Salt stress affects plant photosynthesis,water relationship and ionic balance through ionic toxicity and osmotic stress.Tomato,one of the important vegetables,is seriously affected by salt stress in its yield and quality.The salt tolerance is obviously distinct in different genotypes of tomatoes,and its regulatory mechanism has been explored a lot but is still not completely understood.NHXs mediate Na+,K+/H+transport pathway and plays an important role in salt tolerance.In Arabidopsis,NHX not only regulates the transport of K+and Na+and controls K+homeostasis,but also plays an essential part in plant growth and development.However,in tomato,the possible functions that SlNHXs play on K+transport and growth have not been revealed.In this study,wild tomato（Solanum pennellii）‘LA0716’and cultivated tomato（Solanum lycopersicum）‘M82’were used to explore their responses to Na+and K+stress under different concentrations of Na Cl and KCl.CRISPR/Cas9 gene editing technology was used to obtain mutants including double knockout and triple knockout of SlNHXs in tomato,and SlNHX4 was transformed into Arabidopsis mutant nhx1to study functions of SlNHX1,SlNHX3 and SlNHX4.The main results are as follows:1.After treatments of NaCl and KCl,in two genotypes of tomato,the biological features including biomass,shoot water content and photosynthesis were inhibited in various extent,and the inhibition was strengthened with the increase of salt concentrations.At the same concentration,compared with KCl,the inhibition of Na Cl was stronger.M82 was more sensitive to salt stress than LA0716,and its plant height was apparently declined.The biomass and shoot water content of M82 were decreased significantly under 100 m M Na Cl,while those of LA0716 were not affected and it exibited stronger capability of water retention.Under salt stress,the increases of ionic content and Na+/K+ratio were significantly higher in shoot of LA0716 than M82,indicating that LA0716 had a stronger ion accumulation ability.Under the stress of Na Cl and KCl,the net photosynthetic rate,stomatal conductance and transpiration rate in both genotypes were decreased significantly,and the decrease of LA0716 was greater than that of M82.2.In tomato,the relative expression of SlNHX1 was highest in leaves,but it was lower compared with that of SlNHX3 or SlNHX4.SlNHX3 expression was higher in flowers than other tissues.The transcript levels of SlNHX4 was higher in all aboveground parts especially in flowers and fruits,but the expression in roots was relatively low.Phylogenetic analysis showed that,in tomato,SlNHX1,SlNHX3 and SlNHX4 are closely related to At NHX1,At NHX2 and At NHX4 in Arabidopsis.Analysis of the amino acid sequence by SMART indicated that SlNHX1,SlNHX3,and SlNHX4 all contain the Na+/H+exchange conserved domain.3.Mutants,which include one slnhx1/slnhx3 double knockout line,two slnhx1/slnhx4double knockout lines,three slnhx3/slnhx4 double knockout lines,and two slnhx1/slnhx3/slnhx4 triple knockout lines were obtained through CRISPR/Cas9 gene editing technology.SlNHX4 in one slnhx1/slnhx4 double knockout line and one slnhx3/slnhx4double knockout line were homozygous mutation,and the rest of knockout lines were biallelic mutations or heterozygous mutations.4.Observing SlNHXs knockout lines and WT points out that fruits were small and the number of seeds was decreased in SlNHXs double knockout lines.Under salt stress,the growth and development of the double knockout lines were suppressed and the biological features such as plant height,biomass,photosynthesis and the relative content of chlorophyll were reduced in different extent,and they were more sensitive to K+stress.The number of lateral branches in double knockout and triple knockout lines was increased significantly.Strigolactones（SLs）negatively regulate the formation of lateral branch.The expressing levels of SLs biosynthesis genes were analyzed by q RT-PCR,and the results showed that the transcript levels of SLs biosynthesis genes Sl CCD7 and Sl MAX1 were down-regulated in the three double knockout lines.5.SlNHX4 was transformed into Arabidopsis nhx1 mutant by plant genetic transformation technique to obtain SlNHX4 overexpressing lines.Compared to wild-type,nhx1 had fewer leaves,and was smaller and more sensitive to salt stress.Nevertheless,in some extent,overexpression of SlNHX4 restored the phenotypes of nhx1 and promoted bolting and flowering.This study showed that wild tomato LA0716 had stronger water retention and ionic accumulation ability compared with cultivated tomato M82,thereby being more tolerant to salt stress.Na Cl was more toxic to tomato plants than KCl.The vacuolar-type SlNHXs may play an important role in plant growth and development as well as maintaining K+balance.