Functional screening of salt tolerance genes from a halophyte Sporobolus virginicus and transcriptomic and metabolomic analysis of salt tolerant plants expressing glycine-rich RNA-binding protein.

Sporobolus virginicus is a halophytic C4 grass found worldwide, from tropical to warm temperate regions. One Japanese genotype showed a salinity tolerance up to 1.5 M NaCl, a three-fold higher concentration than the salinity of sea water. To identify the key genes involved in the regulation of salt tolerance in S. virginicus, we produced 3500 independent transgenic Arabidopsis lines expressing random cDNA from S. virginicus and screened 10 lines which showed enhanced salt tolerance compared with the wild type in a medium containing 150 mM NaCl. Among the selected lines, two contained cDNA coding glycine-rich RNA-binding proteins (SvGRP1 and SvGRP2). This is the first reports on the function of GRPs from halophytes in salt tolerance though reports have shown GRPs are involved in diverse biological and biochemical processes including salt tolerance in Arabidopsis and some other glycophytes. Transcriptomic analysis and GO enrichment analysis of SvGRP1-expressing Arabidopsis under salt stress revealed upregulation of polyol and downregulation of glucosinolate and indole acetic acid biosynthesis/metabolic pathways. Metabolomic analysis of the SvGRP1-transformant suggested that the increase in 3-aminoppropanoic acid, citramalic acid, and isocitric acid content was associated with enhanced salt tolerance. These findings could provide novel insight into the roles of GRPs in plant salt tolerance.

Growth and physiological adaptation of whole plants and cultured cells from a halophyte turf grass under salt stress.

Understanding the mechanisms used by halophytic members of the Poaceae to cope with salt stress will contribute to the knowledge necessary to genetically engineer salt-tolerant crops. In this study, we identified a genotype of Sporobolus virginicus, a halophytic turf grass collected in Japan, and investigated its growth rate, ion concentration and secretion, and proline concentration in comparison with the reported properties of genotypes collected from the USA, South Africa and Egypt. Surprisingly, the Japanese genotype showed a salinity tolerance up to 1.5 M NaCl, a 3-fold higher concentration than seawater salinity. Shoot growth was stimulated by 100 mM NaCl and root growth was stimulated at salinities of up to 1 M NaCl. Accumulation of Na(+) and CI(-) in shoots and roots was rapidly elevated by salinity stress but did not exceed the levels required for osmotic adjustment, due in part to ion secretion by salt glands, which are present in genotypes of S. virginicus. However, the Japanese genotypes accumulated K(+) to a higher level than other genotypes, resulting in a relatively high K(+)/Na(+) ratio even under salinity stress. An increase in proline concentration was observed that was proportional to the NaCl concentration in the culture solution and might partially account for osmotic adjustment in the shoots. We also generated and characterized cultured cells of S. virginicus. In 500 mM NaCl, the cultured cells showed an enhanced growth compared with cultured cells of rice. The concentration of Na(+) and CI(-) in the cultured cells in 300-500 mM NaCl was lower than in 100 mM NaCl. Cultured cells of S. virginicus accumulated proline to higher levels than rice cells cultured under salinity stress. The active regulation of Na(+), Cl(-) and K(+) influx/efflux and proline accumulation might be involved in salt tolerance mechanisms at the cellular level as well as in planta.