Constitutive Contribution by the Rice OsHKT1;4 Na+ Transporter to Xylem Sap Desalinization and Low Na+ Accumulation in Young Leaves Under Low as High External Na+ Conditions

Functional analysis on the role of HvHKT1.4 in barley (Hordeum vulgare L.) salinity tolerance

High-affinity potassium transporters (HKTs) are well known proteins that govern the partitioning of Na+ between roots and shoots. Six HvHKTs were identified in barley and designated as HvHKT1.1, HvHKT1.3, HvHKT1.4, HvHKT1.5, HvHKT2.1 and HvHKT2.2 according to their similarity to previously reported OsHKTs. Among these HvHKTs, HvHKT1.4 was highly up-regulated under salinity stress in both leaves and roots of Golden Promise. Subcellular localization analysis showed that HvHKT1.4 is a plasma-membrane-localized protein. The knockout mutants of HvHKT1.4 showed greater salinity sensitivity and higher Na+ concentration in leaves than wild-type plants. Haplotype analysis of HvHKT1.4 in 344 barley accessions showed 15 single nucleotide substitutions in the CDS region, belonging to five haplotypes. Significant differences in mean salinity damage scores, leaf Na+ contents and Na+/K+ were found between Hap5 and other haplotypes with Hap5 showing better salinity tolerance. The results indicated that HvHKT1.4 can be an effective target in improving salinity tolerance through ion homeostasis.

Structures and ion transport mechanisms of plant high-affinity potassium transporters

Plant high-affinity K+ transporters (HKTs) mediate Na+ and K+ uptake, maintain Na+/K+ homeostasis, and therefore play crucial roles in plant salt tolerance. In this study, we present cryoelectron microscopy structures of HKTs from two classes, class I HKT1;1 from Arabidopsis thaliana (AtHKT1;1) and class II HKT2;1 from Triticum aestivum (TaHKT2;1), in both Na+- and K+-bound states at 2.6- to 3.0-Å resolutions. Both AtHKT1;1 and TaHKT2;1 function as homodimers. Each HKT subunit consists of four tandem domain units (D1-D4) with a repeated K+-channel-like M-P-M topology. In each subunit, D1-D4 assemble into an ion conduction pore with a pseudo-four-fold symmetry. Although both TaHKT2;1 and AtHKT1;1 have only one putative Na+ ion bound in the selectivity filter with a similar coordination pattern, the two HKTs display different K+ binding modes in the filter. TaHKT2;1 has three K+ ions bound in the selectivity filter, but AtHKT1;1 has only two K+ ions bound in the filter, which has a narrowed external entrance due to the presence of a Ser residue in the first filter motif. These structures, along with computational, mutational, and electrophysiological analyses, enable us to pinpoint key residues that are critical for the ion selectivity of HKTs. The findings provide new insights into the ion selectivity and ion transport mechanisms of plant HKTs and improve our understanding about how HKTs mediate plant salt tolerance and enhance crop growth.

Comparison between the impact of osmotic and NaCl treatments on the expression of genes coding for ion transporters in Oryza glaberrima Steud

We analyzed the expression of genes coding for Na+ transporters (OsHKT1.5, OsHKT1.1, OsSOS1, OsSOS2, OsNHX1, OsNHX2), Cl- transporter (OsNRT1, OsCLC, OsCCC1) and gene coding for the transcription factor DREB (OsDREB2) involved in response to desiccation in two cultivars of O. glaberrrima differing in salt-resistance (salt-tolerant cultivar (TOG5307) and salt-sensitive (TOG 5949)) exposed to NaCl, PEG or both agents present simultaneously. Seedlings were grown in iso-osmotic nutrient solution (Ψs = -0.47±0.02 MPa) containing PEG 6,000 12.9% (water stress), NaCl 75 mM (salt stress) and PEG 6.4% + NaCl 37.5 mM (MIX-treatment) during 1 and 7 days. Plants were analyzed for gene expression, mineral nutrients, and photosynthetic-related parameters. Na+ and Cl- accumulations in salt-treated plants were lower in roots and shoots of TOG5307 comparatively to TOG5949 while water content decreased in TOG5307. TOG5307 exhibited tolerance to water stress and maintained higher net photosynthesis and water use efficiency than TOG5949 in response to all treatments, but was less efficient for osmotic adjustment. Dehydration tolerance of TOG5307 involves a higher OsDREB2 expression. TOG5307 also exhibited a higher OsSOS1, OsSOS2, OsNHX1 and OsNHX2 expression than TOG5949 in response to salinity. OsHKT1.5 was slightly induced in the shoot. OsHKT1.1 was recorded in the shoots but remained undetectable in the roots. Chloride and sodium accumulations were strongly reduced in the shoots when PEG was present. Salinity resistance in Oryza glaberrima implies tolerance to dehydration as well as complementary strategies of Na+ exclusion through the SOS system and Na+ tolerance through vacuolar sequestration.

