Pysiological And Molecular Mechanisms Underlying Salt Tolerance In Halophyte Kochia Sieversiana

According to ability of salt tolerance,plants can be divided into halophytes and glycophytes.Halophyte is defined as a kind of plants that can survive long-term under more than 200 mM NaCl stress and complete their life cycle.Numerous studies have demonstrated that the salt-tolerant mechanisms of halophytes are significantly different from that of glycophytes.In the past 30 years,the research of salt-tolerant mechanisms of glycophytes have made great progress in many fields such as ion transport,signal transduction,hormone regulation,etc.,but few studies have involved the salt-tolerance molecular mechanisms of extreme halophytes.Our study used RNA sequencing,phenotypic analysis,as well as physiological and biochemical methods to investigate the physiological and molecular mechanisms of salt tolerance of an extreme halophyte,Kochia sieversiana(Pall.)C.A.M.This article paid special attention to the physiological and molecular mechanisms underlying the phenomenon that low salinity stimulates the growth of K.sieversiana.The results showed that low salinity stimulates the growth of K.sieversiana,while high salinity does harm to its growth.In parallel,salt stress can increase the absorption of water in the soil,and the water content increases with the increase of the salt stress concentration.Even under480 mM NaCl,K.sieversiana still maintains extremely high water content.This high water content may be a specific adaptive strategy of K.sieversiana to high salinity.The physiological analysis indicated that increasing succulence and great accumulations of sodium,alanine,sucrose and maltose may be favorable to the water uptake and osmotic regulation of K.sieversiana under high-salinity stress.Transcriptome data showed that the expression of aquaporin genes(AQP)and potassium ion transporter genes was up-regulated under salt stress treatment.These up-regulated genes may be essential for the water uptake and ion balance of K.sieversiana,respectively,which may in turn increase the ability to resist osmotic stress and ion toxicity.The responses of different members of these gene families to salt stress were further compared,and it was found that different members of the same gene family had significantly different effects under different stress intensities.In addition,aquaporin genes in root tissues did not play a role under high salt stress,but they were up-regulated under low salt conditions.Highly-expressed levels of the aquaporin gene allow the K.sieversiana to absorb more water under low salt stress conditions.Compared to high salinity conditions,under low-salt stress conditions,the value of leaf K~+/root K~+increased.It suggests that low-salt stress may promote K~+transport from root tissue to up-ground parts of K.sieversiana.In sum,low-salt conditions stimulate the growth of K.sieversiana,and high-salt conditions inhibit its growth.K.sieversiana promotes water content under low-salt conditions,providing a better cell environment for its growth and development.At the same time,the low-salt environment promotes K.sieversiana to transport more K~+to up-ground tissues,thereby competing with Na~+and reducing Na~+toxicity.However,under high-salt conditions,affected by stress signal proteins,K.sieversiana may employ different mechanisms of ion transport and osmotic adjustment.Thus,although the water content is still increasing,the high concentration of Na~+will still threaten the growth of the plant.In addition,some of the important salt-tolerant genes of K.sieversiana under salt stress may be neo-functionalization or non-functionalization compared to glycophytes,resulting in some changes in salt tolerance strategies.This may be the reason why K.sieversiana can build dominant populations under extreme salt and alkali conditions.