Halophytic Hordeum brevisubulatum HbHAK1 Facilitates Potassium Retention and Contributes to Salt Tolerance

Understanding the salinity resilience and productivity of halophytes in saline environments

The escalating salinity levels in cultivable soil pose a significant threat to agricultural productivity and, consequently, human sustenance. This problem is being exacerbated by natural processes and human activities, coinciding with a period of rapid population growth. Developing halophytic crops is needed to ensure food security is not impaired and land resources can be used sustainably. Evolution has created many close halophyte relatives of our major glycophytic crops, such as Puccinellia tenuiflora (relative of barley and wheat), Oryza coarctata (relative of rice) and Glycine soja (relative of soybean). There are also some halophytes have been subjected to semi-domestication and are considered as minor crops, such as Chenopodium quinoa. In this paper, we examine the prevailing comprehension of robust salinity resilience in halophytes. We summarize the existing strategies and technologies that equip researchers with the means to enhance the salt tolerance capabilities of primary crops and investigate the genetic makeup of halophytes.

Genome-wide identification and expression analysis of the HAK/KUP/KT gene family in Moso bamboo

The K+ uptake permease/high-affinity K+/K+ transporter (KUP/HAK/KT) family is the most prominent group of potassium (K+) transporters, playing a key role in K+ uptake, transport, plant growth and development, and stress tolerance. However, the presence and functions of the KUP/HAK/KT family in Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), the fastest-growing plant, have not been studied. In this study, we identified 41 KUP/HAK/KT genes (PeHAKs) distributed across 18 chromosomal scaffolds of the Moso bamboo genome. PeHAK is a typical membrane protein with a conserved structural domain and motifs. Phylogenetic tree analysis classified PeHAKs into four distinct clusters, while collinearity analysis revealed gene duplications resulting from purifying selection, including both tandem and segmental duplications. Enrichment analysis of promoter cis-acting elements suggested their plausible role in abiotic stress response and hormone induction. Transcriptomic data and STEM analyses indicated that PeHAKs were involved in tissue and organ development, rapid growth, and responded to different abiotic stress conditions. Subcellular localization analysis demonstrated that PeHAKs are predominantly expressed at the cell membrane. In-situ PCR experiments confirmed that PeHAK was mainly expressed in the lateral root primordia. Furthermore, the involvement of PeHAKs in potassium ion transport was confirmed by studying the potassium ion transport properties of a yeast mutant. Additionally, through homology modeling, we revealed the structural properties of HAK as a transmembrane protein associated with potassium ion transport. This research provides a solid basis for understanding the classification, characterization, and functional analysis of the PeHAK family in Moso bamboo.

Positive Regulatory Roles of Manihot esculenta HAK5 under K+ Deficiency or High Salt Stress

HAK/KUP/KT family members have been identified as playing key roles in K+ uptake and salt tolerance in numerous higher plants. However, their functions in cassava (Manihot esculenta Cantz) remain unknown. In this study, a gene encoding for a high-affinity potassium transporter (MeHAK5) was isolated from cassava and its function was investigated. Subcellular localization analysis showed that MeHAK5 is a plasma membrane-localized transporter. RT-PCR and RT-qPCR indicated that MeHAK5 is predominantly expressed in cassava roots, where it is upregulated by low potassium or high salt; in particular, its highest expression levels separately increased by 2.2 and 2.9 times after 50 µM KCl and 150 mM NaCl treatments. When heterologously expressed in yeast, MeHAK5 mediated K+ uptake within the cells of the yeast strain CY162 and rescued the salt-sensitive phenotype of AXT3K yeast. MeHAK5 overexpression in transgenic Arabidopsis plants exhibited improved growth and increased shoot K+ content under low potassium conditions. Under salt stress, MeHAK5 transgenic Arabidopsis plants accumulated more K+ in the shoots and roots and had reduced Na+ content in the shoots. As a result, MeHAK5 transgenic Arabidopsis demonstrated a more salt-tolerant phenotype. These results suggest that MeHAK5 functions as a high-affinity K+ transporter under K+ starvation conditions, improving K+/Na+ homeostasis and thereby functioning as a positive regulator of salt stress tolerance in transgenic Arabidopsis. Therefore, MeHAK5 may be a suitable candidate gene for improving K+ utilization efficiency and salt tolerance.

