Nodulin 26-like Intrinsic Proteins Research Articles

Arabidopsis NIP3;1 Plays an Important Role in Arsenic Uptake and Root-to-Shoot Translocation under Arsenite Stress Conditions

In Arabidopsis, the nodulin 26-like intrinsic protein (NIP) subfamily of aquaporin proteins consists of nine members, five of which (NIP1;1, NIP1;2, NIP5;1, NIP6;1, and NIP7;1) were previously identified to be permeable to arsenite. However, the roles of NIPs in the root-to-shoot translocation of arsenite in plants remain poorly understood. In this study, using reverse genetic strategies, Arabidopsis NIP3;1 was identified to play an important role in both the arsenic uptake and root-to-shoot distribution under arsenite stress conditions. The nip3;1 loss-of-function mutants displayed obvious improvements in arsenite tolerance for aboveground growth and accumulated less arsenic in shoots than those of the wild-type plants, whereas the nip3;1 nip1;1 double mutant showed strong arsenite tolerance and improved growth of both roots and shoots under arsenite stress conditions. A promoter-β-glucuronidase analysis revealed that NIP3;1 was expressed almost exclusively in roots (with the exception of the root tips), and heterologous expression in the yeast Saccharomyces cerevisiae demonstrated that NIP3;1 was able to mediate arsenite transport. Taken together, our results suggest that NIP3;1 is involved in arsenite uptake and root-to-shoot translocation in Arabidopsis, probably as a passive and bidirectional arsenite transporter.

Arsenic stress in rice: Redox consequences and regulation by iron

Arsenic (As) contamination is a serious hazard to human health and agriculture. It has emerged as an important threat for rice cultivation mainly in South Asian countries. In this study, we investigated the effect of iron (Fe) supplementation on arsenic (AsV) induced oxidative stress responses in rice (Oryza sativa L.). Rice seedlings treated with AsV for 24 and 48 h in presence or absence of 2.5 mM Fe after which the root and shoot tissues were harvested for analysis. The results indicate significant (p ≤ 0.05) reduction in root and shoot length/dry biomass. Supplementation of Fe showed improved growth responses under stress as compared to AsV alone. The scanning electron microscopy (SEM) analysis of roots under AsV treatment for 48 h showed major alterations in root structure and integrity, although no noticeable changes were observed in Fe – supplemented seedlings. Significantly high (p ≤ 0.05) accumulation of AsV was observed in root and shoot after 24 and 48 h of stress. However, under Fe – supplementation As accumulation in root and shoot were considerably low after 24 and 48 h of AsV treatment. The hydrogen peroxide (H2O2) and malondialdehyde (MDA) content in both root and shoot increased significantly (p ≤ 0.05) after 24 and 48 h of AsV treatment. In Fe – supplemented seedlings, the levels of H2O2 and MDA were considerably low as compared to AsV alone. Ascorbate (AsA) and glutathione (GSH) levels also increased significantly (p ≤ 0.05) under AsV stress as compared to control and Fe-supplemented seedlings. Activities of catalase (CAT) and superoxide dismutase (SOD) were significantly (p ≤ 0.05) high after 24 and 48 h of AsV treatment as compared to Fe-supplemented seedlings. The gene expression analysis revealed up-regulation of metallothionein (MT1, MT2) and nodulin 26-like intrinsic protein (NIP2;1) genes after 5d of As treatment, while their expressions were repressed under Fe-supplementation. Our results indicate that Fe regulates oxidative stress and promotes growth under As stress.

Identification and Characterization of an Arabidopsis Mutant with Altered Localization of NIP5;1, a Plasma Membrane Boric Acid Channel, Reveals the Requirement for d-Galactose in Endomembrane Organization

