Comparative Physiological and Transcriptome Profiles Uncover Salt Tolerance Mechanisms in Alfalfa

Physiological and transcriptomic analyses reveal the molecular mechanism of PsAMT1.2 in salt tolerance

Soil salinization has become a global problem and high salt concentration in soil negatively affects plant growth. In our previous study, we found that overexpression of PsAMT1.2 from Populus simonii could improve the salt tolerance of poplar, but the physiological and molecular mechanism was not well understood. To explore the regulation pathway of PsAMT1.2 in salt tolerance, we investigated the morphological, physiological and transcriptome differences between the PsAMT1.2 overexpression transgenic poplar and the wild type under salt stress. The PsAMT1.2 overexpression transgenic poplar showed better growth with increased net photosynthetic rate and higher chlorophyll content compared with wild type under salt stress. The overexpression of PsAMT1.2 increased the catalase, superoxide dismutase, peroxidase and ascorbate peroxidase activities, and therefore probably enhanced the reactive oxygen species clearance ability, which also reduced the degree of membrane lipid peroxidation under salt stress. Meanwhile, the PsAMT1.2 overexpression transgenic poplar maintained a relatively high K+/Na+ ratio under salt stress. RNA-seq analysis indicated that PsAMT1.2 might improve plant salt tolerance by regulating pathways related to the photosynthetic system, chloroplast structure, antioxidant activity and anion transport. Among the 1056 differentially expressed genes, genes related to photosystem I and photosystem II were up-regulated and genes related to chloride channel protein-related were down-regulated. The result of the present study would provide new insight into regulation mechanism of PsAMT1.2 in improving salt tolerance of poplar.

Integrative physiology and transcriptome sequencing reveal differences between G. hirsutum and G. barbadense in response to salt stress and the identification of key salt tolerance genes

BACKGROUND

Soil salinity is one of the major abiotic stresses that threatens crop growth. Cotton has some degree of salt tolerance, known as the "pioneer crop" of saline-alkali land. Cultivation of cotton is of great significance to the utilization of saline-alkali land and the development of cotton industry. Gossypium hirsutum and G. barbadense, as two major cotton species, are widely cultivated worldwide. However, until recently, the regulatory mechanisms and specific differences of their responses to salt stress have rarely been reported.

RESULTS

In this study, we comprehensively compared the differences in the responses of G. hirsutum acc. TM-1 and G. barbadense cv. Hai7124 to salt stress. The results showed that Hai7124 exhibited better growth than did TM-1 under salt stress, with greater PRO content and antioxidant capability, whereas TM-1 only presented greater K+ content. Transcriptome analysis revealed significant molecular differences between the two cotton species in response to salt stress. The key pathways of TM-1 induced by salt are mainly related to growth and development, such as porphyrin metabolism, DNA replication, ribosome and photosynthesis. Conversely, the key pathways of Hai7124, such as plant hormone signal transduction, MAPK signaling pathway-plant, and phenylpropanoid biosynthesis, are mainly related to plant defense. Further comparative analyses of differentially expressed genes (DEGs) revealed that antioxidant metabolism, abscisic acid (ABA) and jasmonic acid (JA) signalling pathways were more strongly activated in Hai7124, whereas TM-1 was more active in K+ transporter-related genes and ethylene (ETH) signalling pathway. These differences underscore the various molecular strategies adopted by the two cotton species to navigate through salt stress, and Hai7124 responded more strongly to salt stress, which explains the potential reasons for the greater salt tolerance of Hai7124. Finally, we identified 217 potential salt tolerance-related genes, 167 of which overlapped with the confidence intervals of significant SNPs identified in previous genome-wide association studies (GWASs), indicating the high reliability of these genes.

CONCLUSIONS

These findings provide new insights into the differences in the regulatory mechanisms of salt tolerance between G. hirsutum and G. barbadense, and identify key candidate genes for salt tolerance molecular breeding in cotton.

Transcriptome Analysis for Salt-Responsive Genes in Two Different Alfalfa (Medicago sativa L.) Cultivars and Functional Analysis of MsHPCA1

Alfalfa (Medicago sativa L.) is an important forage legume and soil salinization seriously affects its growth and yield. In a previous study, we identified a salt-tolerant variety 'Gongnong NO.1' and a salt-sensitive variety 'Sibeide'. To unravel the molecular mechanism involved in salt stress, we conducted transcriptomic analysis on these two cultivars grown under 0 and 250 mM NaCl treatments for 0, 12, and 24 h. Totals of 336, and 548 differentially expressed genes (DEGs) in response to NaCl were, respectively, identified in the 'Gongnong NO.1' and 'Sibeide' varieties. The Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) pathway enrichment analysis showed that the DEGs were classified in carbohydrate metabolism, energy production, transcription factor, and stress-associated pathway. Expression of MsHPCA1, encoding a putative H2O2 receptor, was responsive to both NaCl and H2O2 treatment. MsHPCA1 was localized in cell membrane and overexpression of MsHPCA1 in alfalfa increased salt tolerance and H2O2 content. This study will provide new gene resources for the improvement in salt tolerance in alfalfa and legume crops, which has important theoretical significance and potential application value.

