Overexpression of X Intrinsic Protein 1;1 in Nicotiana tabacum and Arabidopsis reduces boron allocation to shoot sink tissues

Overexpression of OsTIP1;2 confers arsenite tolerance in rice and reduces root-to-shoot translocation of arsenic

Tonoplast Intrinsic Proteins (TIPs) are vital in transporting water and solutes across vacuolar membrane. The role of TIPs in the arsenic stress response is largely undefined. Rice shows sensitivity to the arsenite [As[III]] stress and its accumulation at high concentrations in grains poses severe health hazards. In this study, functional characterization of OsTIP1;2 from Oryza sativa indica cultivar Pusa Basmati-1 (PB-1) was done under the As[III] stress. Overexpression of OsTIP1;2 in PB-1 rice conferred tolerance to As[III] treatment measured in terms of enhanced shoot growth, biomass, and shoot/root ratio of overexpression (OE) lines compared to the wild-type (WT) plants. Moreover, seed priming with the IRW100 yeast cells (deficient in vacuolar membrane As[III] transporter YCF1) expressing OsTIP1;2 further increased As[III] stress tolerance of both WT and OE plants. The dithizone assay showed that WT plants accumulated high arsenic in shoots, while OE lines accumulated more arsenic in roots than shoots thereby limiting the translocation of arsenic to shoot. The activity of enzymatic and non-enzymatic antioxidants also increased in the OE lines on exposure to As[III]. The tissue-specific localization showed OsTIP1;2 promoter activity in root and root hairs, indicating its possible root-specific function. After As[III] treatment in hydroponic medium, the arsenic translocation factor (TF) for WT was around 0.8, while that of OE lines was around 0.2. Moreover, the arsenic content in the grains of OE lines reduced significantly compared to WT plants.

Overexpression of MsNIP2 improves salinity tolerance in Medicago sativa

Alfalfa (Medicago sativa) is one of the most widely cultivated forage crops in the world. However, alfalfa yield and quality are adversely affected by salinity stress. Nodulin 26-like intrinsic proteins (NIPs) play essential roles in water and small molecules transport and response to salt stress. Here, we isolated a salt stress responsive MsNIP2 gene and demonstrated its functions by overexpression in alfalfa. The open reading frame of MsNIP2 is 816 bp in length, and it encodes 272 amino acids. It has six transmembrane domains and two NPA motifs. MsNIP2 showed high identity to other known NIP proteins, and its tertiary model was similar to the crystal structure of OsNIP2-1 (7cjs) tetramer. Subcellular localization analysis showed that MsNIP2 protein fused with green fluorescent protein (GFP) was localized to the plasma membrane. Transgenic alfalfa lines overexpressing MsNIP2 showed significantly higher height and branch number compared with the non-transgenic control. The POD and CAT activity of the transgenic alfalfa lines was significantly increased and their MDA content was notably reduced compared with the control group under the treatment of NaCl. The transgenic lines showed higher capability in scavenging oxygen radicals with lighter NBT staining than the control under salt stress. The transgenic lines showed relative lower water loss rate and electrolyte leakage, but relatively higher Na+ content than the control line under salt stress. The relative expression levels of abiotic-stress-related genes (MsHSP23, MsCOR47, MsATPase, and MsRD2) in three transgenic lines were compared with the control, among them, only the expression of MsCOR47 was up-regulated. Consequently, this study offers a novel perspective for exploring the function of MsNIP2 in improving salt tolerance of alfalfa.

Unveiling Innovative Approaches to Mitigate Metals/Metalloids Toxicity for Sustainable Agriculture

Due to anthropogenic activities, environmental pollution of heavy metals/metalloids (HMs) has increased and received growing attention in recent decades. Plants growing in HM-contaminated soils have slower growth and development, resulting in lower agricultural yield. Exposure to HMs leads to the generation of free radicals (oxidative stress), which alters plant morpho-physiological and biochemical pathways at the cellular and tissue levels. Plants have evolved complex defense mechanisms to avoid or tolerate the toxic effects of HMs, including HMs absorption and accumulation in cell organelles, immobilization by forming complexes with organic chelates, extraction via numerous transporters, ion channels, signaling cascades, and transcription elements, among others. Nonetheless, these internal defensive mechanisms are insufficient to overcome HMs toxicity. Therefore, unveiling HMs adaptation and tolerance mechanisms is necessary for sustainable agriculture. Recent breakthroughs in cutting-edge approaches such as phytohormone and gasotransmitters application, nanotechnology, omics, and genetic engineering tools have identified molecular regulators linked to HMs tolerance, which may be applied to generate HMs-tolerant future plants. This review summarizes numerous systems that plants have adapted to resist HMs toxicity, such as physiological, biochemical, and molecular responses. Diverse adaptation strategies have also been comprehensively presented to advance plant resilience to HMs toxicity that could enable sustainable agricultural production.