Endomembrane organization is important for various aspects of cell physiology, including membrane protein trafficking. To explore the molecular mechanisms regulating the trafficking of plasma membrane-localized proteins in plants, we screened for Arabidopsis mutants with defective localization of green fluorescent protein (GFP)-nodulin 26-like intrinsic protein (NIP)5;1. Fluorescence imaging-based screening led to the isolation of a mutant which accumulated abnormal intracellular aggregates labeled by GFP-NIP5;1. The aggregates appeared in epidermal cells in the root elongation zone and included the trans-Golgi network/early endosomes. Rough mapping and whole-genome sequencing identified the mutant as an allele of UDP-glucose 4-epimerase 4 (uge4)/root hair defective 1 (rhd1) /root epidermal bulgar 1 (reb 1), which was originally defined as a cell wall mutant. The responsible gene encodes UDP-glucose 4-epimerase 4 (UGE4), which functions in the biosynthesis of d-galactose, especially for the synthesis of the cell wall polysaccharide xyloglucan and arabinogalactan proteins (AGPs). The endomembrane aggregates in the mutants were absent in the presence of d-galactose, indicative of a requirement for a d-galactose-containing component in endomembrane organization. Genetic and pharmacological analyses suggested that the aggregates were not caused by the disruption of cell wall polysaccharides or the cytoskeleton. Overall, our results suggest that UGE4 activity in d-galactose synthesis is required for the structure of cell wall polysaccharides and endomembrane organization.

Functional and molecular characteristics of rice and barley NIP aquaporins transporting water, hydrogen peroxide and arsenite

Mercury-sensitive water transport activities were detected in seven NIP (Nodulin 26-like intrinsic protein) type aquaporins among eleven NIPs examined. Amino acid substitutions in rice OsNIP3;3 revealed that mercury-sensitivity depended on a histidine (but not on a cysteine) in apoplastic loop C in plant NIP aquaporins, although the cysteine is involved in the mercury-sensitivity of animal aquaporins. Rice OsNIP3;3 was also first identified as a unique aquaporin facilitating all water, hydrogen peroxide and arsenite transports. In rice OsNIP3;2, hydrogen peroxide and arsenite transport activities were detected, but water transport was not. Barley HvNIP1;2- or rice OsNIP2;1-expressing yeast cells showed the arsenite transport activity but not the H2O2 transport activity. The present work revealed novel molecular mechanisms of water and other low molecular weight compounds transport/selection in barley and rice NIP aquaporins, including the histidine-related mercury-sensitivity in the water transport of aquaporins.

Identification and functional characterization of silicon transporters in soybean using comparative genomics of major intrinsic proteins in Arabidopsis and rice

Silicon (Si) confers several benefits to many plant species when absorbed as silicic acid through nodulin 26-like intrinsic proteins (NIPs). The NIPs belong to major intrinsic protein (MIP) family, members of which form channels with high selectivity to control transport of water and different solutes. Here, comparative genomic analysis of the MIPs was performed to investigate the presence of Si transporter MIPs in soybean. Thorough analysis of phylogeny, gene organization, transcriptome profiling and protein modeling was performed to characterize MIPs in rice, Arabidopsis and soybean. Based on several attributes, two putative Si transporter genes, GmNIP2-1 and GmNIP2-2, were identified, characterized and cloned from soybean. Expression of both genes was detected in shoot and root tissues, and decreased as Si increased. The protein encoded by GmNIP2-2 showed functionality for Si transport when expressed in Xenopus oocytes, thus confirming the genetic capability of soybean to absorb the element. Comparative analysis of MIPs in plants provides opportunities to decipher gene evolution, functionality and selectivity of nutrient uptake mechanisms. Exploitation of this strategy has helped to uncover unique features of MIPs in soybean. The identification and functional characterization of Si transporters can be exploited to optimize the benefits that plants can derive from Si absorption.

Isolation and expression analysis of CiNIP5, a citrus boron transport gene involved in tolerance to boron deficiency

It was reported that AtNIP5;1 is responsible for boron (B) uptake into the root under B-deficient conditions. In this study, we cloned an NIP5-like gene (named as CiNIP5) from Trifoliate orange. The full length of CiNIP5 was 1470bp in which 903bp formed the open reading frame, coding a protein of 300 amino acids. Sequence analysis showed that CiNIP5 had a highly conserved Asn-Pro-Ala (NPA) region and aromatic/arginine (ar/R) motif and it belongs to Nodulin 26-like intrinsic protein (NIP) group II. Analysis of gene expression indicated that CiNIP5 expressed mainly in the roots of citrus seedlings, and it increased obviously under B-deficiency but was suppressed under B toxicity. Additionally, the responses of CiNIP5 expression to B deficiency were compared between the roots of Carrizo citrange and Fragrant citrus, in which the former showed more tolerance than the latter to low B condition in field practice. CiNIP5 transcript level in the root of Carrizo citrange was induced and increased continuously to 7.7 times at 48h after B deficient treatment (HABdT) as compared with that at 0 HABdT while in the root of Fragrant citrus it was induced to 4.4 times at 24 HABdT, and then decreased to half at 48 HABdT. The possible role of CiNIP5 was discussed in the mechanism of B-uptake efficiency of different rootstocks under B-deficiency.

