The Molecular Mechanisms Of K~+,Na~+ Absorption And Transport In The Halophyte Suaeda Salsa

Potassium(K)is an essential macronutrient for plant growth and development.Soil salinity is a major limiting factor reducing crop growth and influencing agro-ecological environment.Excessive Na+ accumulation in plants inhibits K+ absorption and transport,resulting in drastic reduction of plant growth and even death.Halophytes have evolved effective mechanisms to adapt to the highly saline conditions.Absorbing a great quantity of Na+ from culture medium which is efficiently transported to the leaves,and maintaining the stability of K+ concentration in those leaves are two major adaptive strategies used by the typical salt-accumulating halophyte Suaeda salsa to adapt to the saline conditions.These results suggest that S.salsa has a strong ability to regulate Na+ and K+ homeostasis thus maintaining plant growth under saline conditions.However,very little is known about the gene regulatory network of K+,Na+ uptake and transport in S.salsa.In the present work,in order to investigate the molecular mechanisms of K+,Na+ uptake and transport,genes were isolated from S.salsa by using RT-PCR methods,also yeast complementation assays and the expression patterns of genes were analyzed.The main results were as follows:1.We cloned SsAKT1 gene encoding the inward-rectifying K+ channel,SsHAK2 gene,SsHAK5 gene,SsHAK6 gene and SsHAK8 partial segment from the KT/HAK/KUP family.The expression of SsAKT1 and SsHAK5 rescued the K+-uptake-defective phenotype of yeast strain CY162,and also suppressed the salt-sensitive phenotype of yeast strain G19,suggesting that SsAKT1 and SsHAK5 mediated K+ transport.2.SsAKT1 was predominantly expressed in roots,and was induced significantly by K+starvation,also its transcript level increased further under various resupplied KCl(0.1-10 mM)or NaCl concentrations(25-250 mM)for 6 h.The results indicated that SsAKT1 might mediate K+uptake in S.salsa.Furthermore,SsAKT1 might be involved in salt tolerance by participating in the maintenance of K+ nutrition in S.salsa under salinity.3.SsHAK2 was highly expressed in roots and leaves,and was significantly induced by different K+ concentrations,especially 2.5 mM K+ condition.Under 2.5 mM K+ condition,the expression of SsHAK2 was induced significantly by 25 mM NaCl,and was inhibited by NH4+application.Under 0.01 mM K+ condition,the expression levels of SsHAK2 decreased in roots,but increased in leaves by 25 or 150 mM NaCl condition.4.SsHAK5 was only expressed in roots,and was induced significantly by different K+concentrations,especially by 0.01 mM K+ condition.Under 2.5 mM K+ treatment,the expression of SsHAK5 in roots was induced by 150 mM Na+ application and was inhibited by NH4+ application.Under 0.01 mM K+ condition,the expression of SsHAK5 in roots was up-regulated by Na+(25 and 150 mM)application in roots and was down-regulated by NH4+ condition.The results indicated that SsHAK5 might mediate K+ uptake in S.salsa,and might be involved in salt tolerance by participating in the maintenance of K+ nutrition in S.salsa under salinity.5.SsHAK6 was highly expressed in roots and leaves,and was significantly induced by different K+ concentrations(0.01 or 2.5 mM).Besides,the expression of SsHAK6 was regulated by 25 mM or 150 mM NaCl,and the effects of the regulation were depend on the K+ concentrations in medium.The expression of SsHAK6 in leaves was down-regulated under 2.5 mM K+ plus 25 mM NaCl treatment,and the transcript levels of SsHAK6 in roots and leaves were decreased under 0.01 mM K+ plus 150 mM NaCl condition.6.SsHKT1;1 was mainly expressed in roots of S.salsa,and was induced by different resupplied KCI(0.1-10 mM)or NaCl concentrations(25-250 mM)for 6 h,suggesting that SsHKT1;1 mediated K+ uptake in roots of S.salsa under normal or saline condition.SsSOSl and SsNHX1 in S.salsa were also induced by different KCl or NaCl concentrations,and were induced for 6 h in roots and 48 h in leaves,respectively.SsSOSl and SsNHX1 were involved in Na+ and K+ transport.7.Under 2.5 mM K+ plus 25 mM NaCl condition:both Na+ and K+ net absorption rate,also Na+ and K+ concentration in shoots were declined by NH4+ application;the expression levels of SsAKT1 and SsHKT1;1 were evidently induced by NH4+ application.Under 2.5 mM K+ plus 150 mM NaCl condition:the Na+ concentration in shoots and Na+ net absorption rate decreased by 5 mM NH4+ application;the expression levels of SsAKT1 and SsHKT1;1 were evidently induced by NH4+ application.The results suggested that,instead of some members of the KUP/HAK/KT family such as SsHAK2,SsHAK5 and SsHAK6,SsAKT1 and SsHKT1;1 might mediate more important K+ uptake.Besides,the Na+ and K+ transport by SsSOS1 and SsNHX1 were regulated by the changes of Na+ and K+ uptake.8.Under 0.01 mM K+ plus 150 mM NaCl condition,the transcript abundance of SsHAK2,SsHAK5,SsHAK6,SsAKT1 and SsHKT1;1 were all significantly inhibited by 5 mM NH4+application.We speculated that other K+ and Na+ transporters might be activated to mediate K+,Na+ uptake and transport under this condition.In conclusion,the inward-rectifying K+ channel SsAKT1,the high-affinity K+ transporters SsHAK2,SsHAK5,SsHAK6 and SsHKT1;1,the plasma membrane Na+/H+ antiporter SsSOS1 and the tonoplast Na+/H+ antiporter SsNHX1 may cooperate together to regulate K+,Na+ uptake and transport in S.salsa.