What Can Boron Deficiency Symptoms Tell Us about Its Function and Regulation?

On the eve of the 100th anniversary of Dr. Warington's discovery of boron (B) as a nutrient essential for higher plants, "boronists" have struggled to demonstrate a role beyond its structural function in cell walls dimerizing pectin molecules of rhamnogalacturonan II (RGII). In this regard, B deficiency has been associated with a plethora of symptoms in plants that include macroscopic symptoms like growth arrest and cell death and biochemical or molecular symptoms that include changes in cell wall pore size, apoplast acidification, or a steep ROS production that leads to an oxidative burst. Aiming to shed light on B functions in plant biology, we proposed here a unifying model integrating the current knowledge about B function(s) in plants to explain why B deficiency can cause such remarkable effects on plant growth and development, impacting crop productivity. In addition, based on recent experimental evidence that suggests the existence of different B ligands other than RGII in plant cells, namely glycolipids, and glycoproteins, we proposed an experimental pipeline to identify putative missing ligands and to determine how they would integrate into the above-mentioned model.

Genome-wide identification and characterization of aquaporins in mangrove plant Kandelia obovata and its role in response to the intertidal environment

Aquaporins (AQPs) play important roles in plant growth, development and tolerance to environmental stresses. To understand the role of AQPs in the mangrove plant Kandelia obovata, which has the ability to acquire water from seawater, we identified 34 AQPs in the K. obovata genome and analysed their structural features. Phylogenetic analysis revealed that KoAQPs are homologous to AQPs of Populus and Arabidopsis, which are evolutionarily conserved. The key amino acid residues were used to assess water-transport ability. Analysis of cis-acting elements in the promoters indicated that KoAQPs may be stress- and hormone-responsive. Subcellular localization of KoAQPs in yeast showed most KoAQPs function in the membrane system. That transgenic yeast with increased cell volume showed that some KoAQPs have significant water-transport activity, and the substrate sensitivity assay indicates that some KoAQPs can transport H₂ O₂ . The transcriptome data were used to analyze the expression patterns of KoAQPs in different tissues and developing fruits of K. obovata. In addition, real-time quantitative PCR analyses combined transcriptome data showed that KoAQPs have complex responses to environmental factors, including salinity, flooding and cold. Collectively, the transport of water and solutes by KoAQPs contributed to the adaptation of K. obovata to the coastal intertidal environment.

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.

Metalloid transporters and their regulation in plants

Transport of metalloids including B, Si, and As is mediated by a combination of channels and efflux transporters in plants, which are strictly regulated in response to environmental changes.

Regulation, Diversity and Evolution of Boron Transporters in Plants

Boron (B) is an essential trace element in plants, and borate cross-linking of pectic polysaccharide rhamnogalacturonan-II (RG-II) in cell walls is required for normal cell growth. High concentrations of B are toxic to cells. Therefore, plants need to control B transport to respond to B conditions in the environment. Over the past two decades, genetic analyses of Arabidopsis thaliana have revealed that B transport is governed by two types of membrane transport molecules: NIPs (nodulin-26-like intrinsic proteins), which facilitate boric acid permeation, and BORs, which export borate from cells. In this article, we review recent findings on the (i) regulation at the cell level, (ii) diversity among plant species and (iii) evolution of these B transporters in plants. We first describe the systems regulating these B transporters at the cell level, focusing on the molecular mechanisms underlying the polar localization of proteins and B-dependent expression, as well as their physiological significance in A. thaliana. Then, we examine the presence of homologous genes and characterize the functions of NIPs and BORs in B homeostasis, in a wide range of plant species, including Brassica napus, Oryza sativa and Zea mays. Finally, we discuss the evolutionary aspects of NIPs and BORs as B transporters, and the possible relationship between the diversification of B transport and the occurrence of RG-II in plants. This review considers the sophisticated systems of B transport that are conserved among various plant species, which were established to meet mineral nutrient requirements.

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.