Isolation and characterization of three maize aquaporin genes, ZmNIP2;1, ZmNIP2;4 and ZmTIP4;4 involved in urea transport

Urea-based nitrogen fertilizer was widely utilized in maize production, but transporters involved in urea uptake, translocation and cellular homeostasis have not been identified. Here, we isolated three maize aquapoin genes, ZmNIP2;1, ZmNIP2;4 and ZmTIP4;4, from a cDNA library by heterogeneous complementation of a urea uptake-defective yeast. ZmNIP2;1 and ZmNIP2;4 belonged to the nodulin 26-like intrinsic proteins (NIPs) localized at plasma membrane, and ZmTIP4;4 belonged to the tonoplast intrinsic protein (TIPs) at vacuolar membrane. Quantitative RT-PCR revealed that ZmNIP2;1 was expressed constitutively in various organs while ZmNIP2;4 and ZmTIP4;4 transcripts were abundant in reproductive organs and roots. Expression of ZmTIP4;4 was significantly increased in roots and expanded leaves under nitrogen starvation, while those of ZmNIP2;1 and ZmNIP2;4 remained unaffected. Functions of maize aquapoin genes in urea transport together with their distinct expression manners suggested that they might play diverse roles on urea uptake and translocation, or equilibrating urea concentration across tonoplast.

Members of rice plasma membrane intrinsic proteins subfamily are involved in arsenite permeability and tolerance in plants

Rice accumulates high level of arsenic (As) in its edible parts and thus plays an important role in the transfer of As into the food chain. However, the mechanisms of As uptake and its detoxification in rice are not well understood. Recently, members of the Nodulin 26-like intrinsic protein (NIP) subfamily of plant aquaporins were shown to transport arsenite in rice and Arabidopsis. Here we report that members of the rice plasma membrane intrinsic protein (PIP) subfamily are also involved in As tolerance and transport. Based on the homology search with the mammalian AQP9 and yeast Fps1 arsenite transporters, we identified and cloned five rice PIP gene subfamily members. qRT-PCR analysis of PIPs in rice root and shoot tissues revealed a significant down regulation of transcripts encoding OsPIP1;2, OsPIP1;3, OsPIP2;4, OsPIP2;6, and OsPIP2;7 in response to arsenite treatment. Heterologous expression of OsPIP2;4, OsPIP2;6, and OsPIP2;7 in Xenopus laevis oocytes significantly increased the uptake of arsenite. Overexpression of OsPIP2;4, OsPIP2;6, and OsPIP2;7 in Arabidopsis yielded enhanced arsenite tolerance and higher biomass accumulation. Further, these transgenic plants showed no significant accumulation of As in shoot and root tissues in long term uptake assays. Whereas, short duration exposure to arsenite caused both active influx and efflux of As in the roots. The data suggests a bidirectional arsenite permeability of rice PIPs in plants. These rice PIPs genes will be highly useful for engineering important food and biofuel crops for enhanced crop productivity on contaminated soils without increasing the accumulation of toxic As in the biomass or edible tissues.

The aromatic/arginine selectivity filter of NIP aquaporins plays a critical role in substrate selectivity for silicon, boron, and arsenic

Nodulin-26-like intrinsic proteins (NIPs) of the aquaporin family are involved in the transport of diverse solutes, but the mechanisms controlling the selectivity of transport substrates are poorly understood. The purpose of this study was to investigate how the aromatic/arginine (ar/R) selectivity filter influences the substrate selectivity of two NIP aquaporins; the silicic acid (Si) transporter OsLsi1 (OsNIP2;1) from rice and the boric acid (B) transporter AtNIP5;1 from Arabidopsis; both proteins are also permeable to arsenite. Native and site-directed mutagenized variants of the two genes were expressed in Xenopus oocytes and the transport activities for Si, B, arsenite, and water were assayed. Substitution of the amino acid at the ar/R second helix (H2) position of OsLsi1 did not affect the transport activities for Si, B, and arsenite, but that at the H5 position resulted in a total loss of Si and B transport activities and a partial loss of arsenite transport activity. Conversely, changes of the AtNIP5;1 ar/R selectivity filter and the NPA motifs to the OsLsi1 type did not result in a gain of Si transport activity. B transport activity was partially lost in the H5 mutant but unaffected in the H2 mutant of AtNIP5;1. In contrast, both the single and double mutations at the H2 and/or H5 positions of AtNIP5;1 did not affect arsenite transport activity. The results reveal that the residue at the H5 position of the ar/R filter of both OsLsi1 and AtNIP5;1 plays a key role in the permeability to Si and B, but there is a relatively low selectivity for arsenite.

