Adaptation of plants to salt stress: the role of the ion transporters

Mechanisms by Which Exogenous Substances Enhance Plant Salt Tolerance through the Modulation of Ion Membrane Transport and Reactive Oxygen Species Metabolism

Soil salinization is one of the major abiotic stresses affecting plant growth and development. Plant salt tolerance is controlled by complex metabolic pathways. Exploring effective methods and mechanisms to improve crop salt tolerance has been a key aspect of research on the utilization of saline soil. Exogenous substances, such as plant hormones and signal transduction substances, can regulate ion transmembrane transport and eliminate reactive oxygen species (ROS) to reduce salt stress damage by activating various metabolic processes. In this review, we summarize the mechanisms by which exogenous substances regulate ion transmembrane transport and ROS metabolism to improve plant salt tolerance. The molecular and physiological relationships among exogenous substances in maintaining the ion balance and enhancing ROS clearance are examined, and trends and research directions for the application of exogenous substances for improving plant salt tolerance are proposed.

Combined transcriptomics and metabolomics analysis reveals salinity stress specific signaling and tolerance responses in the seagrass Zostera japonica

KEY MESSAGE

Multiple regulatory pathways of Zostera japonica to salt stress were identified through growth, physiological, transcriptomic and metabolomic analyses. Seagrasses are marine higher submerged plants that evolved from terrestrial monocotyledons and have fully adapted to the high saline seawater environment during the long evolutionary process. As one of the seagrasses growing in the intertidal zone, Zostera japonica not only has the ability to quickly adapt to short-term salt stress but can also survive at salinities ranging from the lower salinity of the Yellow River estuary to the higher salinity of the bay, making it a good natural model for studying the mechanism underlying the adaptation of plants to salt stress. In this work, we screened the growth, physiological, metabolomic, and transcriptomic changes of Z. japonica after a 5-day exposure to different salinities. We found that high salinity treatment impeded the growth of Z. japonica, hindered its photosynthesis, and elicited oxidative damage, while Z. japonica increased antioxidant enzyme activity. At the transcriptomic level, hypersaline stress greatly reduced the expression levels of photosynthesis-related genes while increasing the expression of genes associated with flavonoid biosynthesis. Meanwhile, the expression of candidate genes involved in ion transport and cell wall remodeling was dramatically changed under hypersaline stress. Moreover, transcription factors signaling pathways such as mitogen-activated protein kinase (MAPK) were also significantly influenced by salt stress. At the metabolomic level, Z. japonica displayed an accumulation of osmolytes and TCA mediators under hypersaline stress. In conclusion, our results revealed a complex regulatory mechanism in Z. japonica under salt stress, and the findings will provide important guidance for improving salt resistance in crops.

Revealing the Mechanism of Aroma Production Driven by High Salt Stress in Trichomonascus ciferrii WLW

Douchi is a Chinese traditional fermented food with a unique flavor. Methyl anthranilate (MA) plays an important role in formation of this flavor. However, the complicated relationship between the MA formation and the metabolic mechanism of the key functional microorganisms remains unclear. Here, we elucidated the response mechanism of aroma production driven by high salt stress in Trichomonascus ciferrii WLW (T. ciferrii WLW), which originates from the douchi fermentation process. The highest production of MA was obtained in a 10% NaCl environment. The enhanced expression of the key enzyme genes of the pentose phosphate pathway and shikimic acid pathway directed carbon flow toward aromatic amino acid synthesis and helped sustain an increased expression of metK to synthesize a large amount of the methyl donor S-adenosylmethionine, which promoted methyl anthranilate yield. This provides a theoretical basis for in-depth research on the applications of the flavor formation mechanisms of fermented foods.

An insight into the different responses to salt stress in growth characteristics of two legume species during seedling growth

Legumes play a crucial role in the restoration and utilization of salinized grassland. To explore the physiological response mechanism of Astragalus membranaceus and Medicago sativa seedlings to salt stress, salt stress culture experiments with five NaCl concentration treatments (0 mmol/L, 50 mmol/L, 100 mmol/L, 200 mmol/L, and 300 mmol/L) were conducted on these two legume seedlings. Morphological characteristics, physiological features, biomass, and the protective enzyme system were measured for both seedlings. Correlation analysis, principal component analysis (PCA), and membership function analysis (MFA) were conducted for each index. Structural equation modeling (SEM) was employed to analyze the salt stress pathways of plants. The results indicated that number of primary branches (PBN), ascorbate peroxidase (APX) activity in stems and leaves, catalase (CAT) activity in roots, etc. were identified as the primary indicators for evaluating the salt tolerance of A. membranaceus during its seedling growth period. And CAT and peroxidase (POD) activity in roots, POD and superoxide dismutase (SOD) activity in stems and leaves, etc. were identified as the primary indicators for evaluating the salt tolerance of M. sativa during its growth period. Plant morphological characteristics, physiological indexes, and underground biomass (UGB) were directly affected by salinity, while physiological indexes indirectly affected the degree of leaf succulence (LSD). Regarding the response of the protective enzyme system to salt stress, the activity of POD and APX increased in A. membranaceus, while the activity of CAT increased in M. sativa. Our findings suggest that salt stress directly affects the growth strategies of legumes. Furthermore, the response of the protective enzyme system and potential cell membrane damage to salinity were very different in the two legumes.

