeds, and in roots and shoots of seedlings, but transcripts of PsNIP-1 were only detected in the seed coat. In mature dry seeds, high hybridization signals were observed with the probe for PsPIP1-1, but transcripts of PsPIP2-1, PsTIP1-1, and PsNIP-1 were not detected. Functional characterization after heterologous expression in Xenopus oocytes showed that PsPIP2-1 and PsTIP1-1 are aquaporins whereas PsNIP-1 is an aquaglyceroporin. PsNIP-1, like several other NIPs, contains a tryptophan residue corresponding with Trp-48 in GlpF (the glycerol facilitator of Escherichia coli) that borders the selectivity filter in the permeation channel. It is suggested that PsPIP1-1 and/or its possible close homologues could play a role in water absorption during seed imbibition, and that PsPIP2-1, possibly together with PsPIP1-1, could be involved in the release of phloem water from the seed coat symplast, which is intimately connected with the release of nutrients for the embryo.

A defect in the yeast plasma membrane urea transporter Dur3p is complemented by CpNIP1, a Nod26-like protein from zucchini (Cucurbita pepo L.), and by Arabidopsis thaliana delta-TIP or gamma-TIP

Dur3 encodes the yeast plasma membrane urea transporter and Deltadur3 mutants are unable to grow on media containing low concentrations of urea as sole nitrogen source. Complementation of the Deltadur3 mutant line with expression libraries generated from whole Arabidopsis thaliana seedlings or from zucchini (Cucurbita pepo L.) vascular tissue yielded numerous lines that had regained the capacity to grow on low urea as sole nitrogen source. Analysis of several of these yeast lines revealed that the Deltadur3 mutation was complemented either by delta-TIP (TIP=tonoplast intrinsic protein) or gamma-TIP from Arabidopsis or by CpNIP1, a new NOD26-like protein from zucchini. delta-TIP (At3g16240) and gamma-TIP (At2g36830) had previously been characterized as proteins facilitating the transport of water across the tonoplast membrane, and Nod26-like proteins were characterized as glycerol transporters. So far, transport of urea has not been described for any of the proteins described in this paper. Further analyses support this function of TIPs and nodulin 26-like intrinsic proteins in urea transport.

Analysis of the lectins from teosinte (Zea diploperennis) and maize (Zea mays) coleoptiles

To identify molecular evidence of the common origin of maize and teosinte, a lectin from teosinte coleoptile (TCL) was purified, through affinity chromatography on a lactosyl-Sepharose column, and some of the physicochemical parameters were compared with those from the maize coleoptile lectin (CCL). TCL is a 92 kDa glycoprotein constituted mainly by aspartic, glutamic, glycine, leucine, and lysine residues; in minor proportion, methionine and cysteine were also found. The glycannic portion of the lectin, which corresponds to 10% w/w, is composed by Gal, Man, and GlcNAc. CCL is an 88.7 kDa glycoprotein that contains 12% sugars by weight; its sugar and amino acid compositions are similar to those of TCL. TCL is formed by two isoforms identified through acidic electrophoresis, whereas CCL is constituted by a single molecular form. The NH(2) termini of both TCL isoforms are blocked, but their amino acid sequences determined from tryptic peptides by matrix-assisted laser desorption ionization time-of-flight) indicated that TCL isoforms have no homology with other mono- or dicotyledonous lectins, including CCL. TCL, just as CCL, showed hemagglutinating activity toward animal erythrocytes, including human A, B, and O. Hapten inhibition assays indicated that although TCL shows broader sugar specificity than CCL, it recognizes Gal in O- and N-glycosidically linked glycans. Both lectins are equally well recognized by antibodies against TCL.