Boron toxicity tolerance in barley through reduced expression of the multifunctional aquaporin HvNIP2;1.

Boron (B) toxicity is a significant limitation to cereal crop production in a number of regions worldwide. Here we describe the cloning of a gene from barley (Hordeum vulgare), underlying the chromosome 6H B toxicity tolerance quantitative trait locus. It is the second B toxicity tolerance gene identified in barley. Previously, we identified the gene Bot1 that functions as an efflux transporter in B toxicity-tolerant barley to move B out of the plant. The gene identified in this work encodes HvNIP2;1, an aquaporin from the nodulin-26-like intrinsic protein (NIP) subfamily that was recently described as a silicon influx transporter in barley and rice (Oryza sativa). Here we show that a rice mutant for this gene also shows reduced B accumulation in leaf blades compared to wild type and that the mutant protein alters growth of yeast (Saccharomyces cerevisiae) under high B. HvNIP2;1 facilitates significant transport of B when expressed in Xenopus oocytes compared to controls and to another NIP (NOD26), and also in yeast plasma membranes that appear to have relatively high B permeability. We propose that tolerance to high soil B is mediated by reduced expression of HvNIP2;1 to limit B uptake, as well as by increased expression of Bot1 to remove B from roots and sensitive tissues. Together with Bot1, the multifunctional aquaporin HvNIP2;1 is an important determinant of B toxicity tolerance in barley.

Overexpressing a Putative Aquaporin Gene from Wheat, TaNIP, Enhances Salt Tolerance in Transgenic Arabidopsis

High soil salinity is a major abiotic stress in plant agriculture worldwide. Here, we report the characterization of a novel aquaporin gene TaNIP (Triticum asetivum L. nodulin 26-like intrinsic protein), which was involved in salt tolerance pathways in plants. TaNIP was identified and cloned through the gene chip expression analysis of a salt-tolerant wheat mutant RH8706-49 under salt stress. Quantitative reverse transcription-PCR (Q-RT-PCR) was used to detect TaNIP expression under salt, drought, cold and ABA treatment. The overexpression of TaNIP in transgenic Arabidopsis produced higher salt tolerance than wild-type plants. Localization analysis showed that TaNIP proteins tagged with green fluorescent protein (GFP) were localized to the cell plasma membrane. Under salt stress treatment, TaNIP-overexpressing Arabidopsis accumulated higher K(+), Ca(2+) and proline contents and lower Na(+) level than the wild-type plants. The overexpression of TaNIP in transgenic Arabidopsis also up-regulated the expression of a number of stress-associated genes. Our results suggest that TaNIP plays an important role in salt tolerance in Arabidopsis and can also enhance plants' tolerance to other abiotic stresses.

Boron transport in plants: co-ordinated regulation of transporters

The essentiality of boron (B) for plant growth was established > 85 years ago. In the last decade, it has been revealed that one of the physiological roles of B is cross-linking the pectic polysaccharide rhamnogalacturonan II in primary cell walls. Borate cross-linking of pectic networks serves both for physical strength of cell walls and for cell adhesion. On the other hand, high concentrations of B are toxic to plant growth. To avoid deficiency and toxicity problems, it is important for plants to maintain their tissue B concentrations within an optimum range by regulating transport processes. Boron transport was long believed to be a passive, unregulated process, but the identification of B transporters has suggested that plants sense and respond to the B conditions and regulate transporters to maintain B homeostasis. Transporters responsible for efficient B uptake by roots, xylem loading and B distribution among leaves have been described. These transporters are required under B limitation for efficient acquisition and utilization of B. Transporters important for tolerating high B levels in the environment have also been identified, and these transporters export B from roots back to the soil. Two types of transporters are involved in these processes: NIPs (nodulin-26-like intrinsic proteins), boric acid channels, and BORs, B exporters. It is demonstrated that the expression of genes encoding these transporters is finely regulated in response to B availability in the environment to ensure tissue B homeostasis. Furthermore, plants tolerant to stress produced by low B or high B in the environment can be generated through altered expression of these transporters. The identification of the first B transporter led to the discovery that B transport was a process mediated not only by passive diffusion but also by transporters whose activity was regulated in response to B conditions. Now it is evident that plants sense internal and external B conditions and regulate B transport by modulating the expression and/or accumulation of these transporters. Results obtained in model plants are applicable to other plant species, and such knowledge may be useful in designing plants or crops tolerant to soils containing low or high B.

