Aquaporins constitute a large and highly divergent protein family in maize

Aquaporins (AQPs) are an ancient family of channel proteins that transport water and neutral solutes through a pore and are found in all eukaryotes and most prokaryotes. A comparison of the amino acid sequences and phylogenetic analysis of 31 full-length cDNAs of maize (Zea mays) AQPs shows that they comprise four different groups of highly divergent proteins. We have classified them as plasma membrane intinsic proteins (PIPs), tonoplast intrinsic proteins, Nod26-like intrinsic proteins, and small and basic intrinsic proteins. Amino acid sequence identities vary from 16% to 100%, but all sequences share structural motifs and conserved amino acids necessary to stabilize the two loops that form the aqueous pore. Most divergent are the small and basic integral proteins in which the first of the two highly conserved Asn-Pro-Ala motifs of the pore is not conserved, but is represented by alanine-proline-threonine or alanine-proline-serine. We present a model of ZmPIP1-2 based on the three-dimensional structure of mammalian AQP1. Tabulation of the number of times that the AQP sequences are found in a collection of databases that comprises about 470,000 maize cDNAs indicates that a few of the maize AQPs are very highly expressed and many are not abundantly expressed. The phylogenetic analysis supports the interpretation that the divergence of PIPs through gene duplication occurred more recently than the divergence of the members of the other three subfamilies. This study opens the way to analyze the function of the proteins in Xenopus laevis oocytes, determine the tissue specific expression of the genes, recover insertion mutants, and determine the in planta function.

Aquaporins: highly regulated channels controlling plant water relations

Plant growth and development are dependent on tight regulation of water movement. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating. At the whole organ level aquaporins modify water conductance and gradients at key "gatekeeper" cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation to determine the degree of isohydry/anisohydry. Molecular, physiological, and biophysical approaches have demonstrated that variations in root and leaf hydraulic conductivity can be accounted for by aquaporins but this must be integrated with anatomical considerations. This Update integrates these data and emphasizes the central role played by aquaporins in regulating plant water relations.

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.

FRET imaging in living maize cells reveals that plasma membrane aquaporins interact to regulate their subcellular localization

Zea mays plasma membrane intrinsic proteins (ZmPIPs) fall into two groups, ZmPIP1s and ZmPIP2s, that exhibit different water channel activities when expressed in Xenopus oocytes. ZmPIP1s are inactive, whereas ZmPIP2s induce a marked increase in the membrane osmotic water permeability coefficient, P(f). We previously showed that, in Xenopus oocytes, ZmPIP1;2 and ZmPIP2;1 interact to increase the cell P(f). Here, we report the localization and interaction of ZmPIP1s and ZmPIP2s in living maize cells. ZmPIPs were fused to monomeric yellow fluorescent protein and/or monomeric cyan fluorescent protein and expressed transiently in maize mesophyll protoplasts. When expressed alone, ZmPIP1 fusion proteins were retained in the endoplasmic reticulum, whereas ZmPIP2s were found in the plasma membrane. Interestingly, when coexpressed with ZmPIP2s, ZmPIP1s were relocalized to the plasma membrane. Using FRET/fluorescence lifetime imaging microscopy, we demonstrated that this relocalization results from interaction between ZmPIP1s and ZmPIP2s. Immunoprecipitation experiments provided additional evidence for the association of ZmPIP1;2 and ZmPIP2;1 in maize roots and suspension cells. These data suggest that PIP1-PIP2 interaction is required for in planta PIP1 trafficking to the plasma membrane to modulate plasma membrane permeability.

Regulation of plant aquaporin activity

Accumulating evidence indicates that aquaporins play a key role in plant water relations. Plant aquaporins are part of a large and highly divergent protein family that can be divided into four subfamilies according to amino acid sequence similarity. As in other organisms, plant aquaporins facilitate the transcellular movement of water, but, in some cases, also the flux of small neutral solutes across a cellular membrane. Plant cell membranes are characterized by a large range of osmotic water permeabilities, and recent data indicate that plant aquaporin activity might be regulated by gating mechanisms. The factors affecting the gating behaviour possibly involve phosphorylation, heteromerization, pH, Ca2+, pressure, solute gradients and temperature. Regulation of aquaporin trafficking may also represent a way to modulate membrane water permeability. The aim of this review is to integrate recent molecular and biophysical data on the mechanisms regulating aquaporin activity in plant membranes and to relate them to putative changes in protein structure.

