Identification of an H2 O2 permeable PIP aquaporin in barley and a serine residue promoting H2 O2 transport

Molecular mechanisms underlying the negative effects of transient heatwaves on crop fertility

Transient heatwaves occurring more frequently as the climate warms, yet their impacts on crop yield are severely underestimated and even overlooked. Heatwaves lasting only a few days or even hours during sensitive stages, such as microgametogenesis and flowering, can significantly reduce crop yield by disrupting plant reproduction. Recent advances in multi-omics and GWAS analysis have shed light on the specific organs (e.g., pollen, lodicule, style), key metabolic pathways (sugar and reactive oxygen species metabolism, Ca2+ homeostasis), and essential genes that are involved in crop responses to transient heatwaves during sensitive stages. This review therefore places particular emphasis on heat-sensitive stages, with pollen development, floret opening, pollination, and fertilization as the central narrative thread. The multifaceted effects of transient heatwaves and their molecular basis are systematically reviewed, with a focus on key structures such as the lodicule and tapetum. A number of heat-tolerance genes associated with these processes have been identified in major crops like maize and rice. The mechanisms and key heat-tolerance genes shared among different stages may facilitate the more precise improvement of heat-tolerant crops.

Analysis of sulfide signaling in rice highlights specific drought responses

Hydrogen sulfide regulates essential plant processes, including adaptation responses to stress situations, and the best characterized mechanism of action of sulfide consists of the post-translational modification of persulfidation. In this study, we reveal the first persulfidation proteome described in rice including 3443 different persulfidated proteins that participate in a broad range of biological processes and metabolic pathways. In addition, comparative proteomics revealed specific proteins involved in sulfide signaling during drought responses. Several proteins are involved in the maintenance of cellular redox homeostasis, the tricarboxylic acid cycle and energy-related pathways, and ion transmembrane transport and cellular water homeostasis, with the aquaporin family showing the highest differential levels of persulfidation. We revealed that water transport activity is regulated by sulfide which correlates with an increasing level of persulfidation of aquaporins. Our findings emphasize the impact of persulfidation on total ATP levels, fatty acid composition, levels of reactive oxygen species, antioxidant enzymatic activities, and relative water content. Interestingly, the role of persulfidation in aquaporin transport activity as an adaptation response in rice differs from current knowledge of Arabidopsis, which highlights the distinct role of sulfide in improving rice tolerance to drought.

A high-throughput yeast approach to characterize aquaporin permeabilities: Profiling the Arabidopsis PIP aquaporin sub-family

INTRODUCTION

Engineering membrane transporters to achieve desired functionality is reliant on availability of experimental data informing structure-function relationships and intelligent design. Plant aquaporin (AQP) isoforms are capable of transporting diverse substrates such as signaling molecules, nutrients, metalloids, and gases, as well as water. AQPs can act as multifunctional channels and their transport function is reliant on many factors, with few studies having assessed transport function of specific isoforms for multiple substrates.

METHODS

High-throughput yeast assays were developed to screen for transport function of plant AQPs, providing a platform for fast data generation and cataloguing of substrate transport profiles. We applied our high-throughput growth-based yeast assays to screen all 13 Arabidopsis PIPs (AtPIPs) for transport of water and several neutral solutes: hydrogen peroxide (H2O2), boric acid (BA), and urea. Sodium (Na+) transport was assessed using elemental analysis techniques.

RESULTS

All AtPIPs facilitated water and H2O2 transport, although their growth phenotypes varied, and none were candidates for urea transport. For BA and Na+ transport, AtPIP2;2 and AtPIP2;7 were the top candidates, with yeast expressing these isoforms having the most pronounced toxicity response to BA exposure and accumulating the highest amounts of Na+. Linking putative AtPIP isoform substrate transport profiles with phylogenetics and gene expression data, enabled us to align possible substrate preferences with known and hypothesized biological roles of AtPIPs.

DISCUSSION

This testing framework enables efficient cataloguing of putative transport functionality of diverse AQPs at a scale that can help accelerate our understanding of AQP biology through big data approaches (e.g. association studies). The principles of the individual assays could be further adapted to test additional substrates. Data generated from this framework could inform future testing of AQP physiological roles, and address knowledge gaps in structure-function relationships to improve engineering efforts.

