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

TIP aquaporins in Cyperus esculentus: genome-wide identification, expression profiles, subcellular localizations, and interaction patterns

BACKGROUND

Tonoplast intrinsic proteins (TIPs), which typically mediate water transport across vacuolar membranes, play an essential role in plant growth, development, and stress responses. However, their characterization in tigernut (Cyperus esculentus L.), an oil-bearing tuber plant of the Cyperaceae family, is still in the infancy.

RESULTS

In this study, a first genome-wide characterization of the TIP subfamily was conducted in tigernut, resulting in ten members representing five previously defined phylogenetic groups, i.e., TIP1-5. Although the gene amounts are equal to that present in two model plants Arabidopsis and rice, the group composition and/or evolution pattern were shown to be different. Except for CeTIP1;3 that has no counterpart in both Arabidopsis and rice, complex orthologous relationships of 1:1, 1:2, 1:3, 2:1, and 2:2 were observed. Expansion of the CeTIP subfamily was contributed by whole-genome duplication (WGD), transposed, and dispersed duplications. In contrast to the recent WGD-derivation of CeTIP3;1/-3;2, synteny analyses indicated that TIP4 and - 5 are old WGD repeats of TIP2, appearing sometime before monocot-eudicot divergence. Expression analysis revealed that CeTIP genes exhibit diverse expression profiles and are subjected to developmental and diurnal fluctuation regulation. Moreover, when transiently overexpressed in tobacco leaves, CeTIP1;1 was shown to locate in the vacuolar membrane and function in homo/heteromultimer, whereas CeTIP2;1 is located in the cell membrane and only function in heteromultimer. Interestingly, CeTIP1;1 could mediate the tonoplast-localization of CeTIP2;1 via protein interaction, implying complex regulatory patterns.

CONCLUSIONS

Our findings provide a global view of CeTIP genes, which provide valuable information for further functional analysis and genetic improvement through manipulating key members in tigernut.

Genome-wide identification of Avicennia marina aquaporins reveals their role in adaptation to intertidal habitats and their relevance to salt secretion and vivipary

Aquaporins (AQPs) regulate the transport of water and other substrates, aiding plants in adapting to stressful environments. However, the knowledge of AQPs in salt-secreting and viviparous Avicennia marina is limited. In this study, 46 AmAQPs were identified in A. marina genome, and their subcellular localisation and function in transporting H2 O2 and boron were assessed through bioinformatics analysis and yeast transformation. Through analysing their expression patterns via RNAseq and real-time quantitative polymerase chain reaction, we found that most AmAQPs were downregulated in response to salt and tidal flooding. AmPIP (1;1, 1;7, 2;8, 2;9) and AmTIP (1;5, 1;6) as salt-tolerant candidate genes may contribute to salt secretion together with Na+ /H+ antiporters. AmPIP2;1 and AmTIP1;5 were upregulated during tidal flooding and may be regulated by anaerobic-responsive element and ethylene-responsive element cis-elements, aiding in adaptation to tidal inundation. Additionally, we found that the loss of the seed desiccation and dormancy-related TIP3 gene, and the loss of the seed dormancy regulator DOG1 gene, or DOG1 protein lack heme-binding capacity, may be genetic factors contributing to vivipary. Our findings shed light on the role of AQPs in A. marina adaptation to intertidal environments and their relevance to salt secretion and vivipary.

Regulation of drought tolerance in Arabidopsis involves the PLATZ4-mediated transcriptional repression of plasma membrane aquaporin PIP2;8

Plant A/T-rich protein and zinc-binding protein (PLATZ) transcription factors play important roles in plant growth, development and abiotic stress responses. However, how PLATZ influences plant drought tolerance remains poorly understood. The present study showed that PLATZ4 increased drought tolerance in Arabidopsis thaliana by causing stomatal closure. Transcriptional profiling analysis revealed that PLATZ4 affected the expression of a set of genes involved in water and ion transport, antioxidant metabolism, small peptides and abscisic acid (ABA) signaling. Among these genes, the direct binding of PLATZ4 to the A/T-rich sequences in the plasma membrane intrinsic protein 2;8 (PIP2;8) promoter was identified. PIP2;8 consistently reduced drought tolerance in Arabidopsis through inhibiting stomatal closure. PIP2;8 was localized in the plasma membrane, exhibited water channel activity in Xenopus laevis oocytes and acted epistatically to PLATZ4 in regulating the drought stress response in Arabidopsis. PLATZ4 increased ABA sensitivity through upregulating the expression of ABSCISIC ACID INSENSITIVE 3 (ABI3), ABI4 and ABI5. The transcripts of PLATZ4 were induced to high levels in vegetative seedlings under drought and ABA treatments within 6 and 3 h, respectively. Collectively, these findings reveal that PLATZ4 positively influences plant drought tolerance through regulating the expression of PIP2;8 and genes involved in ABA signaling.

Genome-Wide Identification and Expression Analyses of the Aquaporin Gene Family in Passion Fruit (Passiflora edulis), Revealing PeTIP3-2 to Be Involved in Drought Stress

Aquaporins (AQPs) in plants can transport water and small molecules, and they play an important role in plant development and abiotic stress response. However, to date, a comprehensive study on AQP family members is lacking. In this study, 27 AQP genes were identified from the passion fruit genome and classified into four groups (NIP, PIP, TIP, SIP) on the basis of their phylogenetic relationships. The prediction of protein interactions indicated that the AQPs of passion fruit were mainly associated with AQP family members and boron protein family genes. Promoter cis-acting elements showed that most PeAQPs contain light response elements, hormone response elements, and abiotic stress response elements. According to collinear analysis, passion fruit is more closely related to Arabidopsis than rice. Furthermore, three different fruit ripening stages and different tissues were analyzed on the basis of the transcriptome sequencing results for passion fruit AQPs under drought, high-salt, cold and high-temperature stress, and the results were confirmed by qRT-PCR. The results showed that the PeAQPs were able to respond to different abiotic stresses, and some members could be induced by and expressed in response to multiple abiotic stresses at the same time. Among the three different fruit ripening stages, 15 AQPs had the highest expression levels in the first stage. AQPs are expressed in all tissues of the passion fruit. One of the passion fruit aquaporin genes, PeTIP3-2, which was induced by drought stress, was selected and transformed into Arabidopsis. The survival rate of transgenic plants under drought stress treatment is higher than that of wild-type plants. The results indicated that PeTIP3-2 was able to improve the drought resistance of plants. Our discovery lays the foundation for the functional study of AQPs in passion fruit.

Transcriptome analysis of Pará rubber tree (H. brasiliensis) seedlings under ethylene stimulation

BACKGROUND

Natural rubber (cis-1,4-polyioprene, NR) is an indispensable industrial raw material obtained from the Pará rubber tree (H. brasiliensis). Natural rubber cannot be replaced by synthetic rubber compounds because of the superior resilience, elasticity, abrasion resistance, efficient heat dispersion, and impact resistance of NR. In NR production, latex is harvested by periodical tapping of the trunk bark. Ethylene enhances and prolongs latex flow and latex regeneration. Ethephon, which is an ethylene-releasing compound, applied to the trunk before tapping usually results in a 1.5- to 2-fold increase in latex yield. However, intense mechanical damage to bark tissues by excessive tapping and/or over-stimulation with ethephon induces severe oxidative stress in laticifer cells, which often causes tapping panel dryness (TPD) syndrome. To enhance NR production without causing TPD, an improved understanding of the molecular mechanism of the ethylene response in the Pará rubber tree is required. Therefore, we investigated gene expression in response to ethephon treatment using Pará rubber tree seedlings as a model system.