Expression of a cauliflower tonoplast aquaporin tagged with GFP in tobacco suspension cells correlates with an increase in cell size

In plants, vacuoles are essential organelles that undergo dynamic volume changes during cell growth due to rapid and high flow of water through tonoplast water-carrying channels composed of integral proteins (tonoplast aquaporins). The tonoplast BobTIP26-1 from cauliflower has previously been shown to be an efficient active aquaporin in Xenopus leavis oocytes. In this study we used tobacco (Nicotiana tabacum cv. Wisconsin 38) suspension cells to examine the effect of BobTIP26-1 expression. In order to follow the intracellular localisation of the protein in real time, the gfp sequence was fused downstream to the BobTIP26-1 coding region. The fusion protein BobTIP26-1::GFP is less active than BobTIP26-1 by itself when expressed in Xenopus oocytes. Nevertheless, this fusion protein is well targeted to the tonoplast of the plant suspension cell when expressed via Agrobacterium co-cultivation. A complex tonoplast labelling is shown when young vacuolated cells are observed. The expression of the fusion protein does not affect the growth rate of the cells but increases their volume. We postulate that the increase in cell volume is triggered by the fusion protein allowing vacuolar volume increase.

Laser-capture microdissection, a tool for the global analysis of gene expression in specific plant cell types: identification of genes expressed differentially in epidermal cells or vascular tissues of maize

Laser-capture microdissection (LCM) allows for the one-step procurement of large homogeneous populations of cells from tissue sections. In mammals, LCM has been used to conduct cDNA microarray and proteomics studies on specific cell types. However, LCM has not been applied to plant cells, most likely because plant cell walls make it difficult to separate target cells from surrounding cells and because ice crystals can form in the air spaces between cells when preparing frozen sections. By fixing tissues, using a cryoprotectant before freezing, and using an adhesive-coated slide system, it was possible to capture large numbers (>10,000) of epidermal cells and vascular tissues (vascular bundles and bundle sheath cells) from ethanol:acetic acid-fixed coleoptiles of maize. RNA extracted from these cells was amplified with T7 RNA polymerase and used to hybridize a microarray containing approximately 8800 maize cDNAs. Approximately 250 of these were expressed preferentially in epidermal cells or vascular tissues. These results demonstrate that the combination of LCM and microarrays makes it feasible to conduct high-resolution global gene expression analyses of plants. This approach has the potential to enhance our understanding of diverse plant cell type-specific biological processes.

Reconstitution of water channel function of an aquaporin overexpressed and purified from Pichia pastoris

The aquaporin PM28A is one of the major integral proteins in spinach leaf plasma membranes. Phosphorylation/dephosphorylation of Ser274 at the C-terminus and of Ser115 in the first cytoplasmic loop has been shown to regulate the water channel activity of PM28A when expressed in Xenopus oocytes. To understand the mechanisms of the phosphorylation-mediated gating of the channel the structure of PM28A is required. In a first step we have used the methylotrophic yeast Pichia pastoris for expression of the pm28a gene. The expressed protein has a molecular mass of 32462 Da as determined by matrix-assisted laser desorption ionization-mass spectrometry, forms tetramers as revealed by electron microscopy and is functionally active when reconstituted in proteoliposomes. PM28A was efficiently solubilized from urea- and alkali-stripped Pichia membranes by octyl-beta-D-thioglucopyranoside resulting in a final yield of 25 mg of purified protein per liter of cell culture.

Comparative transcriptional profiling of placenta and endosperm in developing maize kernels in response to water deficit

The early post-pollination phase of maize (Zea mays) development is particularly sensitive to water deficit stress. Using cDNA microarray, we studied transcriptional profiles of endosperm and placenta/pedicel tissues in developing maize kernels under water stress. At 9 d after pollination (DAP), placenta/pedicel and endosperm differed considerably in their transcriptional responses. In placenta/pedicel, 79 genes were significantly affected by stress and of these 89% were up-regulated, whereas in endosperm, 56 genes were significantly affected and 82% of these were down-regulated. Only nine of the stress-regulated genes were in common between these tissues. Hierarchical cluster analysis indicated that different sets of genes were regulated in the two tissues. After rewatering at 9 DAP, profiles at 12 DAP suggested that two regulons exist, one for genes responding specifically to concurrent imposition of stress, and another for genes remaining affected after transient stress. In placenta, genes encoding recognized stress tolerance proteins, including heat shock proteins, chaperonins, and major intrinsic proteins, were the largest class of genes regulated, all of which were up-regulated. In contrast, in endosperm, genes in the cell division and growth category represented a large class of down-regulated genes. Several cell wall-degrading enzymes were expressed at lower levels than in controls, suggesting that stress delayed normal advance to programmed cell death in the central endosperm. We suggest that the responsiveness of placenta to whole-plant stress factors (water potential, abscisic acid, and sugar flux) and of endosperm to indirect factors may play key roles in determining the threshold for kernel abortion.