Overexpression of an aquaporin gene EsPIP1;4 enhances abiotic stress tolerance and promotes flowering in Arabidopsis thaliana

Aquaporins are water channel proteins that play an essential role in plant growth and development. Despite extensive functional characterization of aquaporins in model plants such as Arabidopsis, their contributions to abiotic stress tolerance in non-model plants are still poorly understood. As a close relative of Arabidopsis thaliana, Eutrema salsugineum is an excellent model for studying salt tolerance. Here, we identified and functionally characterized EsPIP1;4, a gene encoding a plasma membrane intrinsic protein (PIP) aquaporin in E. salsugineum. Overexpression of EsPIP1;4 in Arabidopsis improved seed germination and root growth of transgenic plants under abiotic stress, which was accompanied by an increase in proline accumulation, reduction in MDA, and decrease in the rate of ion leakage. Under abiotic stress, transgenic plants overexpressing EsPIP1;4 also showed increased antioxidant enzyme activity, and enhanced K⁺/Na⁺ ratio compared to control plants. Furthermore, overexpression of EsPIP1;4 promoted flowering by regulating genes in multiple flowering pathways. Together, our results demonstrated that an aquaporin from E. salsugineum improves abiotic stress tolerance and promotes flowering.

The Aquaporin TaPIP2;10 Confers Resistance to Two Fungal Diseases in Wheat

Plants employ aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family to import environmental substrates, thereby affecting various processes, such as the cellular responses regulated by the signaling molecule hydrogen peroxide (H₂O₂). Common wheat (Triticum aestivum) contains 24 candidate members of the PIP family, designated as TaPIP1;1 to TaPIP1;12 and TaPIP2;1 to TaPIP2;12. None of these TaPIP candidates have been characterized for substrate selectivity or defense responses in their source plant. Here, we report that T. aestivum AQP TaPIP2;10 facilitates the cellular uptake of H₂O₂ to confer resistance against powdery mildew and Fusarium head blight, two devastating fungal diseases in wheat throughout the world. In wheat, the apoplastic H₂O₂ signal is induced by fungal attack, while TaPIP2;10 is stimulated to translocate this H₂O₂ into the cytoplasm, where it activates defense responses to restrict further attack. TaPIP2;10-mediated transport of H₂O₂ is essential for pathogen-associated molecular pattern-triggered plant immunity (PTI). Typical PTI responses are induced by the fungal infection and intensified by overexpression of the TaPIP2;10 gene. TaPIP2;10 overexpression causes a 70% enhancement in wheat resistance to powdery mildew and an 86% enhancement in resistance to Fusarium head blight. By reducing the disease severities, TaPIP2;10 overexpression brings about >37% increase in wheat grain yield. These results verify the feasibility of using an immunity-relevant AQP to concomitantly improve crop productivity and immunity.

Rice aquaporin OsPIP2;2 is a water-transporting facilitator in relevance to drought-tolerant responses

In rice (Oryza sativa), the PLASMA MEMBRANE INTRINSIC PROTEIN (PIP) family of aquaporin has 11 members, OsPIP1;1 to OsPIP1;3, and OsPIP2;1 to OsPIP2;8, which are hypothesized to facilitate the transport of H2O and other small compounds across cell membranes. To date, however, only OsPIP1;2, OsPIP2;1, and OsPIP2;4 have been demonstrated for substrate selectivity in their source plant (rice). In this study, OsPIP2;2 was characterized as the most efficient facilitator of H2O transport across cell membranes in comparison with the other 10 OsPIPs. In concomitant tests of all OsPIPs, four genes (OsPIP1;3, OsPIP2;1, OsPIP2;2, and OsPIP2;4) were induced to express in leaves of rice plants following a physiological drought stress, while OsPIP2;2 was expressed to the highest level. After de novo expression in frog oocytes and yeast cells, the four OsPIP proteins were localized to the plasma membranes in trimer and tetramer and displayed the activity to increase the membrane permeability to H2O. In comparison, OsPIP2;2 was most supportive to H2O import to oocytes and yeast cells. After de novo expression in tobacco protoplasts, OsPIP2;2 exceeded OsPIP1;3, OsPIP2;1, and OsPIP2;4 to support H2O transport across the plasma membranes. OsPIP2;2-mediated H2O transport was accompanied by drought-tolerant responses, including increases in concentrations of proline and polyamines, both of which are physiological markers of drought tolerance. In rice protoplasts, H2O transport and drought-tolerant responses, which included expression of marker genes of drought tolerance pathway, were considerably enhanced by OsPIP2;2 overexpression but strongly inhibited by the gene silencing. Furthermore, OsPIP2;2 played a role in maintenance of the cell membrane integrity and effectively protected rice cells from electrolyte leakage caused by the physiological drought stress. These results suggest that OsPIP2;2 is a predominant facilitator of H2O transport in relevance to drought tolerance in the plant.