RESULTS

After ethephon treatment, 3270 genes showed significant differences in expression compared with the mock treatment. Genes associated with carotenoids, flavonoids, and abscisic acid biosynthesis were significantly upregulated by ethephon treatment, which might contribute to an increase in latex flow. Genes associated with secondary cell wall formation were downregulated, which might be because of the reduced sugar supply. Given that sucrose is an important molecule for NR production, a trade-off may arise between NR production and cell wall formation for plant growth and for wound healing at the tapping panel.

CONCLUSIONS

Dynamic changes in gene expression occur specifically in response to ethephon treatment. Certain genes identified may potentially contribute to latex production or TPD suppression. These data provide valuable information to understand the mechanism of ethylene stimulation, and will contribute to improved management practices and/or molecular breeding to attain higher yields of latex from Pará rubber trees.

Genome-Wide Identification, Structure Characterization, and Expression Pattern Profiling of the Aquaporin Gene Family in Betula pendula

Aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life. They play key roles in the flux of water and many solutes across the membranes. The AQP diversity, protein features, and biological functions of silver birch are still unknown. A genome analysis of Betula pendula identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs). The BpeAQP gene structure is conserved within each subfamily, with exon numbers ranging from one to five. The predictions of the aromatic/arginine selectivity filter (ar/R), Froger's positions, specificity-determining positions, and 2D and 3D biochemical properties indicate noticeable transport specificities to various non-aqueous substrates between members and/or subfamilies. Nevertheless, overall, the BpePIPs display mostly hydrophilic ar/R selective filter and lining-pore residues, whereas the BpeTIP, BpeNIP, BpeSIP, and BpeXIP subfamilies mostly contain hydrophobic permeation signatures. Transcriptional expression analyses indicate that 23 BpeAQP genes are transcribed, including five organ-related expressions. Surprisingly, no significant transcriptional expression is monitored in leaves in response to cold stress (6 °C), although interesting trends can be distinguished and will be discussed, notably in relation to the plasticity of this pioneer species, B. pendula. The current study presents the first detailed genome-wide analysis of the AQP gene family in a Betulaceae species, and our results lay a foundation for a better understanding of the specific functions of the BpeAQP genes in the responses of the silver birch trees to cold stress.

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

Grapevine aquaporins: Diversity, cellular functions, and ecophysiological perspectives

High-scored premium wines are typically produced under moderate drought stress, suggesting that the water status of grapevine is crucial for wine quality. Aquaporins greatly influence the plant water status by facilitating water diffusion across the plasma membrane in a tightly regulated manner. They adjust the hydraulic conductance of the plasma membrane rapidly and reversibly, which is essential in specific physiological events, including adaptation to soil water scarcity. The comprehension of the sophisticated plant-water relations at the molecular level are thus important to optimize agricultural practices or to assist plant breeding programs. This review explores the recent progresses in understanding the water transport in grapevine at the cellular level through aquaporins and its regulation. Important aspects, including aquaporin structure, diversity, cellular localization, transport properties, and regulation at the cellular and whole plant level are addressed. An ecophysiological perspective about the roles of grapevine aquaporins in plant response to drought stress is also provided.

Significance of solute specificity, expression, and gating mechanism of tonoplast intrinsic protein during development and stress response in plants

Tonoplast intrinsic proteins (TIPs), belonging to the aquaporin family, are transmembrane channels located mostly at the tonoplast of plant cells. The TIPs are known to transport water and many other small solutes such as ammonia, urea, hydrogen peroxide, and glycerol. In the present review, phylogenetic distribution, structure, transport dynamics, gating mechanism, sub-cellular localization, tissue-specific expression, and co-expression of TIPs are discussed to define their versatile role in plants. Based on the phylogenetic distribution, TIPs are classified into five distinct groups with aromatic-arginine (Ar/R) selectivity filters, typical pore-morphology, and tissue-specific gene expression patterns. The tissue-specific expression of TIPs is conserved among diverse plant species, more particularly for TIP3s, which are expressed exclusively in seeds. Studying TIP3 evolution will help to understand seed development and germination. The solute specificity of TIPs plays an imperative role in physiological processes like stomatal movement and vacuolar sequestration as well as in alleviating environmental stress. TIPs also play an important role in growth and developmental processes like radicle protrusion, anther dehiscence, seed germination, cell elongation, and expansion. The gating mechanism of TIPs regulates the solute flow in response to external signals, which helps to maintain the physiological functions of the cell. The information provided in this review is a base to explore TIP's potential in crop improvement programs.

SlERF52 regulates SlTIP1;1 expression to accelerate tomato pedicel abscission

Abscission of plant organs is induced by developmental signals and diverse environmental stimuli and involves multiple regulatory networks, including biotic or abiotic stress-impaired auxin flux in the abscission zone (AZ). Depletion of auxin activates AZ ethylene (ETH) production and triggers acceleration of abscission, a process that requires hydrogen peroxide (H2O2). However, the interaction between these networks and the underlying mechanisms that control abscission are poorly understood. Here, we found that expression of tonoplast intrinsic proteins, which belong to the aquaporin (AQP) family in the AZ was important for tomato (Solanum lycopersicum) pedicel abscission. Liquid chromatography-tandem mass spectrometry and in situ hybridization revealed that SlTIP1;1 was most abundant and specifically present in the tomato pedicel AZ. SlTIP1;1 localized in the plasma membrane and tonoplast. Knockout of SlTIP1;1 resulted in delayed abscission, whereas overexpression of SlTIP1;1 accelerated abscission. Further analysis indicated that SlTIP1;1 mediated abscission via gating of cytoplasmic H2O2 concentrations and osmotic water permeability (Pf). Elevated cytoplasmic levels of H2O2 caused a suppressed auxin signal in the early abscission stage and enhanced ETH production during abscission. Furthermore, we found that increasing Pf was required to enhance the turgor pressure to supply the break force for AZ cell separation. Moreover, we observed that SlERF52 bound directly to the SlTIP1;1 promoter to regulate its expression, demonstrating a positive loop in which cytoplasmic H2O2 activates ETH production, which activates SlERF52. This, in turn, induces SlTIP1;1, which leads to elevated cytoplasmic H2O2 and water influx.

Response Mechanism of Plants to Drought Stress

With the global climate anomalies and the destruction of ecological balance, the water shortage has become a serious ecological problem facing all mankind, and drought has become a key factor restricting the development of agricultural production. Therefore, it is essential to study the drought tolerance of crops. Based on previous studies, we reviewed the effects of drought stress on plant morphology and physiology, including the changes of external morphology and internal structure of root, stem, and leaf, the effects of drought stress on osmotic regulation substances, drought-induced proteins, and active oxygen metabolism of plants. In this paper, the main drought stress signals and signal transduction pathways in plants are described, and the functional genes and regulatory genes related to drought stress are listed, respectively. We summarize the above aspects to provide valuable background knowledge and theoretical basis for future agriculture, forestry breeding, and cultivation.