Drought and abscisic acid effects on aquaporin content translate into changes in hydraulic conductivity and leaf growth rate: a trans-scale approach

The effects of abscisic acid (ABA) on aquaporin content, root hydraulic conductivity (Lpr), whole plant hydraulic conductance, and leaf growth are controversial. We addressed these effects via a combination of experiments at different scales of plant organization and tested their consistency via a model. We analyzed under moderate water deficit a series of transformed maize (Zea mays) lines, one sense and three antisense, affected in NCED (for 9-cis-epoxycarotenoid dioxygenase) gene expression and that differed in the concentration of ABA in the xylem sap. In roots, the mRNA expression of most aquaporin PIP (for plasma membrane intrinsic protein) genes was increased in sense plants and decreased in antisense plants. The same pattern was observed for the protein contents of four PIPs. This resulted in more than 6-fold differences between lines in Lpr under both hydrostatic and osmotic gradients of water potential. This effect was probably due to differences in aquaporin activity, because it was nearly abolished by a hydrogen peroxide treatment, which blocks the water channel activity of aquaporins. The hydraulic conductance of intact whole plants was affected in the same way when measured either in steady-state conditions or via the rate of recovery of leaf water potential after rewatering. The recoveries of leaf water potential and elongation upon rehydration differed between lines and were accounted for by the experimentally measured Lpr in a model of water transfer. Overall, these results suggest that ABA has long-lasting effects on plant hydraulic properties via aquaporin activity, which contributes to the maintenance of a favorable plant water status.

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.

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.

Plasma membrane intrinsic proteins from maize cluster in two sequence subgroups with differential aquaporin activity

The transport of water through membranes is regulated in part by aquaporins or water channel proteins. These proteins are members of the larger family of major intrinsic proteins (MIPs). Plant aquaporins are categorized as either tonoplast intrinsic proteins (TIPs) or plasma membrane intrinsic proteins (PIPs). Sequence analysis shows that PIPs form several subclasses. We report on the characterization of three maize (Zea mays) PIPs belonging to the PIP1 and PIP2 subfamilies (ZmPIP1a, ZmPIP1b, and ZmPIP2a). The ZmPIP2a clone has normal aquaporin activity in Xenopus laevis oocytes. ZmPIP1a and ZmPIP1b have no activity, and a review of the literature shows that most PIP1 proteins identified in other plants have no or very low activity in oocytes. Arabidopsis PIP1 proteins are the only exception. Control experiments show that this lack of activity of maize PIP1 proteins is not caused by their failure to arrive at the plasma membrane of the oocytes. ZmPIP1b also does not appear to facilitate the transport of any of the small solutes tried (glycerol, choline, ethanol, urea, and amino acids). These results are discussed in relationship to the function and regulation of the PIP family of aquaporins.

Localization and quantification of plasma membrane aquaporin expression in maize primary root: a clue to understanding their role as cellular plumbers

Water movement across root tissues occurs by parallel apoplastic, symplastic, and transcellular pathways that the plant can control to a certain extent. Because water channels or aquaporins (AQPs) play an important role in regulating water flow, studies on AQP mRNA and protein expression in different root tissues are essential. Here, we quantified and localized the expression of Zea mays plasma membrane AQPs (ZmPIPs) in primary root tip using in situ and quantitative RT-PCR and immunodetection approaches. All ZmPIP genes except ZmPIP2;7 were expressed in primary roots. Expression was found to be dependent on the developmental stage of the root, with, in general, an increase in expression towards the elongation and mature zones. Two genes, ZmPIP1;5 and ZmPIP2;5, showed the greatest increase in expression (up to 11- and 17-fold, respectively) in the mature zone, where they accounted for 50% of the total expressed ZmPIPs. The immunocytochemical localization of ZmPIP2;1 and ZmPIP2;5 in the exodermis and endodermis indicated that they are involved in root radial water movement. In addition, we detected a polar localization of ZmPIP2;5 to the external periclinal side of epidermal cells in root apices, suggesting an important role in water uptake from the root surface. Finally, protoplast swelling assays showed that root cells display a variable, but globally low, osmotic water permeability coefficient (Pf < 10 microm/s). However, the presence of a population of cells with a higher Pf (up to 26 microm/s) in mature zone of the root might be correlated with the increased expression of several ZmPIP genes.

