Plant Aquaporins: Roles in Water Homeostasis, Nutrition, and Signaling Processes

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.

Genome-wide analysis and expression of the aquaporin gene family in Avena sativa L

Background

Oat (Avena sativa L.) belongs to the early maturity grass subfamily of the Gramineae subfamily oats (Avena) and has excellent characteristics, such as tolerance to barrenness, salt, cold, and drought. Aquaporin (AQP) proteins belong to the major intrinsic protein (MIP) superfamily, are widely involved in plant growth and development, and play an important role in abiotic stress responses. To date, previous studies have not identified or analyzed the AsAQP gene family system, and functional studies of oat AQP genes in response to drought, cold, and salt stress have not been performed.

Methods

In this study, AQP genes (AsAQP) were identified from the oat genome, and various bioinformatics data on the AQP gene family, gene structure, gene replication, promoters and regulatory networks were analyzed. Quantitative real-time PCR technology was used to verify the expression patterns of the AQP gene family in different oat tissues under different abiotic stresses.

Results

In this study, a total of 45 AQP genes (AsAQP) were identified from the oat reference genome. According to a phylogenetic analysis, 45 AsAQP were divided into 4 subfamilies (PIP, SIP, NIP, and TIP). Among the 45 AsAQP, 23 proteins had interactions, and among these, 5AG0000633.1 had the largest number of interacting proteins. The 20 AsAQP genes were expressed in all tissues, and their expression varied greatly among different tissues and organs. All 20 AsAQP genes responded to salt, drought and cold stress. The NIP subfamily 6Ag0000836.1 gene was significantly upregulated under different abiotic stresses and could be further verified as a key candidate gene.

Conclusion

The findings of this study provide a comprehensive list of members and their sequence characteristics of the AsAQP protein family, laying a solid theoretical foundation for further functional analysis of AsAQP in oats. This research also offers valuable reference for the creation of stress-tolerant oat varieties through genetic engineering techniques.

Chemopriming for induction of disease resistance against pathogens in rice

Rice is an important grain crop of Asian population. Different fungal, bacterial and viral pathogens cause large reduction in rice grain production. Use of chemical pesticides, to provide protection against pathogens, has become incomplete due to pathogens resistance and is cause of environmental concerns. Therefore, induction of resistance in rice against pathogens via biopriming and chemopriming with safe and novel agents has emerged on a global level as ecofriendly alternatives that provide protection against broad spectrum of rice pathogens without any significant yield penalty. In the past three decades, a number of chemicals such as silicon, salicylic acid, vitamins, plant extract, phytohormones, nutrients etc. have been used to induce defense against bacterial, fungal and viral rice pathogens. From the detailed analysis of abiotic agents used, it has been observed that silicon and salicylic acid are two potential chemicals for inducing resistance against fungal and bacterial diseases in rice, respectively. However, an inclusive evaluation of the potential of different abiotic agents to induce resistance against rice pathogens is lacking due to which the studies on induction of defense against rice pathogens via chemopriming has become disproportionate and discontinuous. The present review deals with a comprehensive analysis of different abiotic agents used to induce defense against rice pathogens, their mode of application, mechanism of defense induction and the effect of defense induction on grain yield. It also provides an account of unexplored areas, which might be taken into attention to efficiently manage rice diseases. DATA AVAILABILITY STATEMENT: Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

Genome-wide identification and biological relevance of broccoli aquaporins

Broccoli (Brassica oleracea var. italica) is an important crop worldwide, and its regular consumption is associated with health benefits due to the presence of various bioactive compounds. An optimal water balance and homeostasis are needed for plant growth; in this sense, aquaporins play a crucial role. As a result of a genome-wide search, a total of 65 aquaporin genes were identified in broccoli. The aquaporins were classified according to their phylogenetic relationships with other Brassicas species and Arabidopsis thaliana, and evolutionary events of gene duplication were also assessed, highlighting the tendency of NIPs (Nodulin-26-like Intrinsic Proteins) to duplicate. Also, the chromosomal localization, gene duplication, the study of the conserved motifs, and the tertiary structure were determined in broccoli. Functional predictive analyses were also carried out, which, together with the expression analyses in different broccoli plant tissues, allowed the prediction of the biological role of each aquaporin isoform. BoiPIP1-2a and BoiPIP1-2b showed higher expression in all the plant tissues when compared with other aquaporins. BoiTIP1-2b also showed high expression levels and was associated with nitrogen compounds transport such as urea. However, NIPs, through their differential expression and the tandem duplications of the isoforms, were revealed as the putative main actors in the response of broccoli plants to abiotic stress responses. The results of this work pointed to the physiological significance of each aquaporin isoform of broccoli, opening a new field of knowledge and constituting the first step of further in vivo analyses.