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.

Mechanisms Activating Latent Functions of PIP Aquaporin Water Channels via the Interaction between PIP1 and PIP2 Proteins

Plant plasma membrane-type plasma membrane intrinsic protein (PIP) aquaporins are classified into two groups, PIP1s and PIP2s. In this study, we focused on HvPIP1;2, a PIP1 in barley (Hordeum vulgare), to dissect the molecular mechanisms that evoke HvPIP1-mediated water transport. No HvPIP1;2 protein was localized to the plasma membrane when expressed alone in Xenopus laevis oocytes. By contrast, a chimeric HvPIP1;2 protein (HvPIP1;2_24NC), in which the N- and C-terminal regions were replaced with the corresponding regions from HvPIP2;4, was found to localize to the plasma membrane of oocytes. However, HvPIP1;2_24NC showed no water transport activity in swelling assays. These results suggested that the terminal regions of PIP2 proteins direct PIP proteins to the plasma membrane, but the relocalization of PIP1 proteins was not sufficient to PIP1s functionality as a water channel in a membrane. A single amino acid replacement of threonine by methionine in HvPIP2;4 (HvPIP2;4T229M) abolished water transport activity. Co-expression of HvPIP1;2_24NC either with HvPIP2;4_12NC or with HvPIP2;4TM_12NC, in which the N- and C-terminal regions were replaced with the corresponding regions of HvPIP1;2, increased the water transport activity in oocytes. These data provided evidence that the HvPIP1;2 molecule has own water transport activity and an interaction with the middle part of the HvPIP2;4 protein (except for the N- and C-termini) is required for HvPIP1;2 functionality as a water channel. This molecular mechanism could be applied to other PIP1s and PIP2s in addition to the known mechanism that the terminal regions of some PIP2s lead some PIP1s to the plasma membrane.

Versatile Roles of Aquaporins in Plant Growth and Development

Aquaporins (AQPs) are universal membrane integrated water channel proteins that selectively and reversibly facilitate the movement of water, gases, metalloids, and other small neutral solutes across cellular membranes in living organisms. Compared with other organisms, plants have the largest number of AQP members with diverse characteristics, subcellular localizations and substrate permeabilities. AQPs play important roles in plant water relations, cell turgor pressure maintenance, the hydraulic regulation of roots and leaves, and in leaf transpiration, root water uptake, and plant responses to multiple biotic and abiotic stresses. They are also required for plant growth and development. In this review, we comprehensively summarize the expression and roles of diverse AQPs in the growth and development of various vegetative and reproductive organs in plants. The functions of AQPs in the intracellular translocation of hydrogen peroxide are also discussed.

Genetic regulation of water and nutrient transport in water stress tolerance in roots

Drought stress is one of the major abiotic factors affecting the growth and development of crops. The primary effect of drought is the alteration of water and nutrient uptake and transport by roots, related essentially with aquaporins and ion transporters of the plasma membrane. Therefore, the efficiency of water and nutrient transport across cell layers is a main factor in tolerance mechanisms. The regulation of this transport under water stress - in relation to the differing degrees of tolerance of crops and the effect of arbuscular mycorrhizae, together with signaling mechanisms - is reviewed here. Three different phases in the response to stress (immediate, short-term and long-term), involving different signals and levels of gene regulation, are highlighted.