Arabidopsis SNAREs SYP61 and SYP121 coordinate the trafficking of plasma membrane aquaporin PIP2;7 to modulate the cell membrane water permeability

Plant plasma membrane intrinsic proteins (PIPs) are aquaporins that facilitate the passive movement of water and small neutral solutes through biological membranes. Here, we report that post-Golgi trafficking of PIP2;7 in Arabidopsis thaliana involves specific interactions with two syntaxin proteins, namely, the Qc-SNARE SYP61 and the Qa-SNARE SYP121, that the proper delivery of PIP2;7 to the plasma membrane depends on the activity of the two SNAREs, and that the SNAREs colocalize and physically interact. These findings are indicative of an important role for SYP61 and SYP121, possibly forming a SNARE complex. Our data support a model in which direct interactions between specific SNARE proteins and PIP aquaporins modulate their post-Golgi trafficking and thus contribute to the fine-tuning of the water permeability of the plasma membrane.

Solanaceae XIPs are plasma membrane aquaporins that facilitate the transport of many uncharged substrates

Major intrinsic proteins (MIPs) transport water and uncharged solutes across membranes in all kingdoms of life. Recently, an uncharacterized MIP subfamily was identified in the genomes of plants and fungi and named X Intrinsic Proteins (XIPs). Here, we describe the genetic features, localization, expression, and functions of a group of Solanaceae XIPs. XIP cDNA and gDNA were cloned from tobacco, potato, tomato, and morning glory. A conserved sequence motif in the first intron of Solanaceae XIPs initiates an RNA-processing mechanism that results in two splice variants (α and β). When transiently or stably expressed in tobacco plants, yellow fluorescent protein-tagged NtXIP1;1α and NtXIP1;1β were both localized in the plasma membrane. Transgenic tobacco lines expressing NtXIP1;1-promoter-GUS constructs and RT-PCR studies showed that NtXIP1;1 was expressed in all organs. The NtXIP1;1 promoter was mainly active in cell layers facing the environment in all above-ground tissues. Heterologous expression of Solanaceae XIPs in Xenopus laevis oocytes and various Saccharomyces cerevisiae mutants demonstrated that these isoforms facilitate the transport of bulky solutes, such as glycerol, urea, and boric acid. In contrast, permeability for water was undetectable. These data suggest that XIPs function in the transport of uncharged solutes across the cell plasma membrane in specific plant tissues, including at the interface between the environment and external cell layers.

New insights into the regulation of aquaporins by the arbuscular mycorrhizal symbiosis in maize plants under drought stress and possible implications for plant performance

The relationship between modulation by arbuscular mycorrhizae (AM) of aquaporin expression in the host plant and changes in root hydraulic conductance, plant water status, and performance under stressful conditions is not well known. This investigation aimed to elucidate how the AM symbiosis modulates the expression of the whole set of aquaporin genes in maize plants under different growing and drought stress conditions, as well as to characterize some of these aquaporins in order to shed further light on the molecules that may be involved in the mycorrhizal responses to drought. The AM symbiosis regulated a wide number of aquaporins in the host plant, comprising members of the different aquaporin subfamilies. The regulation of these genes depends on the watering conditions and the severity of the drought stress imposed. Some of these aquaporins can transport water and also other molecules which are of physiological importance for plant performance. AM plants grew and developed better than non-AM plants under the different conditions assayed. Thus, for the first time, this study relates the well-known better performance of AM plants under drought stress to not only the water movement in their tissues but also the mobilization of N compounds, glycerol, signaling molecules, or metalloids with a role in abiotic stress tolerance. Future studies should elucidate the specific function of each aquaporin isoform regulated by the AM symbiosis in order to shed further light on how the symbiosis alters the plant fitness under stressful conditions.

Modulating the expression of aquaporin genes in planta: A key to understand their physiological functions?

Aquaporins (AQPs) are believed to act as "cellular plumbers", allowing plants to rapidly alter their membrane water permeability in response to environmental cues. This study of AQP regulation at both the RNA and protein levels has revealed a large number of possible mechanisms. Currently, modulation of AQP expression in planta is considered the strategy of choice for elucidating the role of AQPs in plant physiology. This review highlights the fact that this strategy is complicated by many factors, such as the incomplete characterization of transport selectivity of the targeted AQP, the fact that AQPs might act as multifunctional channels with multiple physiological roles, and the number of post-translational regulation mechanisms. The classification of AQPs as constitutive or stress-responsive isoforms is also proposed.