Plant transporters involved in combating boron toxicity: beyond 3D structures

Membrane transporters control the movement and distribution of solutes, including the disposal or compartmentation of toxic substances that accumulate in plants under adverse environmental conditions. In this minireview, in the light of the approaching 100th anniversary of unveiling the significance of boron to plants (K. Warington, 1923; Ann. Bot.37, 629) we discuss the current state of the knowledge on boron transport systems that plants utilise to combat boron toxicity. These transport proteins include: (i) nodulin-26-like intrinsic protein-types of aquaporins, and (ii) anionic efflux (borate) solute carriers. We describe the recent progress made on the structure–function relationships of these transport proteins and point out that this progress is integral to quantitative considerations of the transporter's roles in tissue boron homeostasis. Newly acquired knowledge at the molecular level has informed on the transport mechanics and conformational states of boron transport systems that can explain their impact on cell biology and whole plant physiology. We expect that this information will form the basis for engineering transporters with optimised features to alleviate boron toxicity tolerance in plants exposed to suboptimal soil conditions for sustained food production.

Genome-wide association mapping identifies HvNIP2;2/HvLsi6 accounting for efficient boron transport in barley

Boron (B) is an essential mineral element for plant growth, and the seed B pool of crops can be crucial when seedlings need to establish on low-B soils. To date, it is poorly understood how B accumulation in grain crops is genetically controlled. Here, we assessed the genotypic variation of the B concentration in grains of a spring barley (Hordeum vulgare L.) association panel that represents broad genetic diversity. We found a large genetic variation of the grain B concentration and detected in total 23 quantitative trait loci (QTLs) using genome-wide association mapping. HvNIP2;2/HvLsi6, encoding a potential B-transporting membrane protein, mapped closely to a major-effect QTL accounting for the largest proportion of grain B variation. Based on transport studies using heterologous expression systems and gene expression analysis, we demonstrate that HvNIP2;2/HvLsi6 represents a functional B channel and that expression variation in its transcript level associates with root and shoot B concentrations as well as with root dry mass formation under B-deficient conditions.

Evolutionary and Predictive Functional Insights into the Aquaporin Gene Family in the Allotetraploid Plant Nicotiana tabacum

Aquaporins (AQPs) are a class of integral membrane proteins that facilitate the membrane diffusion of water and other small solutes. Nicotiana tabacum is an important model plant, and its allotetraploid genome has recently been released, providing us with the opportunity to analyze the AQP gene family and its evolution. A total of 88 full-length AQP genes were identified in the N. tabacum genome, and the encoding proteins were assigned into five subfamilies: 34 plasma membrane intrinsic proteins (PIPs); 27 tonoplast intrinsic proteins (TIPs); 20 nodulin26-like intrinsic proteins (NIPs); 3 small basic intrinsic proteins (SIPs); 4 uncharacterized X intrinsic proteins (XIPs), including two splice variants. We also analyzed the genomes of two N. tabacum ancestors, Nicotiana tomentosiformis and Nicotiana sylvestris, and identified 49 AQP genes in each species. Functional prediction, based on the substrate specificity-determining positions (SDPs), revealed significant differences in substrate specificity among the AQP subfamilies. Analysis of the organ-specific AQP expression levels in the N. tabacum plant and RNA-seq data of N. tabacum bright yellow-2 suspension cells indicated that many AQPs are simultaneously expressed, but differentially, according to the organs or the cells. Altogether, these data constitute an important resource for future investigations of the molecular, evolutionary, and physiological functions of AQPs in N. tabacum.

Genome Wild Analysis and Molecular Understanding of the Aquaporin Diversity in Olive Trees (Olea Europaea L.)

Cellular aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life, playing important roles in the uptake of water and many solutes across the membranes. In olive trees, AQP diversity, protein features and their biological functions are still largely unknown. This study focuses on the structure and functional and evolution diversity of AQP subfamilies in two olive trees, the wild species Olea europaea var. sylvestris (OeuAQPs) and the domesticated species Olea europaea cv. Picual (OleurAQPs), and describes their involvement in different physiological processes of early plantlet development and in biotic and abiotic stress tolerance in the domesticated species. A scan of genomes from the wild and domesticated olive species revealed the presence of 52 and 79 genes encoding full-length AQP sequences, respectively. Cross-genera phylogenetic analysis with orthologous clustered OleaAQPs into five established subfamilies: PIP, TIP, NIP, SIP, and XIP. Subsequently, gene structures, protein motifs, substrate specificities and cellular localizations of the full length OleaAQPs were predicted. Functional prediction based on the NPA motif, ar/R selectivity filter, Froger's and specificity-determining positions suggested differences in substrate specificities of Olea AQPs. Expression analysis of the OleurAQP genes indicates that some genes are tissue-specific, whereas few others show differential expressions at different developmental stages and in response to various biotic and abiotic stresses. The current study presents the first detailed genome-wide analysis of the AQP gene family in olive trees and it provides valuable information for further functional analysis to infer the role of AQP in the adaptation of olive trees in diverse environmental conditions in order to help the genetic improvement of domesticated olive trees.