Role of a single aquaporin isoform in root water uptake

Aquaporins are ubiquitous channel proteins that facilitate the transport of water across cell membranes. Aquaporins show a typically high isoform multiplicity in plants, with 35 homologs in Arabidopsis. The integrated function of plant aquaporins and the function of each individual isoform remain poorly understood. Matrix-assisted laser desorption/ionization time-of-flight analyses suggested that Plasma Membrane Intrinsic Protein2;2 (PIP2;2) is one of the abundantly expressed aquaporin isoforms in Arabidopsis root plasma membranes. Two independent Arabidopsis knockout mutants of PIP2;2 were isolated using a PCR-based strategy from a library of plant lines mutagenized by the insertion of Agrobacterium tumefaciens T-DNA. Expression in transgenic Arabidopsis of a PIP2;2 promoter-beta-glucuronidase gene fusion indicated that PIP2;2 is expressed predominantly in roots, with a strong expression in the cortex, endodermis, and stele. The hydraulic conductivity of root cortex cells, as measured with a cell pressure probe, was reduced by 25 to 30% in the two allelic PIP2;2 mutants compared with the wild type. In addition, free exudation measurements revealed a 14% decrease, with respect to wild-type values, in the osmotic hydraulic conductivity of roots excised from the two PIP2;2 mutants. Together, our data provide evidence for the contribution of a single aquaporin gene to root water uptake and identify PIP2;2 as an aquaporin specialized in osmotic fluid transport. PIP2;2 has a close homolog, PIP2;3, showing 96.8% amino acid identity. The phenotype of PIP2;2 mutants demonstrates that, despite their high homology and isoform multiplicity, plant aquaporins have evolved with nonredundant functions.

Plasma membrane aquaporins play a significant role during recovery from water deficit

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5- to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants. From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.

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.

The role of aquaporins in root water uptake

The capacity of roots to take up water is determined in part by the resistance of living tissues to radial water flow. Both the apoplastic and cell-to-cell paths mediate water transport in these tissues but the contribution of cell membranes to the latter path has long been difficult to estimate. Aquaporins are water channel proteins that are expressed in various membrane compartments of plant cells, including the plasma and vacuolar membranes. Plant aquaporins are encoded by a large multigene family, with 35 members in Arabidopsis thaliana, and many of these aquaporins show a cell-specific expression pattern in the root. Mercury acts as an efficient blocker of most aquaporins and has been used to demonstrate the significant contribution of water channels to overall root water transport. Aquaporin-rich membranes may be needed to facilitate intense water flow across root tissues and may represent critical points where an efficient and spatially restricted control of water uptake can be exerted. Roots, in particular, show a remarkable capacity to alter their water permeability over the short term (i.e. in a few hours to less than 2-3 d) in response to many stimuli, such as day/night cycles, nutrient deficiency or stress. Recent data suggest that these rapid changes can be mostly accounted for by changes in cell membrane permeability and are mediated by aquaporins. Although the processes that allow perception of environmental changes by root cells and subsequent aquaporin regulation are nearly unknown, the study of root aquaporins provides an interesting model to understand the regulation of water transport in plants and sheds light on the basic mechanisms of water uptake by roots.