Genome-Wide Association Study Identified Candidate Genes for Alkalinity Tolerance in Rice

Alkalinity stress is a major hindrance to enhancing rice production globally due to its damaging effect on plants' growth and development compared with salinity stress. However, understanding of the physiological and molecular mechanisms of alkalinity tolerance is limited. Therefore, a panel of indica and japonica rice genotypes was evaluated for alkalinity tolerance at the seedling stage in a genome-wide association study to identify tolerant genotypes and candidate genes. Principal component analysis revealed that traits such as alkalinity tolerance score, shoot dry weight, and shoot fresh weight had the highest contribution to variations in tolerance, while shoot Na⁺ concentration, shoot Na⁺:K⁺ ratio, and root-to-shoot ratio had moderate contributions. Phenotypic clustering and population structure analysis grouped the genotypes into five subgroups. Several salt-susceptible genotypes such as IR29, Cocodrie, and Cheniere placed in the highly tolerant cluster suggesting different underlying tolerance mechanisms for salinity and alkalinity tolerance. Twenty-nine significant SNPs associated with alkalinity tolerance were identified. In addition to three alkalinity tolerance QTLs, qSNK4, qSNC9, and qSKC10, which co-localized with the earlier reported QTLs, a novel QTL, qSNC7, was identified. Six candidate genes that were differentially expressed between tolerant and susceptible genotypes were selected: LOC_Os04g50090 (Helix-loop-helix DNA-binding protein), LOC_Os08g23440 (amino acid permease family protein), LOC_Os09g32972 (MYB protein), LOC_Os08g25480 (Cytochrome P450), LOC_Os08g25390 (Bifunctional homoserine dehydrogenase), and LOC_Os09g38340 (C2H2 zinc finger protein). The genomic and genetic resources such as tolerant genotypes and candidate genes would be valuable for investigating the alkalinity tolerance mechanisms and for marker-assisted pyramiding of the favorable alleles for improving alkalinity tolerance at the seedling stage in rice.

Morpho-physiological, biochemical and molecular characterization of coastal rice landraces to identify novel genetic sources of salinity tolerance

Soil salinity is a leading cause for yield losses in rice, affecting nearly 6% of global rice cultivable area. India is host to a rich diversity of coastal rice landraces that are naturally tolerant to salinity and an untapped source to identify novel determinants of salinity tolerance. In the present study, we have assessed the relative salinity tolerance of 43 previously genotyped rice landraces at seedling stage, using thirteen morpho-physiological and biochemical parameters using a hydroponics system. Among 43 rice varieties, 25 were tolerant, 15 were moderately tolerant, 1 was moderately susceptible and 2 sensitive checks were found to be highly susceptible based on standard salinity scoring methods. In addition to previously known saline tolerant genotypes (Pokkali, FL478 and Nona Bokra), the present study has novel genotypes such as Katrangi, Orkyma, Aduisen 1, Orumundakan 1, Hoogla, and Talmugur 2 as potential sources of salinity tolerance through measurement of morpho-physiological and biochemical parameters including Na+, K+ estimations and Na+/K+ ratios. Further, Pallipuram Pokkali may be an important source of the tissue tolerance trait under salinity. Four marker trait associations (RM455-root Na+; RM161-shoot and root Na+/K+ ratios; RM237-salinity tolerance index) accounted for phenotypic variations in the range of 20.97-39.82%. A significant increase in root endodermal and exodermal suberization was observed in selected rice landraces under salinity. For the first time, variation in the number of suberized sclerenchymatous layers as well as passage cells is reported, in addition to expression level changes in suberin biosynthetic genes (CYP86A2, CYP81B1, CYP86A8 and PERL).