The Arabidopsis abiotic stress-induced TSPO-related protein reduces cell-surface expression of the aquaporin PIP2;7 through protein-protein interactions and autophagic degradation

The Arabidopsis thaliana multi-stress regulator TSPO is transiently induced by abiotic stresses. The final destination of this polytopic membrane protein is the Golgi apparatus, where its accumulation is strictly regulated, and TSPO is downregulated through a selective autophagic pathway. TSPO-related proteins regulate the physiology of the cell by generating functional protein complexes. A split-ubiquitin screen for potential TSPO interacting partners uncovered a plasma membrane aquaporin, PIP2;7. Pull-down assays and fluorescence imaging approaches revealed that TSPO physically interacts with PIP2;7 at the endoplasmic reticulum and Golgi membranes in planta. Intriguingly, constitutive expression of fluorescently tagged PIP2;7 in TSPO-overexpressing transgenic lines resulted in patchy distribution of the fluorescence, reminiscent of the pattern of constitutively expressed yellow fluorescent protein-TSPO in Arabidopsis. Mutational stabilization of TSPO or pharmacological inhibition of the autophagic pathway affected concomitantly the detected levels of PIP2;7, suggesting that the complex containing both proteins is degraded through the autophagic pathway. Coexpression of TSPO and PIP2;7 resulted in decreased levels of PIP2;7 in the plasma membrane and abolished the membrane water permeability mediated by transgenic PIP2;7. Taken together, these data support a physiological role for TSPO in regulating the cell-surface expression of PIP2;7 during abiotic stress conditions through protein-protein interaction and demonstrate an aquaporin regulatory mechanism involving TSPO.

The expression pattern of plasma membrane aquaporins in maize leaf highlights their role in hydraulic regulation

Leaves are key organs for evaporation and photosynthesis and play a crucial role in plant growth and development. In order to function properly, they need to maintain a balanced water content. Water movement through a leaf occurs by a combination of different pathways: water can follow an apoplastic route through the cell wall or a cell-to-cell route via the symplastic and transcellular paths. As aquaporins (AQPs) play an important role in regulating transcellular water flow and CO(2) conductance, studies on AQP mRNA and protein expression in leaves are essential to better understand their role in these physiological processes. Here, we quantified and localized the expression of Zea mays plasma membrane aquaporins (ZmPIPs, plasma membrane intrinsic proteins) in the leaf using quantitative RT-PCR and immunodetection. All ZmPIP genes except ZmPIP2;7 were expressed in leaves. Expression was found to be dependent on the developmental stage of the leaf tissue, with, in general, an increase in expression at the end of the elongation zone and a decrease in mature leaf tissue. These data correlated with the cell water permeability, as determined using a protoplast swelling assay. The diurnal expression of ZmPIPs was also investigated and expression was found to be higher during the first hours of the light period than at night. Immunocytochemical localization of four ZmPIP isoforms indicated that they are involved in leaf radial water movement, in particular in vascular bundles and the mesophyll.

Inactivation of the β(1,2)-xylosyltransferase and the α(1,3)-fucosyltransferase genes in Nicotiana tabacum BY-2 Cells by a Multiplex CRISPR/Cas9 Strategy Results in Glycoproteins without Plant-Specific Glycans

Plants or plant cells can be used to produce pharmacological glycoproteins such as antibodies or vaccines. However these proteins carry N-glycans with plant-typical residues [β(1,2)-xylose and core α(1,3)-fucose], which can greatly impact the immunogenicity, allergenicity, or activity of the protein. Two enzymes are responsible for the addition of plant-specific glycans: β(1,2)-xylosyltransferase (XylT) and α(1,3)-fucosyltransferase (FucT). Our aim consisted of knocking-out two XylT genes and four FucT genes (12 alleles altogether) in Nicotiana tabacum BY-2 suspension cells using CRISPR/Cas9. Three XylT and six FucT sgRNAs were designed to target conserved regions. After transformation of N. tabacum BY-2 cells with genes coding for sgRNAs, Cas9, and a selectable marker (bar), transgenic lines were obtained and their extracellular as well as intracellular protein complements were analyzed by Western blotting using antibodies recognizing β(1,2)-xylose and α(1,3)-fucose. Three lines showed a strong reduction of β(1,2)-xylose and α(1,3)-fucose, while two lines were completely devoid of them, indicating complete gene inactivation. The absence of these carbohydrates was confirmed by mass spectrometry analysis of the extracellular proteins. PCR amplification and sequencing of the targeted region indicated small INDEL and/or deletions between the target sites. The KO lines did not show any particular morphology and grew as the wild-type. One KO line was transformed with genes encoding a human IgG2 antibody. The IgG2 expression level was as high as in a control transformant which had not been glycoengineered. The IgG glycosylation profile determined by mass spectrometry confirmed that no β(1,2)-xylose or α(1,3)-fucose were present on the glycosylation moiety and that the dominant glycoform was the GnGn structure. These data represent an important step toward humanizing the glycosylation of pharmacological proteins expressed in N. tabacum BY-2 cells.