Genome-wide analysis of the aquaporin genes in melon (Cucumis melo L.)

Melon (Cucumis melo L.) is a very important crop throughout the world and has great economic importance, in part due to its nutritional properties. It prefers well-drained soil with low acidity and has a strong demand for water during fruit set. Therefore, a correct water balance—involving aquaporins—is necessary to maintain the plants in optimal condition. This manuscript describes the identification and comparative analysis of the complete set of aquaporins in melon. 31 aquaporin genes were identified, classified and analysed according to the evolutionary relationship of melon with related plant species. The individual role of each aquaporin in the transport of water, ions and small molecules was discussed. Finally, qPCR revealed that almost all melon aquaporins in roots and leaves were constitutively expressed. However, the high variations in expression among them point to different roles in water and solute transport, providing important features as that CmPIP1;1 is the predominant isoform and CmTIP1;1 is revealed as the most important osmoregulator in the tonoplast under optimal conditions. The results of this work pointing to the physiological importance of each individual aquaporin of melon opening a field of knowledge that deserves to be investigated.

Aquaporins are main contributors to root hydraulic conductivity in pearl millet [Pennisetum glaucum (L) R. Br.]

Pearl millet is a key cereal for food security in arid and semi-arid regions but its yield is increasingly threatened by water stress. Physiological mechanisms relating to conservation of soil water or increased water use efficiency can alleviate that stress. Aquaporins (AQP) are water channels that mediate root water transport, thereby influencing plant hydraulics, transpiration and soil water conservation. However, AQP remain largely uncharacterized in pearl millet. Here, we studied AQP function in root water transport in two pearl millet lines contrasting for water use efficiency (WUE). We observed that these lines also contrasted for root hydraulic conductivity (Lpr) and AQP contribution to Lpr. The line with lower WUE showed significantly higher AQP contribution to Lpr. To investigate AQP isoforms contributing to Lpr, we developed genomic approaches to first identify the entire AQP family in pearl millet and secondly, characterize the plasma membrane intrinsic proteins (PIP) gene expression profile. We identified and annotated 33 AQP genes in pearl millet, among which ten encoded PIP isoforms. PgPIP1-3 and PgPIP1-4 were significantly more expressed in the line showing lower WUE, higher Lpr and higher AQP contribution to Lpr. Overall, our study suggests that the PIP1 AQP family are the main regulators of Lpr in pearl millet and may possibly be associated with mechanisms associated to whole plant water use. This study paves the way for further investigations on AQP functions in pearl millet hydraulics and adaptation to environmental stresses.

Rice aquaporin PIP1;3 and harpin Hpa1 of bacterial blight pathogen cooperate in a type III effector translocation

Varieties of Gram-negative bacterial pathogens infect their eukaryotic hosts by deploying the type III translocon to deliver effector proteins into the cytosol of eukaryotic cells in which effectors execute their pathological functions. The translocon is hypothetically assembled by bacterial translocators in association with the assumed receptors situated on eukaryotic plasma membranes. This hypothesis is partially verified in the present study with genetic, biochemical, and pathological evidence for the role of a rice aquaporin, plasma membrane intrinsic protein PIP1;3, in the cytosolic import of the transcription activator-like effector PthXo1 from the bacterial blight pathogen. PIP1;3 interacts with the bacterial translocator Hpa1 at rice plasma membranes to control PthXo1 translocation from cells of a well-characterized strain of the bacterial blight pathogen into the cytosol of cells of a susceptible rice variety. An extracellular loop sequence of PIP1;3 and the α-helix motif of Hpa1 determine both the molecular interaction and its consequences with respect to the effector translocation and the bacterial virulence on the susceptible rice variety. Overall, these results provide multiple experimental avenues to support the hypothesis that interactions between bacterial translocators and their interactors at the target membrane are essential for bacterial effector translocation.

Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H2O2 transport channel AtPIP1;4, and the associated consequence for H2O2 signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H2O2 is generated in the apoplast upon induction by a pathogen or microbial pattern. H2O2 from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H2O2 transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H2O2 through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H2O2 transport facilitated by gating and trafficking regulation.

Manipulating Endoplasmic Reticulum-Plasma Membrane Tethering in Plants Through Fluorescent Protein Complementation

The bimolecular fluorescence complementation (BiFC) assay has been widely used to examine interactions between integral and peripheral proteins within putative plasma membrane (PM) microdomains. In the course of using BiFC assays to examine the co-localization of plasma membrane (PM) targeted receptor-like kinases (RLKs), such as FLS2, with PM micro-domain proteins such as remorins, we unexpectedly observed heterogeneous distribution patterns of fluorescence on the PM of Nicotiana benthamiana leaf cortical cells. These patterns appeared to co-localize with the endoplasmic reticulum (ER) and with ER-PM contact sites, and closely resembled patterns caused by over-expression of the ER-PM tether protein Synaptotagmin1 (SYT1). Using domain swap experiments with SYT1, we inferred that non-specific dimerization between FLS2-VenusN and VenusC-StRem1.3 could create artificial ER-PM tether proteins analogous to SYT1. The same patterns of ER-PM tethering were produced when a representative set of integral membrane proteins were partnered in BiFC complexes with PM-targeted peripheral membrane proteins, including PtdIns(4)P-binding proteins. We inferred that spontaneous formation of mature fluorescent proteins caused the BiFC complexes to trap the integral membrane proteins in the ER during delivery to the PM, producing a PM-ER tether. This phenomenon could be a useful tool to deliberately manipulate ER-PM tethering or to test protein membrane localization. However, this study also highlights the risk of using the BiFC assay to study membrane protein interactions in plants, due to the possibility of alterations in cellular structures and membrane organization, or misinterpretation of protein-protein interactions. A number of published studies using this approach may therefore need to be revisited.

Cooperativity in Plant Plasma Membrane Intrinsic Proteins (PIPs): Mechanism of Increased Water Transport in Maize PIP1 Channels in Hetero-tetramers

Plant aquaporins (AQPs) play vital roles in several physiological processes. Plasma membrane intrinsic proteins (PIPs) belong to the subfamily of plant AQPs. They are further subdivided into two closely related subgroups PIP1s and PIP2s. While PIP2 members are efficient water channels, PIP1s from some plant species have been shown to be functionally inactive. Aquaporins form tetramers under physiological conditions. PIP2s can enhance the water transport of PIP1s when they form hetero-tetramers. However, the role of monomer-monomer interface and the significance of specific residues in enhancing the water permeation of PIP1s have not been investigated at atomic level. We have performed all-atom molecular dynamics (MD) simulations of homo-tetramers and four different hetero-tetramers containing ZmPIP1;2 and ZmPIP2;5 from Zea mays. ZmPIP1;2 in a tetramer assembly will have two interfaces, one formed by transmembrane segments TM4 and TM5 and the other formed by TM1 and TM2. We have analyzed channel radius profiles, water transport and potential of mean force profiles of ZmPIP1;2 monomers. Results of MD simulations clearly revealed the influence of TM4-TM5 interface in modulating the water transport of ZmPIP1;2. MD simulations indicate the importance of I93 residue from the TM2 segment of ZmPIP2;5 for the increased water transport in ZmPIP1;2.