Functional analysis of water channels in barley roots

We identified three genes homologous to water channels in the plasma membrane type subfamily from roots of barley seedlings. These genes were designated HvPIP2;1, HvPIP1;3, and HvPIP1;5 after comparison to Arabidopsis aquaporins. Competitive reverse transcription (RT)-PCR was applied in order to distinguish and to quantify their transcripts. The HvPIP2;1 transcript was the most abundant among the three in roots. Salt stress (200 mM NaCl) down-regulated HvPIP2;1 (transcript and protein), but had almost no effect on the expressions of HvPIP1;3, or HvPIP1;5. Approximately equal amounts of the transcripts of the three were detected in shoots, and salt stress enhanced the expression of HvPIP2;1 but not of HvPIP1;3, or HvPIP1;5. HvPIP2;1 protein was confirmed to be localized in the plasma membrane. Functional expression of HvPIP2;1 in Xenopus oocytes confirmed that HvPIP2;1 encoded an aquaporin that transports water. This water permeability was reduced by HgCl(2), which is a typical water channel inhibitor. This activity was not modified by some inhibitors against protein kinase and protein phosphatase.

Plant aquaporins

Aquaporins are ubiquitous membrane channel proteins that facilitate and regulate the permeation of water across biological membranes. Aquaporins are members of the MIP family and some of them seem to be also able to transport other molecules such as urea or glycerol. In the plant kingdom, a single plant expresses a considerably large number of MIP homologues. These homologues can be subdivided into four groups (PIP, TIP, NIP, SIP) with highly conserved amino acid sequences and intron positions in each group. Since their discovery, advancing knowledge of their structure led to an understanding of the basic features of the water transport mechanism. An optimal water balance is essential to the homeostasis of most organisms, and aquaporins may be one of the mechanisms involved under changing environmental and developmental conditions. In fact, this may be one reason for the abundance and diversity of aquaporins, in particular in plants. In addition, exposure to different types of stress alters water relations and thus, aquaporins may be involved in stress responses as well. The transcriptional and/or post-translational regulation of aquaporins would determine changes in membrane water permeability. Both phosphorylation and translocation to/from vesicles have been reported as post-translational mechanisms. However, translocation in plants has not yet been shown. Although significant advances have been achieved, complete understanding of aquaporin function and regulation remains elusive.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

High expression of putative aquaporin genes in cells with transporting and nutritive functions during seed development in Norway spruce (Picea abies)

Aquaporins mediate the bidirectional passage of water over membranes and are present in tonoplasts (TIPs) and in plasma membranes (PIPs) of plant cells. Knowing their expression in different tissues is valuable when assessing their contribution to plant water relations. A TIP-gene has been cloned from developing female gametophytes of Picea abies, a conifer displaying an embryology different from the angiosperms. Probes were made from conserved regions of the TIP gene and used for in situ hybridization to examine the gene expression pattern in developing female reproductive structures. Early during development high transcript expression was found in the spongy tissue encasing the developing female gametophyte, in cells of the future seed coat of young ovules and in vascular tissue of the ovuliferous scale. At later stages a strong signal was seen in archegonia jacket cells surrounding egg cells and, still later, at the time of storage protein accumulation, in storage parenchyma cells of the gametophyte as well. These aquaporin-homologues probably participate in regulating water balance in the cells although they could also be permeable to other molecules than water.

A new subfamily of major intrinsic proteins in plants

The major intrinsic proteins (MIPs) form a large protein family of ancient origin and are found in bacteria, fungi, animals, and plants. MIPs act as channels in membranes to facilitate passive transport across the membrane. Some MIPs allow small polar molecules like glycerol or urea to pass through the membrane. However, the majority of MIPs are thought to be aquaporins (AQPs), i.e., they are specific for water transport. Plant MIPs can be subdivided into the plasma membrane intrinsic protein, tonoplast intrinsic protein, and NOD26-like intrinsic protein subfamilies. By database mining and phylogenetic analyses, we have identified a new subfamily in plants, the Small basic Intrinsic Proteins (SIPs). Comparisons of sequences from the new subfamily with conserved amino acid residues in other MIPs reveal characteristic features of SIPs. Possible functional consequences of these features are discussed in relation to the recently solved structures of AQP1 and GlpF. We suggest that substitutions at conserved and structurally important positions imply a different substrate specificity for the new subfamily.