Aquaporin-mediated transport: Insights into metalloid trafficking

Metalloids in plants have diverse physiological effects. From being essential to beneficial to toxic, they have significant effects on many physiological processes, influencing crop yield and quality. Aquaporins are a group of membrane channels that have several physiological substrates along with water. Metalloids have emerged as one of their important substrates and they are found to have a substantial role in regulating plant metalloid homeostasis. The present review comprehensively details the multiple isoforms of aquaporins having specificity for metalloids and being responsible for their influx, distribution or efflux. In addition, it also highlights the usage of aquaporin-mediated transport as a selection marker in toxic screens and as tracer elements for closely related metalloids. Therefore, aquaporins, with their imperative contribution to the regulation of plant growth, development and physiological processes, need more research to unravel the metalloid trafficking mechanisms and their future applications.

Strawberry Proteome Responses to Controlled Hot and Cold Stress Partly Mimic Post-harvest Storage Temperature Effects on Fruit Quality

To determine the effect of different temperature on strawberry after harvest, physiological indicator analysis and proteomics analysis were conducted on ripened strawberry (“Sweet Charlie”) fruit stored at 4, 23, and 37°C for 10 or 20 days. Results showed that 4°C maintained a better visual quality of strawberry, and the weight loss and firmness remained stable within 3 days. Low temperature negatively affected anthocyanin but positively affected soluble sugars. Though anthocyanin content was higher with increasing temperature, anthocyanin synthesis related proteins were downregulated. Higher indole-acetic acid (IAA) content in seeds and lower abscisic acid (ABA) content were found in berry at 4°C. Antioxidant related proteins were upregulated during storage, showing a significant up-regulation of peroxidase (POD) at 4°C, and ascorbate-glutathione (AsA-GSH) cycle related proteins and heat shock proteins (HSPs) at 37°C. In addition, overexpressed sugar phosphate/phosphate translocator, 1-aminocyclopropane-1-carboxylate oxidase, and aquaporin PIP2-2 had a positive effect in response to low temperature stress for containing higher protopectin content and POD activity.

Heavy metal stress in rice: Uptake, transport, signaling, and tolerance mechanisms

Heavy metal contamination of agricultural fields has become a global concern as it causes a direct impact on human health. Rice is the major food crop for almost half of the world population and is grown under diverse environmental conditions, including heavy metal-contaminated soil. In recent years, the impact of heavy metal contamination on rice yield and grain quality has been shown through multiple approaches. In this review article, different aspects of heavy metal stress, that is uptake, transport, signaling and tolerance mechanisms, are comprehensively discussed with special emphasis on rice. For uptake, some of the transporters have specificity to one or two metal ions, whereas many other transporters are able to transport many different ions. After uptake, the intercellular signaling is mediated through different signaling pathways involving the regulation of various hormones, alteration of calcium levels, and the activation of mitogen-activated protein kinases. Heavy metal stress signals from various intermediate molecules activate various transcription factors, which triggers the expression of various antioxidant enzymes. Activated antioxidant enzymes then scavenge various reactive oxygen species, which eventually leads to stress tolerance in plants. Non-enzymatic antioxidants, such as ascorbate, metalloids, and even metal-binding peptides (metallothionein and phytochelatin) can also help to reduce metal toxicity in plants. Genetic engineering has been successfully used in rice and many other crops to increase metal tolerance and reduce heavy metals accumulation. A comprehensive understanding of uptake, transport, signaling, and tolerance mechanisms will help to grow rice plants in agricultural fields with less heavy metal accumulation in grains.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Soybean transporter database: A comprehensive database for identification and exploration of natural variants in soybean transporter genes