Cooperativity in Plant Plasma Membrane Intrinsic Proteins (PIPs): Mechanism of Increased Water Transport in Maize PIP1 Channels in Hetero-tetramers

Plant aquaporins (AQPs) play vital roles in several physiological processes. Plasma membrane intrinsic proteins (PIPs) belong to the subfamily of plant AQPs. They are further subdivided into two closely related subgroups PIP1s and PIP2s. While PIP2 members are efficient water channels, PIP1s from some plant species have been shown to be functionally inactive. Aquaporins form tetramers under physiological conditions. PIP2s can enhance the water transport of PIP1s when they form hetero-tetramers. However, the role of monomer-monomer interface and the significance of specific residues in enhancing the water permeation of PIP1s have not been investigated at atomic level. We have performed all-atom molecular dynamics (MD) simulations of homo-tetramers and four different hetero-tetramers containing ZmPIP1;2 and ZmPIP2;5 from Zea mays. ZmPIP1;2 in a tetramer assembly will have two interfaces, one formed by transmembrane segments TM4 and TM5 and the other formed by TM1 and TM2. We have analyzed channel radius profiles, water transport and potential of mean force profiles of ZmPIP1;2 monomers. Results of MD simulations clearly revealed the influence of TM4-TM5 interface in modulating the water transport of ZmPIP1;2. MD simulations indicate the importance of I93 residue from the TM2 segment of ZmPIP2;5 for the increased water transport in ZmPIP1;2.

The dehydration stress of couch grass is associated with its lipid metabolism, the induction of transporters and the re-programming of development coordinated by ABA

Background

The wild relatives of crop species represent a potentially valuable source of novel genetic variation, particularly in the context of improving the crop’s level of tolerance to abiotic stress. The mechanistic basis of these tolerances remains largely unexplored. Here, the focus was to characterize the transcriptomic response of the nodes (meristematic tissue) of couch grass (a relative of barley) to dehydration stress, and to compare it to that of the barley crown formed by both a drought tolerant and a drought sensitive barley cultivar.

Results

Many of the genes up-regulated in the nodes by the stress were homologs of genes known to be mediated by abscisic acid during the response to drought, or were linked to either development or lipid metabolism. Transporters also featured prominently, as did genes acting on root architecture. The resilience of the couch grass node arise from both their capacity to develop an altered, more effective root architecture, but also from their formation of a lipid barrier on their outer surface and their ability to modify both their lipid metabolism and transporter activity when challenged by dehydration stress.

Conclusions

Our analysis revealed the nature of dehydration stress response in couch grass. We suggested the tolerance is associated with lipid metabolism, the induction of transporters and the re-programming of development coordinated by ABA. We also proved the applicability of barley microarray for couch grass stress-response analysis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4700-3) contains supplementary material, which is available to authorized users.

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

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

Roles of Chloroplast Retrograde Signals and Ion Transport in Plant Drought Tolerance

Worldwide, drought affects crop yields; therefore, understanding plants' strategies to adapt to drought is critical. Chloroplasts are key regulators of plant responses, and signals from chloroplasts also regulate nuclear gene expression during drought. However, the interactions between chloroplast-initiated retrograde signals and ion channels under stress are still not clear. In this review, we summarise the retrograde signals that participate in regulating plant stress tolerance. We compare chloroplastic transporters that modulate retrograde signalling through retrograde biosynthesis or as critical components in retrograde signalling. We also discuss the roles of important plasma membrane and tonoplast ion transporters that are involved in regulating stomatal movement. We propose how retrograde signals interact with ion transporters under stress.

Regulation of aquaporins in plants under stress

Aquaporins (AQP) are channel proteins belonging to the Major Intrinsic Protein (MIP) superfamily that play an important role in plant water relations. The main role of aquaporins in plants is transport of water and other small neutral molecules across cellular biological membranes. AQPs have remarkable features to provide an efficient and often, specific water flow and enable them to transport water into and out of the cells along the water potential gradient. Plant AQPs are classified into five main subfamilies including the plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26 like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) and X intrinsic proteins (XIPs). AQPs are localized in the cell membranes and are found in all living cells. However, most of the AQPs that have been described in plants are localized to the tonoplast and plasma membranes. Regulation of AQP activity and gene expression, are also considered as a part of the adaptation mechanisms to stress conditions and rely on complex processes and signaling pathways as well as complex transcriptional, translational and posttranscriptional factors. Gating of AQPs through different mechanisms, such as phosphorylation, tetramerization, pH, cations, reactive oxygen species, phytohormones and other chemical agents, may play a key role in plant responses to environmental stresses by maintaining the uptake and movement of water in the plant body.

Genome-wide analyses of direct target genes of four rice NAC-domain transcription factors involved in drought tolerance

BACKGROUND

Plant stress responses and mechanisms determining tolerance are controlled by diverse sets of genes. Transcription factors (TFs) have been implicated in conferring drought tolerance under drought stress conditions, and the identification of their target genes can elucidate molecular regulatory networks that orchestrate tolerance mechanisms.

RESULTS

We generated transgenic rice plants overexpressing the 4 rice TFs, OsNAC5, 6, 9, and 10, under the control of the root-specific RCc3 promoter. We showed that they were tolerant to drought stress with reduced loss of grain yield under drought conditions compared with wild type plants. To understand the molecular mechanisms underlying this tolerance, we here performed chromatin immunoprecipitation (ChIP)-Seq and RNA-Seq analyses to identify the direct target genes of the OsNAC proteins using the RCc3:6MYC-OsNAC expressing roots. A total of 475 binding loci for the 4 OsNAC proteins were identified by cross-referencing their binding to promoter regions and the expression levels of the corresponding genes. The binding loci were distributed among the promoter regions of 391 target genes that were directly up-regulated by one of the OsNAC proteins in four RCc3:6MYC-OsNAC transgenic lines. Based on gene ontology (GO) analysis, the direct target genes were related to transmembrane/transporter activity, vesicle, plant hormones, carbohydrate metabolism, and TFs. The direct targets of each OsNAC range from 4.0-8.7% of the total number of up-regulated genes found in the RNA-Seq data sets. Thus, each OsNAC up-regulates a set of direct target genes that alter root system architecture in the RCc3:OsNAC plants to confer drought tolerance. Our results provide a valuable resource for functional dissection of the molecular mechanisms of drought tolerance.

CONCLUSIONS

Many of the target genes, including transmembrane/transporter, vesicle related, auxin/hormone related, carbohydrate metabolic processes, and transcription factor genes, that are up-regulated by OsNACs act as the cellular components which would alter the root architectures of RCc3:OsNACs for drought tolerance.

Genome-wide identification and characterization of aquaporin gene family in Beta vulgaris

Aquaporins (AQPs) are essential channel proteins that execute multi-functions throughout plant growth and development, including water transport, uncharged solutes uptake, stress response, and so on. Here, we report the first genome-wide identification and characterization AQP (BvAQP) genes in sugar beet (Beta vulgaris), an important crop widely cultivated for feed, for sugar production and for bioethanol production. Twenty-eight sugar beet AQPs (BvAQPs) were identified and assigned into five subfamilies based on phylogenetic analyses: seven of plasma membrane (PIPs), eight of tonoplast (TIPs), nine of NOD26-like (NIPs), three of small basic (SIPs), and one of x-intrinsic proteins (XIPs). BvAQP genes unevenly mapped on all chromosomes, except on chromosome 4. Gene structure and motifs analyses revealed that BvAQP have conserved exon-intron organization and that they exhibit conserved motifs within each subfamily. Prediction of BvAQPs functions, based on key protein domains conservation, showed a remarkable difference in substrate specificity among the five subfamilies. Analyses of BvAQPs expression, by mean of RNA-seq, in different plant organs and in response to various abiotic stresses revealed that they were ubiquitously expressed and that their expression was induced by heat and salt stresses. These results provide a reference base to address further the function of sugar beet aquaporins and to explore future applications for plants growth and development improvements as well as in response to environmental stresses.