Halophytes as new model plant species for salt tolerance strategies

Soil salinity is becoming a growing issue nowadays, severely affecting the world's most productive agricultural landscapes. With intersecting and competitive challenges of shrinking agricultural lands and increasing demand for food, there is an emerging need to build resilience for adaptation to anticipated climate change and land degradation. This necessitates the deep decoding of a gene pool of crop plant wild relatives which can be accomplished through salt-tolerant species, such as halophytes, in order to reveal the underlying regulatory mechanisms. Halophytes are generally defined as plants able to survive and complete their life cycle in highly saline environments of at least 200-500 mM of salt solution. The primary criterion for identifying salt-tolerant grasses (STGs) includes the presence of salt glands on the leaf surface and the Na⁺ exclusion mechanism since the interaction and replacement of Na⁺ and K⁺ greatly determines the survivability of STGs in saline environments. During the last decades or so, various salt-tolerant grasses/halophytes have been explored for the mining of salt-tolerant genes and testing their efficacy to improve the limit of salt tolerance in crop plants. Still, the utility of halophytes is limited due to the non-availability of any model halophytic plant system as well as the lack of complete genomic information. To date, although Arabidopsis (Arabidopsis thaliana) and salt cress (Thellungiella halophila) are being used as model plants in most salt tolerance studies, these plants are short-lived and can tolerate salinity for a shorter duration only. Thus, identifying the unique genes for salt tolerance pathways in halophytes and their introgression in a related cereal genome for better tolerance to salinity is the need of the hour. Modern technologies including RNA sequencing and genome-wide mapping along with advanced bioinformatics programs have advanced the decoding of the whole genetic information of plants and the development of probable algorithms to correlate stress tolerance limit and yield potential. Hence, this article has been compiled to explore the naturally occurring halophytes as potential model plant species for abiotic stress tolerance and to further breed crop plants to enhance salt tolerance through genomic and molecular tools.

Characterization of HAK protein family in Casuarina equisetifolia and the positive regulatory role of CeqHAK6 and CeqHAK11 genes in response to salt tolerance

The potassium transporter group of the HAK/KUP/KT (high-affinity K⁺)/KUP (K⁺ uptake)/KT (K⁺ transporter) family plays a crucial role in plant growth and development as well as in environmental adaptation such as tolerance to salt stress. HAK/KUP/KT genes and their functions have been characterized for a number of plant species, but they remain unknown for Casuarina equisetifolia, an important tree species for coastal protection in southern China and many other countries. In this study, 25 HAK genes were identified in the C. equisetifolia genome. Their gene structure, conserved motif, phylogeny, and expression were comprehensively and systematically analyzed to understand their functions. All HAK genes were relatively conserved and could be divided into four clusters. The expression level of two particular genes, CeqHAK11 and CeqHAK6, increased significantly with the duration of salt treatment. To further elucidated their function in response to salt stress, subcellular localization, and their functional analysis were developed. Results revealed that CeqHAK11 and CeqHAK6 were localized on the plasma membrane, which mainly mediated high-affinity K⁺ uptake. Overexpression of CeqHAK6 or CeqHAK11 in Arabidopsis showed higher germination and survival rates and longer root length than wild-type (WT) under salt stress, suggesting that both genes improve tolerance to salt stress. Moreover, CeqHAK6 and CeqHAK11 improved their ability to tolerate salt stress by increasing the K⁺/Na⁺ ratio and antioxidant enzyme activities (CAT, POD, and SOD), and decreasing reactive oxygen species (ROS) accumulation. Consequently, CeqHAK6 and CeqHAK11 were verified as potassium transport proteins and could be applied for further molecular breeding for salt tolerance in C. equisetifolia or other crops to increasing salt tolerance.