Transporters, a class of membrane proteins that facilitate exchange of solutes including diverse molecules and ions across the cellular membrane, are vital component for the survival of all organisms. Understanding plant transporters is important to get insight of the basic cellular processes, physiology, and molecular mechanisms including nutrient uptake, signaling, response to external stress, and many more. In this regard, extensive analysis of transporters predicted in soybean and other plant species was performed. In addition, an integrated database for soybean transporter protein, SoyTD, was developed that will facilitate the identification, classification, and extensive characterization of transporter proteins by integrating expression, gene ontology, conserved domain and motifs, gene structure organization, and chromosomal distribution features. A comprehensive analysis was performed to identify highly confident transporters by integrating various prediction tools. Initially, 7541 transmembrane (TM) proteins were predicted in the soybean genome; out of these, 3306 non-redundant transporter genes carrying two or more transmembrane domains were selected for further analysis. The identified transporter genes were classified according to a standard transporter classification (TC) system. Comparative analysis of transporter genes among 47 plant genomes provided insights into expansion and duplication of transporter genes in land plants. The whole genome resequencing (WGRS) and tissue-specific transcriptome datasets of soybean were integrated to investigate the natural variants and expression profile associated with transporter(s) of interest. Overall, SoyTD provides a comprehensive interface to study genetic and molecular function of soybean transporters. SoyTD is publicly available at http://artemis.cyverse.org/soykb_dev/SoyTD/.

Human Aquaporins: Functional Diversity and Potential Roles in Infectious and Non-infectious Diseases

Aquaporins (AQPs) are integral membrane proteins and found in all living organisms from bacteria to human. AQPs mainly involved in the transmembrane diffusion of water as well as various small solutes in a bidirectional manner are widely distributed in various human tissues. Human contains 13 AQPs (AQP0-AQP12) which are divided into three sub-classes namely orthodox aquaporin (AQP0, 1, 2, 4, 5, 6, and 8), aquaglyceroporin (AQP3, 7, 9, and 10) and super or unorthodox aquaporin (AQP11 and 12) based on their pore selectivity. Human AQPs are functionally diverse, which are involved in wide variety of non-infectious diseases including cancer, renal dysfunction, neurological disorder, epilepsy, skin disease, metabolic syndrome, and even cardiac diseases. However, the association of AQPs with infectious diseases has not been fully evaluated. Several studies have unveiled that AQPs can be regulated by microbial and parasitic infections that suggest their involvement in microbial pathogenesis, inflammation-associated responses and AQP-mediated cell water homeostasis. This review mainly aims to shed light on the involvement of AQPs in infectious and non-infectious diseases and potential AQPs-target modulators. Furthermore, AQP structures, tissue-specific distributions and their physiological relevance, functional diversity and regulations have been discussed. Altogether, this review would be useful for further investigation of AQPs as a potential therapeutic target for treatment of infectious as well as non-infectious diseases.

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.

Functional evolution of nodulin 26-like intrinsic proteins: from bacterial arsenic detoxification to plant nutrient transport

Nodulin 26-like intrinsic proteins (NIPs) play essential roles in transporting the nutrients silicon and boron in seed plants, but the evolutionary origin of this transport function and the co-permeability to toxic arsenic remains enigmatic. Horizontal gene transfer of a yet uncharacterised bacterial AqpN-aquaporin group was the starting-point for plant NIP evolution. We combined intense sequence, phylogenetic and genetic context analyses and a mutational approach with various transport assays in oocytes and plants to resolve the transorganismal and functional evolution of bacterial and algal and terrestrial plant NIPs and to reveal their molecular transport specificity features. We discovered that aqpN genes are prevalently located in arsenic resistance operons of various prokaryotic phyla. We provided genetic and functional evidence that these proteins contribute to the arsenic detoxification machinery. We identified NIPs with the ancestral bacterial AqpN selectivity filter composition in algae, liverworts, moss, hornworts and ferns and demonstrated that these archetype plant NIPs and their prokaryotic progenitors are almost impermeable to water and silicon but transport arsenic and boron. With a mutational approach, we demonstrated that during evolution, ancestral NIP selectivity shifted to allow subfunctionalisations. Together, our data provided evidence that evolution converted bacterial arsenic efflux channels into essential seed plant nutrient transporters.