The outward shaker channel OsK5.2 improves plant salt tolerance by contributing to control of both leaf transpiration and K+ secretion into xylem sap

Soil salinity constitutes a major environmental constraint to crop production worldwide. Leaf K⁺ /Na⁺ homoeostasis, which involves regulation of transpiration, and thus of the xylem sap flow, and control of the ionic composition of the ascending sap, is a key determinant of plant salt tolerance. Here, we show, using a reverse genetics approach, that the outwardly rectifying K⁺ -selective channel OsK5.2, which is involved in both K⁺ release from guard cells for stomatal closure in leaves and K⁺ secretion into the xylem sap in roots, is a strong determinant of rice salt tolerance (plant biomass production and shoot phenotype under saline constraint). OsK5.2 expression was upregulated in shoots from the onset of the saline treatment, and OsK5.2 activity in guard cells led to a fast decrease in transpirational water flow and, therefore, reduced Na⁺ translocation to shoots. In roots, upon saline treatment, OsK5.2 activity in xylem sap K⁺ loading was maintained, and even transiently increased, outperforming the negative effect on K⁺ translocation to shoots resulting from the reduction in xylem sap flow. Thus, the overall activity of OsK5.2 in shoots and roots, which both reduces Na⁺ translocation to shoots and benefits shoot K⁺ nutrition, strongly contributes to leaf K⁺ /Na⁺ homoeostasis.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Comparative Analysis of Root Na+ Relation under Salinity between Oryza sativa and Oryza coarctata

Na⁺ toxicity is one of the major physiological constraints imposed by salinity on plant performance. At the same time, Na⁺ uptake may be beneficial under some circumstances as an easily accessible inorganic ion that can be used for increasing solute concentrations and maintaining cell turgor. Two rice species, Oryza sativa (cultivated rice, salt-sensitive) and Oryza coarctata (wild rice, salt-tolerant), demonstrated different strategies in controlling Na⁺ uptake. Glasshouse experiments and gene expression analysis suggested that salt-treated wild rice quickly increased xylem Na⁺ loading for osmotic adjustment but maintained a non-toxic level of stable shoot Na⁺ concentration by increased activity of a high affinity K⁺ transporter HKT1;5 (essential for xylem Na⁺ unloading) and a Na⁺/H⁺ exchanger NHX (for sequestering Na⁺ and K⁺ into root vacuoles). Cultivated rice prevented Na⁺ uptake and transport to the shoot at the beginning of salt treatment but failed to maintain it in the long term. While electrophysiological assays revealed greater net Na⁺ uptake upon salt application in cultivated rice, O. sativa plants showed much stronger activation of the root plasma membrane Na⁺/H⁺ Salt Overly Sensitive 1 (SOS1) exchanger. Thus, it appears that wild rice limits passive Na⁺ entry into root cells while cultivated rice relies heavily on SOS1-mediating Na⁺ exclusion, with major penalties imposed by the existence of the "futile cycle" at the plasma membrane.

To exclude or to accumulate? Revealing the role of the sodium HKT1;5 transporter in plant adaptive responses to varying soil salinity

Arid/semi-arid and coastal agricultural areas of the world are especially vulnerable to climate change-driven soil salinity. Salinity tolerance in plants is a complex trait, with salinity negatively affecting crop yield. Plants adopt a range of mechanisms to combat salinity, with many transporter genes being implicated in Na⁺-partitioning processes. Within these, the high-affinity K⁺ (HKT) family of transporters play a critical role in K⁺ and Na⁺ homeostasis in plants. Among HKT transporters, Type I transporters are Na⁺-specific. While Arabidopsis has only one Na ⁺ -specific HKT (AtHKT1;1), cereal crops have a multiplicity of Type I and II HKT transporters. AtHKT1; 1 (Arabidopsis thaliana) and HKT1; 5 (cereal crops) 'exclude' Na⁺ from the xylem into xylem parenchyma in the root, reducing shoot Na⁺ and hence, confer sodium tolerance. However, more recent data from Arabidopsis and crop species show that AtHKT1;1/HKT1;5 alleles have a strong genetic association with 'shoot sodium accumulation' and concomitant salt tolerance. The review tries to resolve these two seemingly contradictory effects of AtHKT1;1/HKT1;5 operation (shoot exclusion vs shoot accumulation), both conferring salinity tolerance and suggests that contrasting phenotypes are attributable to either hyper-functional or weak AtHKT1;1/HKT1;5 alleles/haplotypes and are under strong selection by soil salinity levels. It also suggests that opposite balancing mechanisms involving xylem ion loading in these contrasting phenotypes exist that require transporters such as SOS1 and CCC. While HKT1; 5 is a crucial but not sole determinant of salinity tolerance, investigation of the adaptive benefit(s) conferred by naturally occurring intermediate HKT1;5 alleles will be important under a climate change scenario.