Arabidopsis NIP1;1 Transports Antimonite and Determines Antimonite Sensitivity

Antimony (Sb) is toxic to organisms including plants. Although it is not essential to organisms, plants take up Sb from the environment. In this study, we identified an antimonite [Sb(III)] transporter from Arabidopsis thaliana. We examined the Sb(III) tolerance of the disruption mutant plants of arsenite [As(III)] transporters, nodulin 26-like intrinsic proteins (NIPs), since Sb(III) is similar to As(III) in structure. One of the mutants, nip1;1, showed Sb(III) tolerance and accumulated less Sb. Furthermore, yeast expressing NIP1;1 accumulated twice as much Sb as control. These data indicate that NIP1;1 transports Sb(III) and determines the Sb(III) sensitivity of A. thaliana.

Mechanisms to cope with arsenic or cadmium excess in plants

The metalloid arsenic and the heavy metal cadmium have no demonstrated biological function in plants. Both elements are highly toxic and of major concern with respect to their accumulation in soils, in the food-chain or in drinking water. Arsenate is taken up by phosphate transporters and rapidly reduced to arsenite, As(III). In reducing environments, As(III) is taken up by aquaporin nodulin 26-like intrinsic proteins. Cd(2+) enters the root via essential metal uptake systems. As(III) and Cd(2+) share some similarity between their toxicology and sequestration machineries. Recent progress in understanding the mechanisms of As and Cd uptake and detoxification is presented, including the elucidation of why rice takes up so much arsenic from soil and of mechanisms of As and Cd hypertolerance.

Arsenite transport in plants

Arsenic is a metalloid which is toxic to living organisms. Natural occurrence of arsenic and human activities have led to widespread contamination in many areas of the world, exposing a large section of the human population to potential arsenic poisoning. Arsenic intake can occur through consumption of contaminated crops and it is therefore important to understand the mechanisms of transport, metabolism and tolerance that plants display in response to arsenic. Plants are mainly exposed to the inorganic forms of arsenic, arsenate and arsenite. Recently, significant progress has been made in the identification and characterisation of proteins responsible for movement of arsenite into and within plants. Aquaporins of the NIP (nodulin26-like intrinsic protein) subfamily were shown to transport arsenite in planta and in heterologous systems. In this review, we will evaluate the implications of these new findings and assess how this may help in developing safer and more tolerant crops.

NIP1;1, an Aquaporin Homolog, Determines the Arsenite Sensitivity of Arabidopsis thaliana

Arsenite [As(III)] is highly toxic to organisms, including plants. Very recently, transporters in rice responsible for As(III) transport have been described (Ma, J. F., Yamaji, N., Mitani, N., Xu, X. Y., Su, Y. H., McGrath, S. P., and Zhao, F. J. (2008) Proc. Natl. Acad. Sci. U. S. A. 105, 9931-9935), but little is known about As(III) tolerance. In this study, three independent As(III)-tolerant mutants were isolated from ethyl methanesulfonate-mutagenized M2 seeds of Arabidopsis thaliana. All three mutants carried independent mutations in Nodulin 26-like intrinsic protein 1;1 (NIP1;1), a homolog of an aquaporin. Two independent transgenic lines carrying T-DNA in NIP1;1 were highly tolerant to As(III), establishing that NIP1;1 is the causal gene of As(III) tolerance. Because an aquaglyceroporin is able to transport As(III), we measured As(III) transport activity. When expressed in Xenopus oocytes, NIP1;1 was capable of transporting As(III). As content in the mutant plants was 30% lower than in wild-type plants. Promoter beta-glucuronidase and real-time PCR analysis showed that NIP1;1 is highly expressed in roots, and GFP-NIP1;1 is localized to the plasma membrane. These data show that NIP1;1 is involved in As(III) uptake into roots and that disruption of NIP1;1 function confers As(III) tolerance to plants. NIP1;2 and NIP5;1, closely related homologs of NIP1;1, were also permeable to As(III). Although the disruption of these genes reduced the As content in plants, As(III) tolerance was not observed in nip1;2 and nip5;1 mutants. This indicates that As(III) tolerance cannot be simply explained by decreased As contents in plants.