Heterologous Expression of Panax ginseng PgTIP1 Confers Enhanced Salt Tolerance of Soybean Cotyledon Hairy Roots, Composite, and Whole Plants

The Panax ginseng TIP gene PgTIP1 was previously demonstrated to have high water channel activity by its heterologous expression in Xenopus laevis oocytes and in yeast; it also plays a significant role in growth of PgTIP1-transgenic Arabidopsis plants under favorable conditions and has enhanced tolerance toward salt and drought treatment. In this work, we first investigated the physiological effects of heterologous PgTIP1 expression in soybean cotyledon hairy roots or composite plants mediated by Agrobacterium rhizogenes toward enhanced salt tolerance. The PgTIP1-transgenic soybean plants mediated by the pollen tube pathway, represented by the lines N and J11, were analyzed at the physiological and molecular levels for enhanced salt tolerance. The results showed that in terms of root-specific heterologous expression, the PgTIP1-transformed soybean cotyledon hairy roots or composite plants displayed superior salt tolerance compared to the empty vector-transformed ones according to the mitigatory effects of hairy root growth reduction, drop in leaf RWC, and rise in REL under salt stress. Additionally, declines in K+ content, increases in Na+ content and Na+/K+ ratios in the hairy roots, stems, or leaves were effectively alleviated by PgTIP1-transformation, particularly the stems and leaves of composite soybean plants. At the whole plant level, PgTIP1-trasgenic soybean lines were found to possess stronger root vigor, reduced root and leaf cell membrane damage, increased SOD, POD, CAT, and APX activities, steadily increased leaf Tr, RWC, and Pn values, and smaller declines in chlorophyll and carotenoid content when exposed to salt stress compared to wild type. Moreover, the distribution patterns of Na+, K+, and Cl- in the roots, stems, and leaves of salt-stressed transgenic plants were readjusted, in that the absorbed Na+ and Cl- were mainly restricted to the roots to reduce their transport to the shoots, and the transport of root-absorbed K+ to the shoots was simultaneously promoted. PgTIP1 transformation into soybean plants enhanced the expression of some stress-related genes (GmPOD, GmAPX1, GmSOS1, and GmCLC1) in the roots and leaves under salt treatment. This indicates that the causes of enhanced salt tolerance of heterologous PgTIP1-transformed soybean are associated with the positive regulation on water relations, ion homeostasis, and ROS scavenging under salt stress both at root-specific and whole plant levels.

Analysis of aquaporins in Brassicaceae species reveals high-level of conservation and dynamic role against biotic and abiotic stress in canola

Aquaporins (AQPs) are of vital importance in the cellular transport system of all living organisms. In this study, genome-wide identification, distribution, and characterization of AQPs were determined in Arabidopsis lyrata, Capsella grandiflora, C. rubella, Eutrema salsugineum, Brassica rapa, B. oleracea, and B. napus (canola). Classification and phylogeny of AQPs revealed the loss of XIPs and NIP-IIIs in all species. Characterization of distinctive AQP features showed a high level of conservation in spacing between NPA-domains, and selectivity filters. Interestingly, TIP3s were found to be highly expressed in developing seeds, suggesting their role in seed desiccation. Analysis of available RNA-seq data obtained under biotic and abiotic stresses led to the identification of AQPs involved in stress tolerance mechanisms in canola. In addition, analysis of the effect of ploidy level, and resulting gene dose effect performed with the different combinations of Brassica A and C genomes revealed that more than 70% of AQPs expression were dose-independent, thereby supporting their role in stress alleviation. This first in-depth characterization of Brassicaceae AQPs highlights transport mechanisms and related physiological processes that could be exploited in breeding programs of stress-tolerant cultivars.

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

MAIN CONCLUSION

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

Dual-targeting of Arabidopsis DMP1 isoforms to the tonoplast and the plasma membrane

The reports of dual-targeted proteins in plants have steadily increased over the past years. The vast majority of these proteins are soluble proteins distributed between compartments of the non-secretory pathway, predominantly chloroplasts and mitochondria. In contrast, dual-targeted transmembrane proteins, especially of the secretory pathway, are rare and the mechanisms leading to their differential targeting remain largely unknown. Here, we report dual-targeting of the Arabidopsis DUF679 Membrane Protein 1 (DMP1) to the tonoplast (TP) and the plasma membrane (PM). In Arabidopsis and tobacco two equally abundant DMP1 isoforms are synthesized by alternative translation initiation: a full length protein, DMP1.1, and a truncated one, DMP1.2, which lacks the N-terminal 19 amino acids including a TP-targeting dileucine motif. Accumulation of DMP1.1 and DMP1.2 in the TP and the PM, respectively, is Brefeldin A-sensitive, indicating transit via the Golgi. However, DMP1.2 interacts with DMP1.1, leading to extensive rerouting of DMP1.2 to the TP and “eclipsed” localization of DMP1.2 in the PM where it is barely visible by confocal laser scanning microscopy but clearly detectable by membrane fractionation. It is demonstrated that eGFP fusion to either DMP1 terminus can cause mistargeting artifacts: C-terminal fusion to DMP1.1 or DMP1.2 results in altered ER export and N-terminal fusion to DMP1.1 causes mistargeting to the PM, presumably by masking of the TP targeting signal. These results illustrate how the interplay of alternative translation initiation, presence or absence of targeting information and rerouting due to protein-protein interaction determines the ultimate distribution of a transmembrane protein between two membranes.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Genome-Wide Identification and Characterization of the Aquaporin Gene Family and Transcriptional Responses to Boron Deficiency in Brassica napus

Aquaporins (AQPs) are an abundant protein family and play important roles to facilitate small neutral molecule transport across membranes. Oilseed rape (Brassica napus L.) is an important oil crop in China and elsewhere in the world, and is very sensitive to low boron (B) stress. Several AQP family genes have been reported to be involved in B transport across plasma membranes in plants. In this study, a total of 121 full-length AQPs were identified and characterized in B. napus (AC genome), and could be classified into four sub-families, including 43 PIPs (plasma membrane intrinsic proteins), 35 TIPs (tonoplast intrinsic proteins), 32 NIPs (NOD26-like intrinsic proteins), and 11 SIPs (small basic intrinsic proteins). The gene characteristics of BnaAQPs were similar to those of BraAQPs (A genome) and BolAQPs (C genome) including the composition of each sub-family, gene structure, and substrate selectivity filters. The BnaNIP was the most complex AQP sub-family, reflecting the composition of substrate selectivity filter structures which affect the permeation of solution molecules. In this study, the seedlings of both B-efficient (QY10) and B-inefficient (W10) cultivars were treated with two boron (B) levels: deficient (0.25 μM B) and sufficient (25 μM B). The transcription of AQP genes in root (R), juvenile leaf (JL), and old leaf (OL) tissues of both cultivars was investigated under B deficient and sufficient conditions. Transcription of most BnaPIPs and BnaTIPs was significantly increased compared with other BnaAQPs in all the three tissues, especially in the roots, of both B-efficient and B-inefficient cultivars under both B conditions. With B deprivation, the expression of the majority of the BnaPIPs and BnaTIPs was down-regulated in the roots. However, the BnaNIPs were up-regulated. In addition, the BnaCnn_random.PIP1;4b, BnaPIP2;4s, BnaC04.TIP4;1a, BnaAnn_random.TIP1;1b, and BnaNIP5;1s (except for BnaA07.NIP5;1c and BnaC06.NIP5;1c) exhibited obvious differences at low B between B-efficient and B-inefficient cultivars. These results will help us to understand boron homeostasis in B. napus.

