Metabolomic and transcriptomic analyses reveal the reasons why Hordeum marinum has higher salt tolerance than Hordeum vulgare

Sea barley (Hordeum marinum) is a wild species belonging to halophytes, while other Hordeum species are glycophytes. In this study, we aimed to understand molecular differences of transcriptomic and metabolomic responses and uncover complex mechanisms underlying salt tolerance between H. marinum and H. vulgare. Physiological studies revealed that sea barley accession H559 had less growth inhibition, higher root K+ concentration, lower shoot Na (+) accumulation and higher shoot osmotic potential than barley genotypes XZ113 and Golden Promise under 400 mM salinity stress. The superior Na+/K+ homeostasis in H559 may depend on the regulation of ion transporters including SOS1, HKT1;1, HKT1;5 and HKT2;2 according to RNA-seq analysis. The comparison of metabolite and transcript profiles among three genotypes indicated that H559 used less energy consuming strategies including a larger scale gene down-regulation and utilization of inorganic ions (i.e. Na+ and K+) as "cheap" osmotica for root tolerance, and enhanced glycolysis and the TCA cycle for energy supply and compatible solutes accumulation for shoot tolerance. Moreover, some genes involved in the ROS detoxification were also identified in H559. We propose that the salt tolerance in the halophyte sea barley is correlated with superior Na+/K+ homeostasis, energy-saving strategy and osmotic adjustments in tissues. The present results provide the fundamental evidences at molecular level for explaining why H. marinum has higher salt tolerance than H. vulgare.