Wheat potassium transporter TaHAK13 mediates K+ absorption and maintains potassium homeostasis under low potassium stress

Potassium (K) is an essential nutrient for plant physiological processes. Members of the HAK/KUP/KT gene family act as potassium transporters, and the family plays an important role in potassium uptake and utilization in plants. In this study, the TaHAK13 gene was cloned from wheat and its function characterized. Real-time quantitative PCR (RT-qPCR) revealed that TaHAK13 expression was induced by environmental stress and up-regulated under drought (PEG6000), low potassium (LK), and salt (NaCl) stress. GUS staining indicated that TaHAK13 was mainly expressed in the leaf veins, stems, and root tips in Arabidopsis thaliana, and expression varied with developmental stage. TaHAK13 mediated K⁺ absorption when heterologously expressed in yeast CY162 strains, and its activity was slightly stronger than that of a TaHAK1 positive control. Subcellular localization analysis illustrated that TaHAK13 was located to the plasma membrane. When c(K⁺) ≤0.01 mM, the root length and fresh weight of TaHAK13 transgenic lines (athak5/TaHAK13, Col/TaHAK13) were significantly higher than those of non-transgenic lines (athak5, Col). Non-invasive micro-test technology (NMT) indicated that the net K influx of the transgenic lines was also higher than that of the non-transgenic lines. This suggests that TaHAK13 promotes K⁺ absorption, especially in low potassium media. Membrane-based yeast two-hybrid (MbY2H) and luciferase complementation assays (LCA) showed that TaHAK13 interacted with TaNPF5.10 and TaNPF6.3. Our findings have helped to clarify the biological functions of TaHAK13 and established a theoretical framework to dissect its function in wheat.

Proteomic Analysis of Roots Response to Potassium Deficiency and the Effect of TaHAK1-4A on K+ Uptake in Wheat

Potassium (K+) is essential for plant growth and stress responses. A deficiency in soil K+ contents can result in decreased wheat quality and productivity. Thus, clarifying the molecular mechanism underlying wheat responses to low-K+ (LK) stress is critical. In this study, a tandem mass tag (TMT)-based quantitative proteomic analysis was performed to investigate the differentially abundant proteins (DAPs) in roots of the LK-tolerant wheat cultivar “KN9204” at the seedling stage after exposure to LK stress. A total of 104 DAPs were identified in the LK-treated roots. The DAPs related to carbohydrate and energy metabolism, transport, stress responses and defense, and post-translational modifications under LK conditions were highlighted. We identified a high-affinity potassium transporter (TaHAK1-4A) that was significantly up-regulated after the LK treatment. Additionally, TaHAK1-4A was mainly expressed in roots, and the encoded protein was localized in the plasma membrane. The complementation assay in yeast suggested that TaHAK1-4A mediates K+ uptake under extreme LK conditions. The overexpression of TaHAK1-4A increased the fresh weight and root length of Arabidopsis under LK conditions and improved the growth of Arabidopsis athak5 mutant seedlings, which grow poorly under LK conditions. Moreover, silencing of TaHAK1-4A in wheat roots treated with LK stress decreased the root length, dry weight, K+ concentration, and K+ influx. Accordingly, TaHAK1-4A is important for the uptake of K+ by roots exposed to LK stress. Our results reveal the protein metabolic changes in wheat induced by LK stress. Furthermore, we identified a candidate gene potentially relevant for developing wheat lines with increased K+ use efficiency.

A Novel Transcriptional Regulator HbERF6 Regulates the HbCIPK2-Coordinated Pathway Conferring Salt Tolerance in Halophytic Hordeum brevisubulatum