Drought response transcriptomes are altered in poplar with reduced tonoplast sucrose transporter expression

Transgenic Populus tremula x alba (717-1B4) plants with reduced expression of a tonoplast sucrose efflux transporter, PtaSUT4, exhibit reduced shoot growth compared to wild type (WT) under sustained mild drought. The present study was undertaken to determine whether SUT4-RNAi directly or indirectly altered poplar predisposition and/or response to changes in soil water availability. While sucrose and hexose levels were constitutively elevated in shoot organs, expression responses to drought were most altered in the root tips of SUT4-RNAi plants. Prior to any drought treatment, constitutively elevated transcript levels of abscisic acid biosynthetic genes and bark/vegetative storage proteins suggested altered metabolism in root tips of RNAi plants. Stronger drought-stimulation of stress-inducible genes encoding late-embryogenesis-abundant proteins in transgenic roots was consistent with increased vulnerability to soil drying. Transcript evidence suggested an RNAi effect on intercellular water trafficking by aquaporins in stem xylem during soil drying and recovery. Co-expression network analysis predicted altered integration of abscisic acid sensing/signaling with ethylene and jasmonate sensing/signaling in RNAi compared to WT roots. The overall conclusion is that steepened shoot-root sugar gradient in RNAi plants increased sensitivity of root tips to decreasing soil water availability.

Genome-Wide Characterization of Major Intrinsic Proteins in Four Grass Plants and Their Non-Aqua Transport Selectivity Profiles with Comparative Perspective

Major intrinsic proteins (MIPs), commonly known as aquaporins, transport not only water in plants but also other substrates of physiological significance and heavy metals. In most of the higher plants, MIPs are divided into five subfamilies (PIPs, TIPs, NIPs, SIPs and XIPs). Herein, we identified 68, 42, 38 and 28 full-length MIPs, respectively in the genomes of four monocot grass plants, specifically Panicum virgatum, Setaria italica, Sorghum bicolor and Brachypodium distachyon. Phylogenetic analysis showed that the grass plants had only four MIP subfamilies including PIPs, TIPs, NIPs and SIPs without XIPs. Based on structural analysis of the homology models and comparing the primary selectivity-related motifs [two NPA regions, aromatic/arginine (ar/R) selectivity filter and Froger's positions (FPs)] of all plant MIPs that have been experimentally proven to transport non-aqua substrates, we predicted the transport profiles of all MIPs in the four grass plants and also in eight other plants. Groups of MIP subfamilies based on ar/R selectivity filter and FPs were linked to the non-aqua transport profiles. We further deciphered the substrate selectivity profiles of the MIPs in the four grass plants and compared them with their counterparts in rice, maize, soybean, poplar, cotton, Arabidopsis thaliana, Physcomitrella patens and Selaginella moellendorffii. In addition to two NPA regions, ar/R filter and FPs, certain residues, especially in loops B and C, contribute to the functional distinctiveness of MIP groups. Expression analysis of transcripts in different organs indicated that non-aqua transport was related to expression of MIPs since most of the unexpressed MIPs were not predicted to facilitate the transport of non-aqua molecules. Among all MIPs in every plant, TIP (BdTIP1;1, SiTIP1;2, SbTIP2;1 and PvTIP1;2) had the overall highest mean expression. Our study generates significant information for understanding the diversity, evolution, non-aqua transport profiles and insight into comparative transport selectivity of plant MIPs, and provides tools for the development of transgenic plants.

Overexpression of MfPIP2-7 from Medicago falcata promotes cold tolerance and growth under NO3 (-) deficiency in transgenic tobacco plants

BACKGROUND

Plasma membrane intrinsic proteins (PIPs), which belong to aquaporins (AQPs) superfamily, are subdivided into two groups, PIP1 and PIP2, based on sequence similarity. Several PIP2s function as water channels, while PIP1s have low or no water channel activity, but have a role in water permeability through interacting with PIP2. A cold responsive PIP2 named as MfPIP2-7 was isolated from Medicago falcata (hereafter falcata), a forage legume with great cold tolerance, and transgenic tobacco plants overexpressing MfPIP2-7 were analyzed in tolerance to multiple stresses including freezing, chilling, and nitrate reduction in this study.

RESULTS

MfPIP2-7 transcript was induced by 4 to 12 h of cold treatment and 2 h of abscisic acid (ABA) treatment. Pretreatment with inhibitor of ABA synthesis blocked the cold induced MfPIP2-7 transcript, indicating that ABA was involved in cold induced transcription of MfPIP2-7 in falcata. Overexpression of MfPIP2-7 resulted in enhanced tolerance to freezing, chilling and NO3 (-) deficiency in transgenic tobacco (Nicotiana tabacum L.) plants as compared with the wild type. Moreover, MfPIP2-7 was demonstrated to facilitate H2O2 diffusion in yeast. Higher transcript levels of several stress responsive genes, such as NtERD10B, NtERD10C, NtDREB1, and 2, and nitrate reductase (NR) encoding genes (NtNIA1, and NtNIA2) were observed in transgenic plants as compared with the wild type with dependence upon H2O2. In addition, NR activity was increased in transgenic plants, which led to alterations in free amino acid components and concentrations.

CONCLUSIONS

The results suggest that MfPIP2-7 plays an important role in plant tolerance to freezing, chilling, and NO3 (-) deficiency by promoted H2O2 diffusion that in turn up-regulates expression of NIAs and multiple stress responsive genes.

Aquaporin gene expression and physiological responses of Robinia pseudoacacia L. to the mycorrhizal fungus Rhizophagus irregularis and drought stress

The influence of arbuscular mycorrhiza (AM) and drought stress on aquaporin (AQP) gene expression, water status, and photosynthesis was investigated in black locust (Robinia pseudoacacia L.). Seedlings were grown in potted soil inoculated without or with the AM fungus Rhizophagus irregularis, under well-watered and drought stress conditions. Six full-length AQP complementary DNAs (cDNAs) were isolated from Robinia pseudoacacia, named RpTIP1;1, RpTIP1;3, RpTIP2;1, RpPIP1;1, RpPIP1;3, and RpPIP2;1. A phylogenetic analysis of deduced amino acid sequences demonstrated that putative proteins coded by these RpAQP genes belong to the water channel protein family. Expression analysis revealed higher RpPIP expression in roots while RpTIP expression was higher in leaves, except for RpTIP1;3. AM symbiosis regulated host plant AQPs, and the expression of RpAQP genes in mycorrhizal plants depended on soil water condition and plant tissue. Positive effects were observed for plant physiological parameters in AM plants, which had higher dry mass and lower water saturation deficit and electrolyte leakage than non-AM plants. Rhizophagus irregularis inoculation also slightly increased leaf net photosynthetic rate and stomatal conductance under well-watered and drought stress conditions. These findings suggest that AM symbiosis can enhance the drought tolerance in Robinia pseudoacacia plants by regulating the expression of RpAQP genes, and by improving plant biomass, tissue water status, and leaf photosynthesis in host seedlings.