Plasma membrane aquaporins in the motor cells of Samanea saman: diurnal and circadian regulation

Leaf-moving organs, remarkable for the rhythmic volume changes of their motor cells, served as a model system in which to study the regulation of membrane water fluxes. Two plasma membrane intrinsic protein homolog genes, SsAQP1 and SsAQP2, were cloned from these organs and characterized as aquaporins in Xenopus laevis oocytes. Osmotic water permeability (P(f)) was 10 times higher in SsAQP2-expressing oocytes than in SsAQP1-expressing oocytes. SsAQP1 was found to be glycerol permeable, and SsAQP2 was inhibited by 0.5 mM HgCl(2) and by 1 mM phloretin. The aquaporin mRNA levels differed in their spatial distribution in the leaf and were regulated diurnally in phase with leaflet movements. Additionally, SsAQP2 transcription was under circadian control. The P(f) of motor cell protoplasts was regulated diurnally as well: the morning and/or evening P(f) increases were inhibited by 50 microM HgCl(2), by 2 mM cycloheximide, and by 250 microM phloretin to the noon P(f) level. Our results link SsAQP2 to the physiological function of rhythmic cell volume changes.

Plant aquaporins: multifunctional water and solute channels with expanding roles

There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.

From genome to function: the Arabidopsis aquaporins

BACKGROUND

In the post-genomic era newly sequenced genomes can be used to deduce organismal functions from our knowledge of other systems. Here we apply this approach to analyzing the aquaporin gene family in Arabidopsis thaliana. The aquaporins are intrinsic membrane proteins that have been characterized as facilitators of water flux. Originally termed major intrinsic proteins (MIPs), they are now also known as water channels, glycerol facilitators and aqua-glyceroporins, yet recent data suggest that they facilitate the movement of other low-molecular-weight metabolites as well.

RESULTS

The Arabidopsis genome contains 38 sequences with homology to aquaporin in four subfamilies, termed PIP, TIP, NIP and SIP. We have analyzed aquaporin family structure and expression using the A. thaliana genome sequence, and introduce a new NMR approach for the purpose of analyzing water movement in plant roots in vivo.

CONCLUSIONS

Our preliminary data indicate a strongly transcellular component for the flux of water in roots.

An aquaglyceroporin is abundantly expressed early in the development of the suspensor and the embryo proper of loblolly pine

In contrast to angiosperms, pines and other gymnosperms form well-developed suspensors in somatic embryogenic cultures. This creates a useful system to study suspensor biology. In a study of gene expression during the early stages of conifer embryogenesis, we identified a transcript, PtNIP1;1, that is abundant in immature loblolly pine (Pinus taeda) zygotic and somatic embryos, but is undetectable in later-stage embryos, megagametophytes, and roots, stems, and needles from 1 year-old seedlings. Analysis of PtNIP1;1 transcript in embryo proper and suspensor tissues by reverse transcription-polymerase chain reaction suggests preferential expression in the suspensor. Based on comparisons of derived amino acid sequences, PtNIP1;1 belongs to the nodulin-like members of the major intrinsic protein superfamily branch of the aquaporin (major intrinsic protein) superfamily. Through heterologous expression in Xenopus laevis oocytes and the yeast (Saccharomyces cerevisiae) fps1(-) mutant, PtNIP1;1 has been shown to be an active aquaglyceroporin.

Strategies to identify transport systems in plants

Since the first molecular structures of plant transporters were discovered over a decade ago, considerable advances have been made in the study of plant membrane transport, but we still do not understand transport regulation. The genes encoding the transport systems in the various cell membranes are still to be identified, as are the physiological roles of most transport systems. A wide variety of complementary strategies are now available to study transport systems in plants, including forward and reverse genetics, proteomics, and in silico exploitation of the huge amount of information contained in the completely known genomic sequence of Arabidopsis.