In rose, transcription factor PTM balances growth and drought survival via PIP2;1 aquaporin

Plants have evolved sophisticated systems in response to environmental changes, and growth arrest is a common strategy used to enhance stress tolerance. Despite the growth-survival trade-off being essential to the shaping of plant productivity, the mechanisms balancing growth and survival remain largely unknown. Aquaporins play a crucial role in growth and stress responses by controlling water transport across membranes. Here, we show that RhPIP2;1, an aquaporin from rose (Rosa sp.), interacts with a membrane-tethered MYB protein, RhPTM. Water deficiency triggers nuclear translocation of the RhPTM C terminus. Silencing of RhPTM causes continuous growth under drought stress and a consequent decrease in survival rate. RNA sequencing (RNA-seq) indicated that RhPTM influences the expression of genes related to carbohydrate metabolism. Water deficiency induces phosphorylation of RhPIP2;1 at Ser 273, which is sufficient to promote nuclear translocation of the RhPTM C terminus. These results indicate that the RhPIP2;1-RhPTM module serves as a key player in orchestrating the trade-off between growth and stress survival in Rosa.

Silicon-mediated Improvement in Plant Salinity Tolerance: The Role of Aquaporins

Silicon (Si) is an abundant and differentially distributed element in soils that is believed to have important biological functions. However, the benefits of Si and its essentiality in plants are controversial due to differences among species in their ability to take up this element. Despite this, there is a consensus that the application of Si improves the water status of plants under abiotic stress conditions. Hence, plants treated with Si are able to maintain a high stomatal conductance and transpiration rate under salt stress, suggesting that a reduction in Na⁺ uptake occurs due to deposition of Si in the root. In addition, root hydraulic conductivity increases when Si is applied. As a result, a Si-mediated upregulation of aquaporin (PIP) gene expression is observed in relation to increased root hydraulic conductivity and water uptake. Aquaporins of the subclass nodulin 26-like intrinsic proteins are further involved in allowing Si entry into the cell. Therefore, on the basis of available published results and recent developments, we propose a model to explain how Si absorption alleviates stress in plants grown under saline conditions through the conjugated action of different aquaporins.

Essential and Beneficial Trace Elements in Plants, and Their Transport in Roots: a Review

The essentiality of 14 mineral elements so far have been reported in plant nutrition. Eight of these elements were known as micronutrients due to their lower concentrations in plants (usually ≤100 mg/kg/dw). However, it is still challenging to mention an exact number of plant micronutrients since some elements have not been strictly proposed yet either as essential or beneficial. Micronutrients participate in very diverse metabolic processes, including from the primary and secondary metabolism to the cell defense, and from the signal transduction to the gene regulation, energy metabolism, and hormone perception. Thus, the attempt to understand the molecular mechanism(s) behind their transport has great importance in terms of basic and applied plant sciences. Moreover, their deficiency or toxicity also caused serious disease symptoms in plants, even plant destruction if not treated, and many people around the world suffer from the plant-based dietary deficiencies or metal toxicities. In this sense, shedding some light on this issue, the 13 mineral elements (Fe, B, Cu, Mn, Mo, Si, Zn, Ni, Cl, Se, Na, Al, and Co), required by plants at trace amounts, has been reviewed with the primary focus on the transport proteins (transporters/channels) in plant roots. So, providing the compiled but extensive information about the structural and functional roles of micronutrient transport genes/proteins in plant roots.