Regulation of plasma membrane aquaporins by inoculation with a Bacillus megaterium strain in maize (Zea mays L.) plants under unstressed and salt-stressed conditions

It is documented that some plant-growth-promoting rhizobacteria (PGPR) enhance plant salt tolerance. However, as to how PGPR may influence two crucial components of plant salt tolerance such as, root hydraulic characteristics and aquaporin regulation has been almost unexplored. Here, maize (Zea mays L.) plants were inoculated with a Bacillus megaterium strain previously isolated from a degraded soil and characterized as PGPR. Inoculated plants were found to exhibit higher root hydraulic conductance (L) values under both unstressed and salt-stressed conditions. These higher L values in inoculated plants correlated with higher plasma membrane type two (PIP2) aquaporin amount in their roots under salt-stressed conditions. Also, ZmPIP1;1 protein amount under salt-stressed conditions was higher in inoculated leaves than in non-inoculated ones. Hence, the different regulation of PIP aquaporin expression and abundance by the inoculation with the B. megaterium strain could be one of the causes of the different salt response in terms of root growth, necrotic leaf area, leaf relative water content and L by the inoculation treatment.

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions

BACKGROUND AND AIMS

The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants.

METHODS

We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured.

KEY RESULTS

Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways.

CONCLUSIONS

The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.

Selective regulation of maize plasma membrane aquaporin trafficking and activity by the SNARE SYP121

Plasma membrane intrinsic proteins (PIPs) are aquaporins facilitating the diffusion of water through the cell membrane. We previously showed that the traffic of the maize (Zea mays) PIP2;5 to the plasma membrane is dependent on the endoplasmic reticulum diacidic export motif. Here, we report that the post-Golgi traffic and water channel activity of PIP2;5 are regulated by the SNARE (for soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) SYP121, a plasma membrane resident syntaxin involved in vesicle traffic, signaling, and regulation of K(+) channels. We demonstrate that the expression of the dominant-negative SYP121-Sp2 fragment in maize mesophyll protoplasts or epidermal cells leads to a decrease in the delivery of PIP2;5 to the plasma membrane. Protoplast and oocyte swelling assays showed that PIP2;5 water channel activity is negatively affected by SYP121-Sp2. A combination of in vitro (copurification assays) and in vivo (bimolecular fluorescence complementation, Förster resonance energy transfer, and yeast split-ubiquitin) approaches allowed us to demonstrate that SYP121 and PIP2;5 physically interact. Together with previous data demonstrating the role of SYP121 in regulating K(+) channel trafficking and activity, these results suggest that SYP121 SNARE contributes to the regulation of the cell osmotic homeostasis.

Comparative landscape genetic analyses show a Belgian motorway to be a gene flow barrier for red deer (Cervus elaphus), but not wild boars (Sus scrofa)

While motorways are often assumed to influence the movement behaviour of large mammals, there are surprisingly few studies that show an influence of these linear structures on the genetic make-up of wild ungulate populations. Here, we analyse the spatial genetic structure of red deer (Cervus elaphus) and wild boars (Sus scrofa) along a stretch of motorway in the Walloon part of Belgium. Altogether, 876 red deer were genotyped at 13 microsatellite loci, and 325 wild boars at 14 loci. In the case of the red deer, different genetic clustering tools identified two genetic subpopulations whose borders matched the motorway well. Conversely, no genetic structure was identified in the case of the wild boar. Analysis of isolation-by-distance patterns of pairs of individuals on the same side and on different sides of the motorway also suggested that the road was a barrier to red deer, but not to wild boar movement. While telemetry studies seem to confirm that red deer are more affected by motorways than wild boar, the red deer sample size was also much larger than that of the wild boars. We therefore repeated the analysis of genetic structure in the red deer with randomly sub-sampled data sets of decreasing size. The power to detect the genetic structure using clustering methods decreased with decreasing sample size.