Genome-Wide Identification of Jatropha curcas Aquaporin Genes and the Comparative Analysis Provides Insights into the Gene Family Expansion and Evolution in Hevea brasiliensis

Aquaporins (AQPs) are channel-forming integral membrane proteins that transport water and other small solutes across biological membranes. Despite the vital role of AQPs, to date, little is known in physic nut (Jatropha curcas L., Euphorbiaceae), an important non-edible oilseed crop with great potential for the production of biodiesel. In this study, 32 AQP genes were identified from the physic nut genome and the family number is relatively small in comparison to 51 in another Euphorbiaceae plant, rubber tree (Hevea brasiliensis Muell. Arg.). Based on the phylogenetic analysis, the JcAQPs were assigned to five subfamilies, i.e., nine plasma membrane intrinsic proteins (PIPs), nine tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), two X intrinsic proteins (XIPs), and four small basic intrinsic proteins (SIPs). Like rubber tree and other plant species, functional prediction based on the aromatic/arginine selectivity filter, Froger's positions, and specificity-determining positions showed a remarkable difference in substrate specificity among subfamilies of JcAQPs. Genome-wide comparative analysis revealed the specific expansion of PIP and TIP subfamilies in rubber tree and the specific gene loss of the XIP subfamily in physic nut. Furthermore, by analyzing deep transcriptome sequencing data, the expression evolution especially the expression divergence of duplicated HbAQP genes was also investigated and discussed. Results obtained from this study not only provide valuable information for future functional analysis and utilization of Jc/HbAQP genes, but also provide a useful reference to survey the gene family expansion and evolution in Euphorbiaceae plants and other plant species.

Crystal Structure of an Ammonia-Permeable Aquaporin

Aquaporins of the TIP subfamily (Tonoplast Intrinsic Proteins) have been suggested to facilitate permeation of water and ammonia across the vacuolar membrane of plants, allowing the vacuole to efficiently sequester ammonium ions and counteract cytosolic fluctuations of ammonia. Here, we report the structure determined at 1.18 Å resolution from twinned crystals of Arabidopsis thaliana aquaporin AtTIP2;1 and confirm water and ammonia permeability of the purified protein reconstituted in proteoliposomes as further substantiated by molecular dynamics simulations. The structure of AtTIP2;1 reveals an extended selectivity filter with the conserved arginine of the filter adopting a unique unpredicted position. The relatively wide pore and the polar nature of the selectivity filter clarify the ammonia permeability. By mutational studies, we show that the identified determinants in the extended selectivity filter region are sufficient to convert a strictly water-specific human aquaporin into an AtTIP2;1-like ammonia channel. A flexible histidine and a novel water-filled side pore are speculated to deprotonate ammonium ions, thereby possibly increasing permeation of ammonia. The molecular understanding of how aquaporins facilitate ammonia flux across membranes could potentially be used to modulate ammonia losses over the plasma membrane to the atmosphere, e.g., during photorespiration, and thereby to modify the nitrogen use efficiency of plants.

The structure of an ammonia channel from plants extends our understanding of substrate specificity in different types of aquaporins and reveals an intriguing side pore that raises new questions.

Ammonia is a central molecule in nitrogen metabolism. Aquaporins are integral membrane proteins that form channels that accelerate the passive permeation of small polar uncharged molecules, like water and ammonia, across lipid membranes of the cell. Structural information of ammonia-permeable aquaporins has been lacking. Here, we report a high-resolution structure of the ammonia-permeable aquaporin AtTIP2;1 and explore it by functional assays of mutants and by molecular dynamics simulations. Our data uncover unexpected features of the substrate selectivity filter, including a conserved arginine in a new orientation that is stabilized by interactions to a histidine that is linked to ammonia specificity. An additional histidine in a different part of AtTIP2;1 fortifies the position of the arginine and interacts directly with the substrate in the channel. This histidine is therefore included in an extended selectivity filter, which should prompt a reinterpretation of the determinants of specificity in all types of aquaporins. We speculate that an intriguing water-filled side pore, next to the substrate-binding histidine, participates in deprotonating ammonium ions, which could increase the net permeation of ammonia. Understanding the principles of ammonia permeability may, in the future, allow us to modulate the passage of ammonia and generate crops with higher nitrogen-use efficiency.

An aquaporin PvTIP4;1 from Pteris vittata may mediate arsenite uptake

The fern Pteris vittata is an arsenic hyperaccumulator. The genes involved in arsenite (As(III)) transport are not yet clear. Here, we describe the isolation and characterization of a new P. vittata aquaporin gene, PvTIP4;1, which may mediate As(III) uptake. PvTIP4;1 was identified from yeast functional complement cDNA library of P. vittata. Arsenic toxicity and accumulating activities of PvTIP4;1 were analyzed in Saccharomyces cerevisiae and Arabidopsis. Subcellular localization of PvTIP4;1-GFP fusion protein in P. vittata protoplast and callus was conducted. The tissue expression of PvTIP4;1 was investigated by quantitative real-time PCR. Site-directed mutagenesis of the PvTIP4;1 aromatic/arginine (Ar/R) domain was studied. Heterologous expression in yeast demonstrates that PvTIP4;1 was able to facilitate As(III) diffusion. Transgenic Arabidopsis showed that PvTIP4;1 increases arsenic accumulation and induces arsenic sensitivity. Images and FM4-64 staining suggest that PvTIP4;1 localizes to the plasma membrane in P. vittata cells. A tissue location study shows that PvTIP4;1 transcripts are mainly expressed in roots. Site-directed mutation in yeast further proved that the cysteine at the LE1 position of PvTIP4;1 Ar/R domain is a functional site. PvTIP4;1 is a new represented tonoplast intrinsic protein (TIP) aquaporin from P. vittata and the function and location results imply that PvTIP4;1 may be involved in As(III) uptake.

Harpin Hpa1 Interacts with Aquaporin PIP1;4 to Promote the Substrate Transport and Photosynthesis in Arabidopsis

Harpin proteins produced by plant-pathogenic Gram-negative bacteria are the venerable player in regulating bacterial virulence and inducing plant growth and defenses. A major gap in these effects is plant sensing linked to cellular responses, and plant sensor for harpin Hpa1 from rice bacterial blight pathogen points to plasma membrane intrinsic protein (PIP). Here we show that Arabidopsis AtPIP1;4 is a plasma membrane sensor of Hpa1 and plays a dual role in plasma membrane permeability of CO2 and H2O. In particular, AtPIP1;4 mediates CO2 transport with a substantial contribute to photosynthesis and further increases this function upon interacting with Hpa1 at the plasma membrane. As a result, leaf photosynthesis rates are increased and the plant growth is enhanced in contrast to the normal process without Hpa1-AtPIP1;4 interaction. Our findings demonstrate the first case that plant sensing of a bacterial harpin protein is connected with photosynthetic physiology to regulate plant growth.

