A Survey of Barley PIP Aquaporin Ionic Conductance Reveals Ca2+-Sensitive HvPIP2;8 Na+ and K+ Conductance

Water for agriculture: more crop per drop

Global crop production in agriculture depends on water availability. Future scenarios predict increasing occurrence of flash floods and rapidly developing droughts accompanied by heatwaves in humid regions that rely on rain-fed agriculture. It is challenging to maintain high crop yields, even in arid and drought-prone regions that depend on irrigation. The average water demand of crops varies significantly, depending on plant species, development stage, and climate. Most crops, such as maize and wheat, require relatively more water during the vegetative phase compared to the ripening phase. In this review, we explain WUE and options to improve water use and thus crop yield. Nutrient management might represent another possibility to manipulate water uptake and use by plants. An emerging topic involves agroforest co-cultivation, where trees in the system facilitate water transfer through hydraulic lift, benefiting neighbouring crops. Other options to enhance crop yield per water use are discussed.

A new method to measure aquaporin-facilitated membrane diffusion of hydrogen peroxide and cations in plant suspension cells

Plant aquaporins (AQPs) facilitate the membrane diffusion of water and small solutes, including hydrogen peroxide (H2 O2 ) and, possibly, cations, essential signalling molecules in many physiological processes. While the determination of the channel activity generally depends on heterologous expression of AQPs in Xenopus oocytes or yeast cells, we established a genetic tool to determine whether they facilitate the diffusion of H2 O2 through the plasma membrane in living plant cells. We designed genetic constructs to co-express the fluorescent H2 O2 sensor HyPer and AQPs, with expression controlled by a heat shock-inducible promoter in Nicotiana tabacum BY-2 suspension cells. After induction of ZmPIP2;5 AQP expression, a HyPer signal was recorded when the cells were incubated with H2 O2 , suggesting that ZmPIP2;5 facilitates H2 O2 transmembrane diffusion; in contrast, the ZmPIP2;5W85A mutated protein was inactive as a water or H2 O2 channel. ZmPIP2;1, ZmPIP2;4 and AtPIP2;1 also facilitated H2 O2 diffusion. Incubation with abscisic acid and the elicitor flg22 peptide induced the intracellular H2 O2 accumulation in BY-2 cells expressing ZmPIP2;5. We also monitored cation channel activity of ZmPIP2;5 using a novel fluorescent photo-switchable Li+ sensor in BY-2 cells. BY-2 suspension cells engineered for inducible expression of AQPs as well as HyPer expression and the use of Li+ sensors constitute a powerful toolkit for evaluating the transport activity and the molecular determinants of PIPs in living plant cells.

The differential expressions of aquaporins underline the diverse strategies of cucumber and tomato against salinity and zinc stress

Salinity and excess zinc are two main problems that have limited agriculture in recent years. Aquaporins are crucial in regulating the passage of water and solutes through cells and may be essential for mitigating abiotic stresses. In the present study, the adaptive response to moderate salinity (60 mM NaCl) and excess Zn (1 mM ZnSO4 ) were compared alone and in combination in Cucumis sativus L. and Solanum lycopersicum L. Water relations, gas exchange and the differential expression of all aquaporins were analysed. The results showed that cucumber plants under salinity maintained the internal movement of water through osmotic adjustment and the overexpression of specific PIPs aquaporins, following a "conservation strategy". As tomato has a high tolerance to salinity, the physiological parameters and the expression of most aquaporins remained unchanged. ZnSO4 was shown to be stressful for both plant species. While cucumber upregulated 7 aquaporin isoforms, the expression of aquaporins increased in a generalized manner in tomato. Despite the differences, water relations and transpiration were adjusted in both plants, allowing the RWC in the shoot to be maintained. The aquaporin regulation in cucumber plants facing NaCl+ZnSO4 stress was similar in the two treatments containing NaCl, evidencing the predominance of salt in stress. However, in tomato, the induced expression of specific isoforms to deal with the combined stress differed from independent stresses. The results clarify the key role of aquaporin regulation in facing abiotic stresses and their possible use as markers of tolerance to salinity and heavy metals in plants.

Analysis of aquaporins in northern grasses reveal functional importance of Puccinellia nuttalliana PIP2;2 in salt tolerance

To better understand the roles of aquaporins in salt tolerance, we cloned PIP2;1, PIP2;2, PIP2;3, PIP1;1, PIP1;3, and TIP1;1 aquaporins from three northern grasses varying is salt tolerance including the halophytic grass Puccinellia nuttalliana, moderately salt tolerant Poa juncifolia, and relatively salt sensitive Poa pratensis. We analysed aquaporin expression in roots by exposing the plants to 0 and 150 mM for 6 days in hydroponic culture. NaCl treatment upregulated several PIP transcripts in P. nuttalliana while decreasing PnuTIP1;1. The PnuPIP2;2 transcripts increased by about six-fold in P. nuttalliana, two-fold in Poa juncifolia, and did not change in Poa pratensis. The NaCl treatment enhanced the rate of water transport in yeast expressing PnuPIP2;2 by 56% compared with control. PnuPIP2,2 expression also resulted in a higher Na⁺ uptake in yeast cells compared with an empty vector suggesting that PnuPIP2;2 may have both water and ion transporting functions. Structural analysis revealed that the transport properties of PnuPIP2;2 could be affected by its unique pore characteristics, which include a combination of hourglass, cylindrical, and increasing diameter conical entrance shape with pore hydropathy of -0.22.