Genetic structure and assignment tests demonstrate illegal translocation of red deer (Cervus elaphus) into a continuous population

Molecular forensic methods are being increasingly used to help enforce wildlife conservation laws. Using multilocus genotyping, illegal translocation of an animal can be demonstrated by excluding all potential source populations as an individual's population of origin. Here, we illustrate how this approach can be applied to a large continuous population by defining the population genetic structure and excluding suspect animals from each identified cluster. We aimed to test the hypothesis that recreational hunters had illegally introduced a group of red deer into a hunting area in Luxembourg. Reference samples were collected over a large area in order to test the possibility that the suspect individuals might be recent immigrants. Due to isolation-by-distance relationships in the data set, inferring the number of genetic clusters using Bayesian methods was not straightforward. Biologically meaningful clusters were only obtained by simultaneously analysing spatial and genetic information using the program baps 4.1. We inferred the presence of three genetic clusters in the study region. Using partial Mantel tests, we detected barriers to gene flow other than distance, probably created by a combination of urban areas, motorways and a river valley used for viticulture. The four focal animals could be excluded with a high certainty from the three genetic subpopulations and it was therefore likely that they had been released illegally.

Characterization of a new vacuolar membrane aquaporin sensitive to mercury at a unique site

The membranes of plant and animal cells contain aquaporins, proteins that facilitate the transport of water. In plants, aquaporins are found in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1, a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala motif. Here, we report the molecular analysis of a new Arabidopsis aquaporin, delta-TIP (for tonoplast intrinsic protein), and show that it is located in the tonoplast. The water channel activity of delta-TIP is sensitive to mercury. However, the mercury-sensitive cysteine residue found in mammalian aquaporins is not present in delta-TIP, or in gamma-TIP, a previously characterized mercury-sensitive tonoplast aquaporin. Site-directed mutagenesis was used to identify the mercury-sensitive site in these two aquaporins as Cys-116 and Cys-118 for delta-TIP and gamma-TIP, respectively. These mutations are at a conserved position in a presumed membrane-spanning domain not previously known to have a role in aquaporin mercury sensitivity. Comparing the tissue expression patterns of delta-TIP with gamma-TIP and alpha-TIP showed that the TIPs are differentially expressed.

Short-term control of maize cell and root water permeability through plasma membrane aquaporin isoforms

Although it is widely accepted that aquaporins are involved in the regulation of root water uptake, the role of specific isoforms in this process is poorly understood. The mRNA expression and protein level of specific plasma membrane intrinsic proteins (PIPs) were analysed in Zea mays in relation to cell and root hydraulic conductivity. Plants were analysed during the day/night period, under different growth conditions (aeroponics/hydroponics) and in response to short-term osmotic stress applied through polyethylene glycol (PEG). Higher protein levels of ZmPIP1;2, ZmPIP2;1/2;2, ZmPIP2;5 and ZmPIP2;6 during the day coincided with a higher water permeability of root cortex cells during the day compared with night period. Similarly, plants which were grown under aeroponic conditions and which developed a hypodermis ('exodermis') with Casparian bands, effectively forcing more water along a membranous uptake path across roots, showed increased levels of ZmPIP2;5 and ZmPIP1;2 in the rhizodermis and exodermis. When PEG was added to the root medium (2-8 h), expression of PIPs and cell water permeability in roots increased. These data support a role of specific PIP isoforms, in particular ZmPIP1;2 and ZmPIP2;5, in regulating root water uptake and cortex cell hydraulic conductivity in maize.

A novel plant major intrinsic protein in Physcomitrella patens most similar to bacterial glycerol channels

A gene encoding a novel fifth type of major intrinsic protein (MIP) in plants has been identified in the moss Physcomitrella patens. Phylogenetic analyses show that this protein, GlpF-like intrinsic protein (GIP1;1), is closely related to a subclass of glycerol transporters in bacteria that in addition to glycerol are highly permeable to water. A likely explanation of the occurrence of this bacterial-like MIP in P. patens is horizontal gene transfer. The expressed P. patens GIP1;1 gene contains five introns and encodes a unique C-loop extension of approximately 110 amino acid residues that has no obvious similarity with any other known protein. Based on alignments and structural comparisons with other MIPs, GIP1;1 is suggested to have retained the permeability for glycerol but not for water. Studies on heterologously expressed GIP1;1 in Xenopus laevis oocytes confirm the predicted substrate specificity. Interestingly, proteins of one of the plant-specific subgroups of MIPs, the NOD26-like intrinsic proteins, are also facilitating the transport of glycerol and have previously been suggested to have evolved from a horizontally transferred bacterial gene. Further studies on localization and searches for GIP1;1 homologs in other plants will clarify the function and significance of this new plant MIP.