Molecular mechanisms of foliar water uptake in a desert tree

Water deficits severely affect growth, particularly for the plants in arid and semiarid regions of the world. In addition to precipitation, other subsidiary water, such as dew, fog, clouds and small rain showers, may also be absorbed by leaves in a process known as foliar water uptake. With the severe scarcity of water in desert regions, this process is increasingly becoming a necessity. Studies have reported on physical and physiological processes of foliar water uptake. However, the molecular mechanisms remain less understood. As major channels for water regulation and transport, aquaporins (AQPs) are involved in this process. However, due to the regulatory complexity and functional diversity of AQPs, their molecular mechanism for foliar water uptake remains unclear. In this study, Tamarix ramosissima, a tree species widely distributed in desert regions, was investigated for gene expression patterns of AQPs and for sap flow velocity. Our results suggest that the foliar water uptake of T. ramosissima occurs in natural fields at night when the humidity is over a threshold of 85 %. The diurnal gene expression pattern of AQPs suggests that most AQP gene expressions display a circadian rhythm, and this could affect both photosynthesis and transpiration. At night, the PIP2-1 gene is also upregulated with increased relative air humidity. This gene expression pattern may allow desert plants to regulate foliar water uptake to adapt to extreme drought. This study suggests a molecular basis of foliar water uptake in desert plants.

Gene Structures, Evolution, Classification and Expression Profiles of the Aquaporin Gene Family in Castor Bean (Ricinus communis L.)

Aquaporins (AQPs) are a class of integral membrane proteins that facilitate the passive transport of water and other small solutes across biological membranes. Castor bean (Ricinus communis L., Euphobiaceae), an important non-edible oilseed crop, is widely cultivated for industrial, medicinal and cosmetic purposes. Its recently available genome provides an opportunity to analyze specific gene families. In this study, a total of 37 full-length AQP genes were identified from the castor bean genome, which were assigned to five subfamilies, including 10 plasma membrane intrinsic proteins (PIPs), 9 tonoplast intrinsic proteins (TIPs), 8 NOD26-like intrinsic proteins (NIPs), 6 X intrinsic proteins (XIPs) and 4 small basic intrinsic proteins (SIPs) on the basis of sequence similarities. Functional prediction based on the analysis of the aromatic/arginine (ar/R) selectivity filter, Froger's positions and specificity-determining positions (SDPs) showed a remarkable difference in substrate specificity among subfamilies. Homology analysis supported the expression of all 37 RcAQP genes in at least one of examined tissues, e.g., root, leaf, flower, seed and endosperm. Furthermore, global expression profiles with deep transcriptome sequencing data revealed diverse expression patterns among various tissues. The current study presents the first genome-wide analysis of the AQP gene family in castor bean. Results obtained from this study provide valuable information for future functional analysis and utilization.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.

Genome-Wide Characterization and Expression Analysis of Major Intrinsic Proteins during Abiotic and Biotic Stresses in Sweet Orange (Citrus sinensis L. Osb.)

The family of aquaporins (AQPs), or major intrinsic proteins (MIPs), includes integral membrane proteins that function as transmembrane channels for water and other small molecules of physiological significance. MIPs are classified into five subfamilies in higher plants, including plasma membrane (PIPs), tonoplast (TIPs), NOD26-like (NIPs), small basic (SIPs) and unclassified X (XIPs) intrinsic proteins. This study reports a genome-wide survey of MIP encoding genes in sweet orange (Citrus sinensis L. Osb.), the most widely cultivated Citrus spp. A total of 34 different genes encoding C. sinensis MIPs (CsMIPs) were identified and assigned into five subfamilies (CsPIPs, CsTIPs, CsNIPs, CsSIPs and CsXIPs) based on sequence analysis and also on their phylogenetic relationships with clearly classified MIPs of Arabidopsis thaliana. Analysis of key amino acid residues allowed the assessment of the substrate specificity of each CsMIP. Gene structure analysis revealed that the CsMIPs possess an exon-intron organization that is highly conserved within each subfamily. CsMIP loci were precisely mapped on every sweet orange chromosome, indicating a wide distribution of the gene family in the sweet orange genome. Investigation of their expression patterns in different tissues and upon drought and salt stress treatments, as well as with ‘Candidatus Liberibacter asiaticus’ infection, revealed a tissue-specific and coordinated regulation of the different CsMIP isoforms, consistent with the organization of the stress-responsive cis-acting regulatory elements observed in their promoter regions. A special role in regulating the flow of water and nutrients is proposed for CsTIPs and CsXIPs during drought stress, and for most CsMIPs during salt stress and the development of HLB disease. These results provide a valuable reference for further exploration of the CsMIPs functions and applications to the genetic improvement of both abiotic and biotic stress tolerance in citrus.

Trafficking of plant plasma membrane aquaporins: multiple regulation levels and complex sorting signals

Aquaporins are small channel proteins which facilitate the diffusion of water and small neutral molecules across biological membranes. Compared with animals, plant genomes encode numerous aquaporins, which display a large variety of subcellular localization patterns. More specifically, plant aquaporins of the plasma membrane intrinsic protein (PIP) subfamily were first described as plasma membrane (PM)-resident proteins, but recent research has demonstrated that the trafficking and subcellular localization of these proteins are complex and highly regulated. In the past few years, PIPs emerged as new model proteins to study subcellular sorting and membrane dynamics in plant cells. At least two distinct sorting motifs (one cytosolic, the other buried in the membrane) are required to direct PIPs to the PM. Hetero-oligomerization and interaction with SNAREs (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptors) also influence the subcellular trafficking of PIPs. In addition to these constitutive processes, both the progression of PIPs through the secretory pathway and their dynamics at the PM are responsive to changing environmental conditions.

Identification of rice cornichon as a possible cargo receptor for the Golgi-localized sodium transporter OsHKT1;3

Membrane proteins are synthesized and folded in the endoplasmic reticulum (ER), and continue their path to their site of residence along the secretory pathway. The COPII system has been identified as a key player for selecting and directing the fate of membrane and secretory cargo proteins. Selection of cargo proteins within the COPII vesicles is achieved by cargo receptors. The cornichon cargo receptor belongs to a conserved protein family found in eukaryotes that has been demonstrated to participate in the selection of integral membrane proteins as cargo for their correct targeting. Here it is demonstrated at the cellular level that rice cornichon OsCNIH1 interacts with OsHKT1;3 and, in yeast cells, enables the expression of the sodium transporter to the Golgi apparatus. Physical and functional HKT-cornichon interactions are confirmed by the mating-based split ubiquitin system, bimolecular fluorescence complementation, and Xenopus oocyte and yeast expression systems. The interaction between the two proteins occurs in the ER of plant cells and their co-expression in oocytes leads to the sequestration of the transporter in the ER. In the yeast cornichon mutant erv14, OsHKT1;3 is mistargeted, preventing the toxic effects of sodium transport in the cell observed in wild-type cells or in the erv14 mutant that co-expressed OsHKT1;3 with either OsCNIH1 or Erv14p. Identification and characterization of rice cornichon as a possible cargo receptor opens up the opportunity to improve our knowledge on membrane protein targeting in plant cells.