Arsenic uptake and metabolism in plants

Arsenic (As) is an element that is nonessential for and toxic to plants. Arsenic contamination in the environment occurs in many regions, and, depending on environmental factors, its accumulation in food crops may pose a health risk to humans.Recent progress in understanding the mechanisms of As uptake and metabolism in plants is reviewed here. Arsenate is taken up by phosphate transporters. A number of the aquaporin nodulin26-like intrinsic proteins (NIPs) are able to transport arsenite,the predominant form of As in reducing environments. In rice (Oryza sativa), arsenite uptake shares the highly efficient silicon (Si) pathway of entry to root cells and efflux towards the xylem. In root cells arsenate is rapidly reduced to arsenite, which is effluxed to the external medium, complexed by thiol peptides or translocated to shoots. One type of arsenate reductase has been identified, but its in planta functions remain to be investigated. Some fern species in the Pteridaceae family are able to hyperaccumulate As in above-ground tissues. Hyperaccumulation appears to involve enhanced arsenate uptake, decreased arsenite-thiol complexation and arsenite efflux to the external medium, greatly enhanced xylem translocation of arsenite, and vacuolar sequestration of arsenite in fronds. Current knowledge gaps and future research directions are also identified.

A subgroup of plant aquaporins facilitate the bi-directional diffusion of As(OH)3 and Sb(OH)3 across membranes.

BackgroundArsenic is a toxic and highly abundant metalloid that endangers human health through drinking water and the food chain. The most common forms of arsenic in the environment are arsenate (As(V)) and arsenite (As(III)). As(V) is a non-functional phosphate analog that enters the food chain via plant phosphate transporters. Inside cells, As(V) becomes reduced to As(III) for subsequent extrusion or compartmentation. Although much is known about As(III) transport and handling in microbes and mammals, the transport systems for As(III) have not yet been characterized in plants.ResultsHere we show that the Nodulin26-like Intrinsic Proteins (NIPs) AtNIP5;1 and AtNIP6;1 from Arabidopsis thaliana, OsNIP2;1 and OsNIP3;2 from Oryza sativa, and LjNIP5;1 and LjNIP6;1 from Lotus japonicus are bi-directional As(III) channels. Expression of these NIPs sensitized yeast cells to As(III) and antimonite (Sb(III)), and direct transport assays confirmed their ability to facilitate As(III) transport across cell membranes. On medium containing As(V), expression of the same NIPs improved yeast growth, probably due to increased As(III) efflux. Our data furthermore provide evidence that NIPs can discriminate between highly similar substrates and that they may have differential preferences in the direction of transport. A subgroup of As(III) permeable channels that group together in a phylogenetic tree required N-terminal truncation for functional expression in yeast.ConclusionThis is the first molecular identification of plant As(III) transport systems and we propose that metalloid transport through NIPs is a conserved and ancient feature. Our observations are potentially of great importance for improved remediation and tolerance of plants, and may provide a key to the development of low arsenic crops for food production.

Characterization of substrate specificity of a rice silicon transporter, Lsi1

Lsi1 (OsNIP2;1) is the first silicon (silicic acid) transporter identified in plant, which belongs to the nodulin 26-like intrinsic membrane protein (NIP) subfamily. In this study, we characterized the function of this transporter by using the Xenopus laevis oocyte expression system. The transport activity of Lsi1 for silicic acid was significantly inhibited by HgCl2 but not by low temperature. Lsi1 also showed an efflux transport activity for silicic acid. The substrate specificity study showed that Lsi1 was able to transport urea and boric acid; however, the transport activity for silicic acid was not affected by the presence of equimolar urea and was decreased only slightly by boric acid. Furthermore, among the NIPs subgroup, OsNIP2;2 showed transport activity for silicic acid, whereas OsNIP1;1 and OsNIP3;1 did not. We propose that Lsi1 and its close homologues form a unique subgroup of NIP with a distinct ar/R selectivity filter, which is located in the narrowest region on the extra-membrane mouth and govern the substrate specificity of the pore.

Expanding roles of plant aquaporins in plasma membranes and cell organelles.

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