Halophytic Hordeum brevisubulatum is a perennial grass which has evolved many distinctive salt-adaptive mechanisms. Our previous studies indicated it could thrive under salt stress through maintaining better K⁺ and Na⁺ homeostasis. Stress-responsive HbCIPK2 can phosphorylate K⁺ channel HbVGKC1 and Na⁺ transporter HbSOS1L to prevent Na⁺ accumulation and K⁺ reduction, hence pathway was not detected in glycophytic plants. In this study, we cloned the inducible promoter of HbCIPK2 by genome-walking, and identified a novel transcriptional regulator HbERF6 through yeast one-hybrid screening. HbERF6 functioned as a transcription factor which can bind to the GCC-box of the HbCIPK2 promoter to activate its expression. HbERF6 transgenic lines in Arabidopsis improved salt tolerance compared with wild type, and especially induced AtCIPK24 (SOS2) expression, resulting in K⁺/Na⁺ homeostasis to enhance salt tolerance. All the results confirmed the inducible function of HbERF6 for CIPK genes during salt tolerance. This regulatory network that integrates transcriptional regulation and post-translation modification will unravel a novel salt stress-responsive mechanism, highlighting the value and utilization of the halophytic resource.

Transcriptome and structure analysis in root of Casuarina equisetifolia under NaCl treatment

Background

High soil salinity seriously affects plant growth and development. Excessive salt ions mainly cause damage by inducing osmotic stress, ion toxicity, and oxidation stress. Casuarina equisetifolia is a highly salt-tolerant plant, commonly grown as wind belts in coastal areas with sandy soils. However, little is known about its physiology and the molecular mechanism of its response to salt stress.

Results

Eight-week-old C. equisetifolia seedlings grown from rooted cuttings were exposed to salt stress for varying durations (0, 1, 6, 24, and 168 h under 200 mM NaCl) and their ion contents, cellular structure, and transcriptomes were analyzed. Potassium concentration decreased slowly between 1 h and 24 h after initiation of salt treatment, while the content of potassium was significantly lower after 168 h of salt treatment. Root epidermal cells were shed and a more compact layer of cells formed as the treatment duration increased. Salt stress led to deformation of cells and damage to mitochondria in the epidermis and endodermis, whereas stele cells suffered less damage. Transcriptome analysis identified 10,378 differentially expressed genes (DEGs), with more genes showing differential expression after 24 h and 168 h of exposure than after shorter durations of exposure to salinity. Signal transduction and ion transport genes such as HKT and CHX were enriched among DEGs in the early stages (1 h or 6 h) of salt stress, while expression of genes involved in programmed cell death was significantly upregulated at 168 h, corresponding to changes in ion contents and cell structure of roots. Oxidative stress and detoxification genes were also expressed differentially and were enriched among DEGs at different stages.

Conclusions

These results not only elucidate the mechanism and the molecular pathway governing salt tolerance, but also serve as a basis for identifying gene function related to salt stress in C. equisetifolia.

Adaptive Mechanisms of Halophytes and Their Potential in Improving Salinity Tolerance in Plants

Soil salinization, which is aggravated by climate change and inappropriate anthropogenic activities, has emerged as a serious environmental problem, threatening sustainable agriculture and future food security. Although there has been considerable progress in developing crop varieties by introducing salt tolerance-associated traits, most crop cultivars grown in saline soils still exhibit a decline in yield, necessitating the search for alternatives. Halophytes, with their intrinsic salt tolerance characteristics, are known to have great potential in rehabilitating salt-contaminated soils to support plant growth in saline soils by employing various strategies, including phytoremediation. In addition, the recent identification and characterization of salt tolerance-related genes encoding signaling components from halophytes, which are naturally grown under high salinity, have paved the way for the development of transgenic crops with improved salt tolerance. In this review, we aim to provide a comprehensive update on salinity-induced negative effects on soils and plants, including alterations of physicochemical properties in soils, and changes in physiological and biochemical processes and ion disparities in plants. We also review the physiological and biochemical adaptation strategies that help halophytes grow and survive in salinity-affected areas. Furthermore, we illustrate the halophyte-mediated phytoremediation process in salinity-affected areas, as well as their potential impacts on soil properties. Importantly, based on the recent findings on salt tolerance mechanisms in halophytes, we also comprehensively discuss the potential of improving salt tolerance in crop plants by introducing candidate genes related to antiporters, ion transporters, antioxidants, and defense proteins from halophytes for conserving sustainable agriculture in salinity-prone areas.