Molecular membrane separation: plants inspire new technologies

Plants draw up their surrounding soil solution to gain water and nutrients required for growth, development and reproduction. Obtaining adequate water and nutrients involves taking up both desired and undesired elements from the soil solution and separating resources from waste. Desirable and undesirable elements in the soil solution can share similar chemical properties, such as size and charge. Plants use membrane separation mechanisms to distinguish between different molecules that have similar chemical properties. Membrane separation enables distribution or retention of resources and efflux or compartmentation of waste. Plants use specialised membrane separation mechanisms to adapt to challenging soil solution compositions and distinguish between resources and waste. Coordination and regulation of these mechanisms between different tissues, cell types and subcellular membranes supports plant nutrition, environmental stress tolerance and energy management. This review considers membrane separation mechanisms in plants that contribute to specialised separation processes and highlights mechanisms of interest for engineering plants with enhanced performance in challenging conditions and for inspiring the development of novel industrial membrane separation technologies. Knowledge gained from studying plant membrane separation mechanisms can be applied to developing precision separation technologies. Separation technologies are needed for harvesting resources from industrial wastes and transitioning to a circular green economy.

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.

Transcriptomic Analysis of Distal Parts of Roots Reveals Potentially Important Mechanisms Contributing to Limited Flooding Tolerance of Canola (Brassica napus) Plants

Since most of the root metabolic activities as well as root elongation and the uptake of water and mineral nutrients take place in the distal parts of roots, we aimed to gain insight into the physiological and transcriptional changes induced by root hypoxia in the distal parts of roots in canola (Brassica napus) plants, which are relatively sensitive to flooding conditions. Plants were subject to three days of root hypoxia via lowering oxygen content in hydroponic medium, and various physiological and anatomical features were examined to characterize plant responses. Untargeted transcriptomic profiling approaches were also applied to investigate changes in gene expression that took place in the distal root tissues in response to hypoxia. Plants responded to three days of root hypoxia by reducing growth and gas exchange rates. These changes were accompanied by decreases in leaf water potential (Ψ_leaf) and root hydraulic conductivity (L_pr). Increased deposition of lignin and suberin was also observed in the root tissues of hypoxic plants. The transcriptomic data demonstrated that the effect of hypoxia on plant water relations involved downregulation of most BnPIPs in the root tissues with the exception of BnPIP1;3 and BnPIP2;7, which were upregulated. Since some members of the PIP1 subfamily of aquaporins are known to transport oxygen, the increase in BnPIP1;3 may represent an important hypoxia tolerance strategy in plants. The results also demonstrated substantial rearrangements of different signaling pathways and transcription factors (TFs), which resulted in alterations of genes involved in the regulation of L_pr, TCA (tricarboxylic acid) cycle-related enzymes, antioxidant enzymes, and cell wall modifications. An integration of these data enabled us to draft a comprehensive model of the molecular pathways involved in the responses of distal parts of roots in B. napus. The model highlights systematic transcriptomic reprogramming aimed at explaining the relative sensitivity of Brassica napus to root hypoxia.

Salinity Tolerance of Halophytic Grass Puccinellia nuttalliana Is Associated with Enhancement of Aquaporin-Mediated Water Transport by Sodium

In salt-sensitive plants, root hydraulic conductivity is severely inhibited by NaCl, rapidly leading to the loss of water balance. However, halophytic plants appear to effectively control plant water flow under salinity conditions. In this study, we tested the hypothesis that Na⁺ is the principal salt factor responsible for the enhancement of aquaporin-mediated water transport in the roots of halophytic grasses, and this enhancement plays a significant role in the maintenance of water balance, gas exchange, and the growth of halophytic plants exposed to salinity. We examined the effects of treatments with 150 mM of NaCl, KCl, and Na₂SO₄ to separate the factors that affect water relations and, consequently, physiological and growth responses in three related grass species varying in salt tolerance. The grasses included relatively salt-sensitive Poa pratensis, moderately salt-tolerant Poa juncifolia, and the salt-loving halophytic grass Puccinellia nuttalliana. Our study demonstrated that sustained growth, chlorophyll concentrations, gas exchange, and water transport in Puccinellia nuttalliana were associated with the presence of Na in the applied salt treatments. Contrary to the other examined grasses, the root cell hydraulic conductivity in Puccinellia nuttalliana was enhanced by the 150 mM NaCl and 150 mM Na₂SO₄ treatments. This enhancement was abolished by the 50 µM HgCl₂ treatment, demonstrating that Na was the factor responsible for the increase in mercury-sensitive, aquaporin-mediated water transport. The observed increases in root Ca and K concentrations likely played a role in the transcriptional and (or) posttranslational regulation of aquaporins that enhanced root water transport capacity in Puccinellia nuttalliana. The study demonstrates that Na plays a key role in the aquaporin-mediated root water transport of the halophytic grass Puccinellia nuttalliana, contributing to its salinity tolerance.