The role of ABA and the transpiration stream in the regulation of the osmotic water permeability of leaf cells

The transpiration stream that passes through a plant may follow an apoplastic route, with low resistance to flow, or a cell-to-cell route, in which cellular membranes impede water flow. However, passage of water through membranes can be facilitated by aquaporins thereby decreasing resistance. We investigated the relationship between transpiration, which can be down-regulated by abscisic acid (ABA) or by high humidity, and the osmotic water permeability (P(os)) of protoplasts. By using leaf protoplasts of wild-type (wt) Arabidopsis thaliana plants and of mutants that are low in ABA (aba1) or insensitive to ABA (abi1 and abi2), we found that protoplasts from aba1 and abi mutants have very low P(os) values compared with those from wt plants when the plants are grown at 45% relative humidity. High values of P(os) were found 3 h after the addition of ABA to the culture medium of aba1 plants; addition of ABA to abi plants did not restore the P(os) to wt levels. There was no such increase in P(os) when excised leaves of aba1 plants were treated with ABA. When the transpiration stream was attenuated by growing the plants at 85% relative humidity, the P(os) of protoplasts from all plants (wt and mutants) was higher. We suggest that attenuation of the transpiration stream in whole plants is required for the up-regulation of the P(os) of the membranes, and that this up-regulation, which does not require ABA, is mediated by the activation of aquaporins in the plasma membrane.

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.

The complete set of genes encoding major intrinsic proteins in Arabidopsis provides a framework for a new nomenclature for major intrinsic proteins in plants

Major intrinsic proteins (MIPs) facilitate the passive transport of small polar molecules across membranes. MIPs constitute a very old family of proteins and different forms have been found in all kinds of living organisms, including bacteria, fungi, animals, and plants. In the genomic sequence of Arabidopsis, we have identified 35 different MIP-encoding genes. Based on sequence similarity, these 35 proteins are divided into four different subfamilies: plasma membrane intrinsic proteins, tonoplast intrinsic proteins, NOD26-like intrinsic proteins also called NOD26-like MIPs, and the recently discovered small basic intrinsic proteins. In Arabidopsis, there are 13 plasma membrane intrinsic proteins, 10 tonoplast intrinsic proteins, nine NOD26-like intrinsic proteins, and three small basic intrinsic proteins. The gene structure in general is conserved within each subfamily, although there is a tendency to lose introns. Based on phylogenetic comparisons of maize (Zea mays) and Arabidopsis MIPs (AtMIPs), it is argued that the general intron patterns in the subfamilies were formed before the split of monocotyledons and dicotyledons. Although the gene structure is unique for each subfamily, there is a common pattern in how transmembrane helices are encoded on the exons in three of the subfamilies. The nomenclature for plant MIPs varies widely between different species but also between subfamilies in the same species. Based on the phylogeny of all AtMIPs, a new and more consistent nomenclature is proposed. The complete set of AtMIPs, together with the new nomenclature, will facilitate the isolation, classification, and labeling of plant MIPs from other species.

The complete set of genes encoding major intrinsic proteins in Arabidopsis provides a framework for a new nomenclature for major intrinsic proteins in plants

Major intrinsic proteins (MIPs) facilitate the passive transport of small polar molecules across membranes. MIPs constitute a very old family of proteins and different forms have been found in all kinds of living organisms, including bacteria, fungi, animals, and plants. In the genomic sequence of Arabidopsis, we have identified 35 different MIP-encoding genes. Based on sequence similarity, these 35 proteins are divided into four different subfamilies: plasma membrane intrinsic proteins, tonoplast intrinsic proteins, NOD26-like intrinsic proteins also called NOD26-like MIPs, and the recently discovered small basic intrinsic proteins. In Arabidopsis, there are 13 plasma membrane intrinsic proteins, 10 tonoplast intrinsic proteins, nine NOD26-like intrinsic proteins, and three small basic intrinsic proteins. The gene structure in general is conserved within each subfamily, although there is a tendency to lose introns. Based on phylogenetic comparisons of maize (Zea mays) and Arabidopsis MIPs (AtMIPs), it is argued that the general intron patterns in the subfamilies were formed before the split of monocotyledons and dicotyledons. Although the gene structure is unique for each subfamily, there is a common pattern in how transmembrane helices are encoded on the exons in three of the subfamilies. The nomenclature for plant MIPs varies widely between different species but also between subfamilies in the same species. Based on the phylogeny of all AtMIPs, a new and more consistent nomenclature is proposed. The complete set of AtMIPs, together with the new nomenclature, will facilitate the isolation, classification, and labeling of plant MIPs from other species.