Improving the genetic potential of okra (Abelmoschus esculentus L.) germplasm to tolerate salinity stress

Okra (Abelmoschus esculentus L.) is the most consumed vegetable worldwide with the potential for diverse ecological adaptation. However, increasing salinization and changing climatic conditions are posing serious threats to the growth, yield, and quality of okra. Therefore, to mitigate increasing soil salinization and ensure sustainable okra production under rapidly changing climatic conditions, evaluation of new okra germplasm to develop salt tolerant cultivars is direly needed. The present study was designed to evaluate the genetic resources of okra genotypes for salt tolerance at growth and reproductive phases. Based on mophological and physio-biochemical responses of plants under stress condition, genotypes were divided into salt tolerant and succeptible groups. The experiment was comprised of 100 okra genotypes and each genotype was grown under control conditions and 6.5 dS m-1 NaCl concentration in a pot having 10 kg capacity. The experiment was conducted in a completely randomized design and each treatment was replicated three times. The results showed vast genetic variability among the evaluated okra germplasm traits like days to emergence, pod length, pod diameter, plant height, stem girth, and other yield-related parameters. Correlation analysis showed a highly significant positive association among the number of leaves at first flower and plant height at first flower.Likewise, pod weight also revealed a highly significant positive relationship for pod weight plant-1, pod length, and K+: Na+. Principal Component Analysis (PCA) revealed that out of 16 principal components (PCs), five components showed more than one eigenvalue and the first six PCs contributed 67.2% of the variation. Bi-plot analysis illustrated that genotypes 95, 111, 133, 99, and 128, under salt stress conditions, exhibited both high yield per plant and salt-tolerant behavior in other yield-related traits. On the basis of all studied traits, a salt susceptible group and a salt-tolerant group were formed. The salt tolerant group comprised of 97, 68, 95, 114, 64, 99, 111, 133, 128, and 109 genotypes, whereas, the salt susceptible group contained 137, 139, 130, 94, and 125 genotypes. Salt-tolerant okra genotypes were suggested to be used in further breeding programs aimed to develop salt tolerance in okra. These insights will empower precision breeding, underscore the importance of genetic diversity, and bear the potential to address the challenges of salt-affected soils while promoting broader agricultural resilience, economic prosperity, and food security.

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 Analysis of MBF1 Family Genes in Five Solanaceous Plants and Functional Analysis of SlER24 in Salt Stress

Multiprotein bridging factor 1 (MBF1) is an ancient family of transcription coactivators that play a crucial role in the response of plants to abiotic stress. In this study, we analyzed the genomic data of five Solanaceae plants and identified a total of 21 MBF1 genes. The expansion of MBF1a and MBF1b subfamilies was attributed to whole-genome duplication (WGD), and the expansion of the MBF1c subfamily occurred through transposed duplication (TRD). Collinearity analysis within Solanaceae species revealed collinearity between members of the MBF1a and MBF1b subfamilies, whereas the MBF1c subfamily showed relative independence. The gene expression of SlER24 was induced by sodium chloride (NaCl), polyethylene glycol (PEG), ABA (abscisic acid), and ethrel treatments, with the highest expression observed under NaCl treatment. The overexpression of SlER24 significantly enhanced the salt tolerance of tomato, and the functional deficiency of SlER24 decreased the tolerance of tomato to salt stress. SlER24 enhanced antioxidant enzyme activity to reduce the accumulation of reactive oxygen species (ROS) and alleviated plasma membrane damage under salt stress. SlER24 upregulated the expression levels of salt stress-related genes to enhance salt tolerance in tomato. In conclusion, this study provides basic information for the study of the MBF1 family of Solanaceae under abiotic stress, as well as a reference for the study of other plants.

Association of jasmonic acid priming with multiple defense mechanisms in wheat plants under high salt stress

Salinity is a global conundrum that negatively affects various biometrics of agricultural crops. Jasmonic acid (JA) is a phytohormone that reinforces multilayered defense strategies against abiotic stress, including salinity. This study investigated the effect of JA (60 μM) on two wheat cultivars, namely ZM9 and YM25, exposed to NaCl (14.50 dSm^-1) during two consecutive growing seasons. Morphologically, plants primed with JA enhanced the vegetative growth and yield components. The improvement of growth by JA priming is associated with increased photosynthetic pigments, stomatal conductance, intercellular CO₂, maximal photosystem II efficiency, and transpiration rate of the stressed plants. Furthermore, wheat cultivars primed with JA showed a reduction in the swelling of the chloroplast, recovery of the disintegrated thylakoids grana, and increased plastoglobuli numbers compared to saline-treated plants. JA prevented dehydration of leaves by increasing relative water content and water use efficiency via reducing water and osmotic potential using proline as an osmoticum. There was a reduction in sodium (Na⁺) and increased potassium (K⁺) contents, indicating a significant role of JA priming in ionic homeostasis, which was associated with induction of the transporters, viz., SOS1, NHX2, and HVP1. Exogenously applied JA mitigated the inhibitory effect of salt stress in plants by increasing the endogenous levels of cytokinins and indole acetic acid, and reducing the abscisic acid (ABA) contents. In addition, the oxidative stress caused by increasing hydrogen peroxide in salt-stressed plants was restrained by JA, which was associated with increased α-tocopherol, phenolics, and flavonoids levels and triggered the activities of superoxide dismutase and ascorbate peroxidase activity. This increase in phenolics and flavonoids could be explained by the induction of phenylalanine ammonia-lyase activity. The results suggest that JA plays a key role at the morphological, biochemical, and genetic levels of stressed and non-stressed wheat plants which is reflected in yield attributes. Hierarchical cluster analysis and principal component analyses showed that salt sensitivity was associated with the increments of Na⁺, hydrogen peroxide, and ABA contents. The regulatory role of JA under salinity stress was interlinked with increased JA level which consequentially improved ion transporting, osmoregulation, and antioxidant defense.