Aquaporin ion conductance properties defined by membrane environment, protein structure, and cell physiology

Aquaporins (AQPs) are multifunctional transmembrane channel proteins permeable to water and an expanding array of solutes. AQP-mediated ion channel activity was first observed when purified AQP0 from bovine lens was incorporated into lipid bilayers. Electrophysiological properties of ion-conducting AQPs since discovered in plants, invertebrates, and mammals have been assessed using native, reconstituted, and heterologously expressed channels. Accumulating evidence is defining amino acid residues that govern differential solute permeability through intrasubunit and central pores of AQP tetramers. Rings of charged and hydrophobic residues around pores influence AQP selectivity, and are candidates for further work to define motifs that distinguish ion conduction capability, versus strict water and glycerol permeability. Similarities between AQP ion channels thus far include large single channel conductances and long open times, but differences in ionic selectivity, permeability to divalent cations, and mechanisms of gating (e.g., by voltage, pH, and cyclic nucleotides) are unique to subtypes. Effects of lipid environments in modulating parameters such as single channel amplitude could explain in part the variations in AQP ion channel properties observed across preparations. Physiological roles of the ion-conducting AQP classes span diverse processes including regulation of cell motility, organellar pH, neural development, signaling, and nutrient acquisition. Advances in computational methods can generate testable predictions of AQP structure-function relationships, which combined with innovative high-throughput assays could revolutionize the field in defining essential properties of ion-conducting AQPs, discovering new AQP ion channels, and understanding the effects of AQP interactions with proteins, signaling cascades, and membrane lipids.

Hormonal and environmental signaling pathways target membrane water transport

Plant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses.

Screening and identification of salt-tolerance genes in Sophora alopecuroides and functional verification of SaAQP

Overexpression of SaAQP can improve the salt tolerance of transgenic soybean hairy roots and A. thaliana. Salt stress severely affects crop yield and food security. There is a need to improve the salt tolerance of crops, but the discovery and utilization of salt-tolerance genes remains limited. Owing to its strong stress tolerance, Sophora alopecuroides is ideal for the identification of salt-tolerance genes. Therefore, we aimed to screen and identify the salt-tolerance genes in S. alopecuroides. With a yeast expression library of seedlings, salt-tolerant genes were screened using a salt-containing medium to simulate salt stress. By combining salt-treatment screening and transcriptome sequencing, 11 candidate genes related to salt tolerance were identified, including genes for peroxidase, inositol methyltransferase, aquaporin, cysteine synthase, pectinesterase, and WRKY. The expression dynamics of candidate genes were analyzed after salt treatment of S. alopecuroides, and salt tolerance was verified in yeast BY4743. The candidate genes participated in the salt-stress response in S. alopecuroides, and their overexpression significantly improved the salt tolerance of yeast. Salt tolerance mediated by SaAQP was further verified in soybean hairy roots and Arabidopsis thaliana, and it was found that SaAQP might enhance the salt tolerance of A. thaliana by participating in a reactive oxygen species scavenging mechanism. This result provides new genetic resources in plant breeding for salt resistance.

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.

An algal PIP-like aquaporin facilitates water transport and ionic conductance

Aquaporins are water and solute channel proteins found throughout the kingdoms of life. Ion-conducting aquaporins (icAQPs) have been identified in both plants and animals indicating that this function may be conserved through evolution. In higher plants icAQP function has been demonstrated for isoforms from two of five aquaporin subfamilies indicating that this function could have existed before the divergence of higher plants from green algae. Here a PIP-like aquaporin from the charophytic alga Klebsormidium nitens was functionally characterised in Xenopus laevis oocytes and its expression was found to induce water and ion conductance.

Functions and structure of roots and their contributions to salinity tolerance in plants

Soil salinity is an increasing threat to the productivity of glycophytic crops worldwide. The root plays vital roles under various stress conditions, including salinity, as well as has diverse functions in non-stress soil environments. In this review, we focus on the essential functions of roots such as in ion homeostasis mediated by several different membrane transporters and signaling molecules under salinity stress and describe recent advances in the impacts of quantitative trait loci (QTLs) or genetic loci (and their causal genes, if applicable) on salinity tolerance. Furthermore, we introduce important literature for the development of barriers against the apoplastic flow of ions, including Na⁺, as well as for understanding the functions and components of the barrier structure under salinity stress.