An Overview of Abiotic Stress in Cereal Crops: Negative Impacts, Regulation, Biotechnology and Integrated Omics

Abiotic stresses (AbS), such as drought, salinity, and thermal stresses, could highly affect the growth and development of plants. For decades, researchers have attempted to unravel the mechanisms of AbS for enhancing the corresponding tolerance of plants, especially for crop production in agriculture. In the present communication, we summarized the significant factors (atmosphere, soil and water) of AbS, their regulations, and integrated omics in the most important cereal crops in the world, especially rice, wheat, sorghum, and maize. It has been suggested that using systems biology and advanced sequencing approaches in genomics could help solve the AbS response in cereals. An emphasis was given to holistic approaches such as, bioinformatics and functional omics, gene mining and agronomic traits, genome-wide association studies (GWAS), and transcription factors (TFs) family with respect to AbS. In addition, the development of omics studies has improved to address the identification of AbS responsive genes and it enables the interaction between signaling pathways, molecular insights, novel traits and their significance in cereal crops. This review compares AbS mechanisms to omics and bioinformatics resources to provide a comprehensive view of the mechanisms. Moreover, further studies are needed to obtain the information from the integrated omics databases to understand the AbS mechanisms for the development of large spectrum AbS-tolerant crop production.

Plant HKT Channels: An Updated View on Structure, Function and Gene Regulation

HKT channels are a plant protein family involved in sodium (Na⁺) and potassium (K⁺) uptake and Na⁺-K⁺ homeostasis. Some HKTs underlie salt tolerance responses in plants, while others provide a mechanism to cope with short-term K⁺ shortage by allowing increased Na⁺ uptake under K⁺ starvation conditions. HKT channels present a functionally versatile family divided into two classes, mainly based on a sequence polymorphism found in the sequences underlying the selectivity filter of the first pore loop. Physiologically, most class I members function as sodium uniporters, and class II members as Na⁺/K⁺ symporters. Nevertheless, even within these two classes, there is a high functional diversity that, to date, cannot be explained at the molecular level. The high complexity is also reflected at the regulatory level. HKT expression is modulated at the level of transcription, translation, and functionality of the protein. Here, we summarize and discuss the structure and conservation of the HKT channel family from algae to angiosperms. We also outline the latest findings on gene expression and the regulation of HKT channels.

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.

Functional analysis of TmHKT1;4-A2 promoter through deletion analysis provides new insight into the regulatory mechanism underlying abiotic stress adaptation

Bioinformatic, molecular, and biochemical analysis were performed to get more insight into the regulatory mechanism by which TmHKT1;4-A2 is regulated. HKT transporters from different plant species have been shown to play important role in plant response to salt. In previous work, TmHKT1;4-A2 gene from Triticum monococcum has been characterized as a major gene for Nax1 QTL (Tounsi et al. Plant Cell Physiol 57:2047-2057, 2016). So far, little is known about its regulatory mechanism. In this study, the promoter region of TmHKT1;4-A2 (1400 bp) was isolated and considered as the full-length promoter (PA2-1400). In silico analysis revealed the presence of important cis-acting elements related to abiotic stresses and phytohormones. Interestingly, our real-time RT-PCR analysis provided evidence that TmHKT1;4-A2 is regulated not only by salt stress but also by osmotic, heavy metal, oxidative, and hormones stresses. In transgenic Arabidopsis plants, TmHKT1;4-A2 is strongly active in vascular tissues of roots and leaves. Through 5'-end deletion analysis, we showed that PA2-1400 promoter is able to drive strong GUS activity under normal conditions and in response to different stresses compared to PA2-824 and PA2-366 promoters. These findings provide new information on the regulatory mechanism of TmHKT1;4-A2 and shed more light on its role under different stresses.

Investigation of Salt Tolerance Mechanisms Across a Root Developmental Gradient in Almond Rootstocks

The intensive use of groundwater in agriculture under the current climate conditions leads to acceleration of soil salinization. Given that almond is a salt-sensitive crop, selection of salt-tolerant rootstocks can help maintain productivity under salinity stress. Selection for tolerant rootstocks at an early growth stage can reduce the investment of time and resources. However, salinity-sensitive markers and salinity tolerance mechanisms of almond species to assist this selection process are largely unknown. We established a microscopy-based approach to investigate mechanisms of stress tolerance in and identified cellular, root anatomical, and molecular traits associated with rootstocks exhibiting salt tolerance. We characterized three almond rootstocks: Empyrean-1 (E1), Controller-5 (C5), and Krymsk-86 (K86). Based on cellular and molecular evidence, our results show that E1 has a higher capacity for salt exclusion by a combination of upregulating ion transporter expression and enhanced deposition of suberin and lignin in the root apoplastic barriers, exodermis, and endodermis, in response to salt stress. Expression analyses revealed differential regulation of cation transporters, stress signaling, and biopolymer synthesis genes in the different rootstocks. This foundational study reveals the mechanisms of salinity tolerance in almond rootstocks from cellular and structural perspectives across a root developmental gradient and provides insights for future screens targeting stress response.