Plant Aquaporins: Diversity, Evolution and Biotechnological Applications

The plasma membrane forms a permeable barrier that separates the cytoplasm from the external environment, defining the physical and chemical limits in each cell in all organisms. The movement of molecules and ions into and out of cells is controlled by the plasma membrane as a critical process for cell stability and survival, maintaining essential differences between the composition of the extracellular fluid and the cytosol. In this process aquaporins (AQPs) figure as important actors, comprising highly conserved membrane proteins that carry water, glycerol and other hydrophilic molecules through biomembranes, including the cell wall and membranes of cytoplasmic organelles. While mammals have 15 types of AQPs described so far (displaying 18 paralogs), a single plant species can present more than 120 isoforms, providing transport of different types of solutes. Such aquaporins may be present in the whole plant or can be associated with different tissues or situations, including biotic and especially abiotic stresses, such as drought, salinity or tolerance to soils rich in heavy metals, for instance. The present review addresses several aspects of plant aquaporins, from their structure, classification, and function, to in silico methodologies for their analysis and identification in transcriptomes and genomes. Aspects of evolution and diversification of AQPs (with a focus on plants) are approached for the first time with the aid of the LCA (Last Common Ancestor) analysis. Finally, the main practical applications involving the use of AQPs are discussed, including patents and future perspectives involving this important protein family.

Plant water transport and aquaporins in oxygen-deprived environments

Oxygen deprivation commonly affects plants exposed to flooding and soil compaction. The resulting root hypoxia has an immediate effect on plant water relations and upsets water balance. Hypoxia inhibits root water transport and triggers stomatal closure. The processes contributing to the inhibition of root hydraulic conductivity and conductance (hydraulic conductivity of the whole root system) are complex and involve changes in root morphology and the functions of aquaporins. Aquaporins (AQPs) comprise a group of membrane intrinsic proteins that are responsible for the transport of water, as well as some small neutral solutes and ions. They respond to a wide range of environmental stresses including O₂ deprivation, but the underlying functional mechanisms are still elusive. The aquaporin-mediated water transport is affected by the acidification of the cytoplasm and depletion of ATP that is required for aquaporin phosphorylation and membrane functions. Cytoplasmic pH, phosphorylation, and intracellular Ca²⁺ concentration directly control AQP gating, all of which are related to O₂ deprivation. This review addresses the structural determinants that are essential for pore conformational changes in AQPs, to highlight the underlying mechanisms triggered by O₂ deprivation stress. Gene expression of AQPs is modified in hypoxic plants, which may constitute an important, yet little explored, mechanism of hypoxia tolerance. In addition to water transport, AQPs may contribute to hypoxia tolerance by transporting O₂, H₂O₂, and lactic acid. Responses of plants to O₂ deprivation, and especially those that contribute to maintenance of water transport, are highly complex and entail the signals originating in roots and shoots that lead to and follow the stomatal closure. These complex responses may involve ethylene, abscisic acid, and possibly other hormonal factors and signaling molecules in ways that remain to be elucidated.

Aquaporins facilitate hydrogen peroxide entry into guard cells to mediate ABA- and pathogen-triggered stomatal closure

Stomatal movements are crucial for the control of plant water status and protection against pathogens. Assays on epidermal peels revealed that, similar to abscisic acid (ABA), pathogen-associated molecular pattern (PAMP) flg22 requires the AtPIP2;1 aquaporin to induce stomatal closure. Flg22 also induced an increase in osmotic water permeability (P_f) of guard cell protoplasts through activation of AtPIP2;1. The use of HyPer, a genetic probe for intracellular hydrogen peroxide (H₂O₂), revealed that both ABA and flg22 triggered an accumulation of H₂O₂ in wild-type but not pip2;1 guard cells. Pretreatment of guard cells with flg22 or ABA facilitated the influx of exogenous H₂O₂ Brassinosteroid insensitive 1-associated receptor kinase 1 (BAK1) and open stomata 1 (OST1)/Snf1-related protein kinase 2.6 (SnRK2.6) were both necessary to flg22-induced P_f and both phosphorylated AtPIP2;1 on Ser121 in vitro. Accumulation of H₂O₂ and stomatal closure as induced by flg22 was restored in pip2;1 guard cells by a phosphomimetic form (Ser121Asp) but not by a phosphodeficient form (Ser121Ala) of AtPIP2;1. We propose a mechanism whereby phosphorylation of AtPIP2;1 Ser121 by BAK1 and/or OST1 is triggered in response to flg22 to activate its water and H₂O₂ transport activities. This work establishes a signaling role of plasma membrane aquaporins in guard cells and potentially in other cellular context involving H₂O₂ signaling.