MsSPL12 is a positive regulator in alfalfa (Medicago sativa L.) salt tolerance

KEY MESSAGE

Over expression of MsSPL12 improved alfalfa salt tolerance by reducing Na+ accumulation and increasing antioxidant enzyme activity and regulating down-stream gene expression. Improvement of salt tolerance is one of the major goals in alfalfa breeding. Here, we demonstrated that MsSPL12, an alfalfa transcription factor gene highly expressed in the stem cells, plays a positive role in alfalfa salt tolerance. MsSPL12 is localized in the nucleus and shows transcriptional activity in the presence of its C-terminus. To investigate MsSPL12 function in plant response to salt stress, we generated transgenic plants overexpressing either MsSPL12 or a chimeric MsSPL12-SRDX gene that represses the function of MsSPL12 by using the Chimeric REpressor gene-Silencing Technology (CRES-T), and observed that overexpression of MsSPL12 increased the salt tolerance of alfalfa transgenic plants associated with an increase in K+/Na+ ratio and relative water content (RWC) under salt stress treatment, but a reduction in electrolyte leakage (EL), reactive oxygen species (ROS), malondialdehyde (MDA), and proline (Pro) compared to wild type (WT) plants. However, transgenic plants overexpressing MsSPL12-SRDX showed an inhibited plant growth and a reduced salt tolerance. RNA-sequencing and quantitative real-time PCR analyses revealed that MsSPL12 affected the expression of plant abiotic resistance-related genes in multiple physiological pathways. The potential MsSPL12-mediated regulatory pathways based on the differentially expressed genes between the MsSPL12 overexpression transgenics and WT controls were predicted. In summary, our study proves that MsSPL12 is a positive regulator in alfalfa salt tolerance and can be used as a new candidate for manipulation to develop forage crops with enhanced salt tolerance.

Elucidation of Physiological, Transcriptomic and Metabolomic Salinity Response Mechanisms in Medicago sativa

Alfalfa (Medicago sativa L.) is a widely grown perennial leguminous forage crop with a number of positive attributes. However, despite its moderate ability to tolerate saline soils, which are increasing in prevalence worldwide, it suffers considerable yield declines under these growth conditions. While a general framework of the cascade of events involved in plant salinity response has been unraveled in recent years, many gaps remain in our understanding of the precise molecular mechanisms involved in this process, particularly in non-model yet economically important species such as alfalfa. Therefore, as a means of further elucidating salinity response mechanisms in this species, we carried out in-depth physiological assessments of M. sativa cv. Beaver, as well as transcriptomic and untargeted metabolomic evaluations of leaf tissues, following extended exposure to salinity (grown for 3-4 weeks under saline treatment) and control conditions. In addition to the substantial growth and photosynthetic reductions observed under salinity treatment, we identified 1233 significant differentially expressed genes between growth conditions, as well as 60 annotated differentially accumulated metabolites. Taken together, our results suggest that changes to cell membranes and walls, cuticular and/or epicuticular waxes, osmoprotectant levels, antioxidant-related metabolic pathways, and the expression of genes encoding ion transporters, protective proteins, and transcription factors are likely involved in alfalfa's salinity response process. Although some of these alterations may contribute to alfalfa's modest salinity resilience, it is feasible that several may be disadvantageous in this context and could therefore provide valuable targets for the further improvement of tolerance to this stress in the future.

Transcriptome and Metabolome Analysis of Selenium Treated Alfalfa Reveals Influence on Phenylpropanoid Biosynthesis to Enhance Growth

Selenium (Se) plays an important role in the growth of plants. Alfalfa (Medicago sativa L.) is a perennial legume forage crop with high nutritional value and Se-rich functions. Many studies have shown that selenium can promote alfalfa growth, but few have explored the molecular biology mechanisms behind this effect. In this study, alfalfa was divided into two groups. One group was sprayed with sodium selenite (Na2SeO3) and the other group was sprayed with distilled water as a control. This study determined the growth, reproductive traits, physiological changes, transcriptome and metabolome of both groups of alfalfa. We found that foliar spraying of 100 mg/L Na2SeO3 could significantly increase the growth rate, dry weight, total Se content, amount of pollen per flower, pollen viability, pod spirals, and seed number per pod of alfalfa plants. The level of chlorophyll, soluble protein, proline, and glutathione also increased dramatically in Na2SeO3-sprayed alfalfa seedlings. After transcriptome and metabolome analysis, a total of 614 differentially expressed genes (DEGs) and 1500 differentially expressed metabolites (DEMs), including 26 secondary differentially metabolites were identified. The DEGs were mainly enriched in MAPK signaling pathway, phenylpropanoid biosynthesis, isoflavonoid biosynthesis, cutin, suberine, and wax biosynthesis, and glycerolipid metabolism. The DEMs were mainly enriched in flavone and flavonol biosynthesis, carbon metabolism, glyoxylate and dicarboxylate metabolism, nitrogen metabolism, and phenylpropanoid biosynthesis. Integrative analysis of transcriptome and metabolome showed that the foliar spraying of Na2SeO3 mainly affects phenylpropanoid biosynthesis to promote alfalfa growth.

Transcriptome and GWAS Analyses Reveal Candidate Gene for Root Traits of Alfalfa during Germination under Salt Stress

Alfalfa growth and production in China are negatively impacted by high salt concentrations in soils, especially in regions with limited water supplies. Few reliable genetic markers are currently available for salt tolerance selection. As a result, molecular breeding strategies targeting alfalfa are hindered. Therefore, with the continuous increase in soil salinity in agricultural lands, it is indispensable that a salt-tolerant variety of alfalfa is produced. We collected 220 alfalfa varieties around the world for resequencing and performed genome-wide association studies (GWASs). Alfalfa seeds were germinated in saline water with different concentrations of NaCl, and the phenotypic differences in several key root traits were recorded. In the phenotypic analysis, the breeding status and geographical origin strongly affected the salt tolerance of alfalfa. Forty-nine markers were significantly associated with salt tolerance, and 103 candidate genes were identified based on linkage disequilibrium. A total of 2712 differentially expressed genes were upregulated and 3570 were downregulated based on transcriptomic analyses. Some candidate genes that affected root development in the seed germination stage were identified through the combination of GWASs and transcriptome analyses. These genes could be used for molecular breeding strategies to increase alfalfa’s salt tolerance and for further research on salt tolerance in general.