Fungal aquaporins: cellular functions and ecophysiological perspectives

Three aspects have to be taken into consideration when discussing cellular water and solute permeability of fungal cells: cell wall properties, membrane permeability, and transport through proteinaceous pores (the main focus of this review). Yet, characterized major intrinsic proteins (MIPs) can be grouped into three functional categories: (mainly) water transporting aquaporins, aquaglyceroporins that confer preferentially solute permeability (e.g., glycerol and ammonia), and bifunctional aquaglyceroporins that can facilitate efficient water and solute transfer. Two ancestor proteins, a water (orthodox aquaporin) and a solute facilitator (aquaglyceroporin), are supposed to give rise to today's MIPs. Based on primary sequences of fungal MIPs, orthodox aquaporins/X-intrinsic proteins (XIPs) and FPS1-like/Yfl054-like/other aquaglyceroporins are supposed to be respective sister groups. However, at least within the fungal kingdom, no easy functional conclusion can be drawn from the phylogenetic position of a given protein within the MIP pedigree. In consequence, ecophysiological prediction of MIP relevance is not feasible without detailed functional analysis of the respective protein and expression studies. To illuminate the diverse MIP implications in fungal lifestyle, our current knowledge about protein function in two organisms, baker's yeast and the Basidiomycotic Laccaria bicolor, an ectomycorrhizal model fungus, was exemplarily summarized in this review. MIP function has been investigated in such a depth in Saccharomyces cerevisiae that a system-wide view is possible. Yeast lifestyle, however, is special in many circumstances. Therefore, L. bicolor as filamentous Basidiomycete was added and allows insight into a very different way of life. Special emphasis was laid in this review onto ecophysiological interpretation of MIP function.

Discovery of a multigene family of aquaporin silicon transporters in the primitive plant Equisetum arvense

Plants benefit greatly from silicon (Si) absorption provided that they contain Si transporters. The latter have recently been identified in the roots of some higher plants known to accumulate high concentrations of Si, and all share a high level of sequence identity. In this study, we searched for transporters in the primitive vascular plant Equisetum arvense (horsetail), which is a valuable but neglected model plant for the study of Si absorption, as it has one of the highest Si concentrations in the plant kingdom. Our initial attempts to identify Si transporters based on sequence homology with transporters from higher plants proved unsuccessful, suggesting a divergent structure or property in horsetail transporters. Subsequently, through sequencing of the horsetail root transcriptome and a search using amino acid sequences conserved in plant aquaporins, we were able to identify a multigene family of aquaporin Si transporters. Comparison of known functional domains and phylogenetic analysis of sequences revealed that the horsetail proteins belong to a different group than higher-plant Si transporters. In particular, the newly identified proteins contain a STAR pore as opposed to the GSGR pore common to all previously identified Si transporters. In order to determine its functionality, the proteins were heterologously expressed in both Xenopus oocytes and Arabidopsis, and the results showed that the horsetail proteins are extremely efficient a transporting Si. These findings offer new insights into the elusive properties of Si and its absorption by plants.

Vacuolar transporters in their physiological context

Vacuoles in vegetative tissues allow the plant surface to expand by accumulating energetically cheap inorganic osmolytes, and thereby optimize the plant for absorption of sunlight and production of energy by photosynthesis. Some specialized cells, such as guard cells and pulvini motor cells, exhibit rapid volume changes. These changes require the rapid release and uptake of ions and water by the vacuole and are a prerequisite for plant survival. Furthermore, seed vacuoles are important storage units for the nutrients required for early plant development. All of these fundamental processes rely on numerous vacuolar transporters. During the past 15 years, the transporters implicated in most aspects of vacuolar function have been identified and characterized. Vacuolar transporters appear to be integrated into a regulatory network that controls plant metabolism. However, little is known about the mode of action of these fundamental processes, and deciphering the underlying mechanisms remains a challenge for the future.

The role of nitric oxide in the germination of plant seeds and pollen

Two complex physiological processes, with opposite positions in the plant's life-cycle, seed and pollen germination, are vital to the accomplishment of successful plant growth and reproduction. This review summarizes the current state of knowledge of the intersection of NO signalling with the signalling pathways of ABA, GA, and ethylene; plant hormones that control the release of plant seeds from dormancy and germination. The cross-talk of NO and ROS is involved in the light- and hormone-specific regulation of seeds' developmental processes during the initiation of plant ontogenesis. Similarly to seed germination, the mechanisms of plant pollen hydration, germination, tube growth, as well as pollen-stigma recognition are tightly linked to the proper adjustment of NO and ROS levels. The interaction of NO with ROS and secondary messengers such as Ca(2+), cAMP and cGMP discovered in pollen represent a common mechanism of NO signalling. The involvement of NO in both breakpoints of plant physiology, as well as in the germination of spores within fungi and oomycetes, points toward NO as a component of an evolutionary conserved signalling pathway.