Genome-wide Identification Of The Bile Acid Sodium Symporter Gene Family And Functional Characterization Of Their Roles In Response To Abiotic Stress

Salt stress is one of the major abiotic stresses restricting cotton(Gossypium spp.)production,and there is an urgent need to identify relevant transporters involved in the uptake and distribution of Na+ in cotton.Bile acid sodium symporter(BASS)family proteins encode a class of sodium/solute symporters.Even though the sodium transporting property of BASSs in mammals was well studied,their sodium transportability and functional roles in plant salt tolerance remained largely unknown.Here,BASS family members from 4 cotton species,as well as 30 other species were identified.Then,they were designated as members of BASS 1 to BASS5 subfamilies according to their sequence similarity and phylogenetic relationships.And the evolutionary study showed that all recent BASS genes were derived from an ancestral BASS4 gene.Interestingly,except three C4 monocots in which BASS3 was absent,all angiosperms selected in this study contained members of all five BASS subfamilies.In four cotton species(G.raimondii,G.arboreum,G.hirsutum and G.barbadense),there were 11,8,16 and 18 putative BASS genes,respectively.While whole-genome duplications(WGD)and segmental duplications rendered the expansion of the BASS gene family in cotton,BASS gene losses occurred in the tetraploid cotton during the evolution from diploids to allotetraploids.Besides,BASS orthologs of each subfamily retained very conserved chromosomal localization,gene architecture and motif patterns,particularly the conserved ten transmembrane segments and two conserved Na+binding sites.Concerning functional characterizations,the transcript profiling of GhBASSs revealed that they not only preferred tissue-specific expression but also were differently induced by various stressors and phytohormones.Moreover,the functions of BASS genes in the upland cotton during salt stress were characterized.Gene silencing and overexpression experiments showed that GhBASSl and GhBASS3 positively regulated,whereas GhBASS2.GhBASS4 and GhBASS5 negatively regulated plant salt tolerance.Taken together.BASS family genes have evolved before the divergence from the common ancestor of prokaryotes and eukaryotes,so they are very conserved across all organisms,as well as GhBASSs are plastidial sodium-dependent metabolite co-transporters that can influence plant salt tolerance.Next,we deeply focused to study the GhBASS5 gene,and the results showed that GhBASS5 served as a plastidic Na+ transporter.It was highly expressed in cotton roots and predominantly existed in the vascular cylinder.Heterogenous expression of GhBASS5 in Arabidopsis chloroplasts promoted Na+ uptake into chloroplasts,which contributed to an increased cytoplasmic Na+ concentration.And GhBASS5-overexpressed transgenic plants showed an increase in Na+ translocation from roots to shoots and an elevated Na+content in both roots and shoots,but a dramatic decrease in the Na+efflux from root tissues and the K+/Na+ ratio,especially under salt stress conditions.Furthermore,overexpressing GhBASS5 greatly damaged plastid functions and enhanced salt sensitivity in transgenic Arabidopsis when compared with wild-type plants under salt stress.Additionally,the salt-responsive transporter genes that regulate K+/Na+homeostasis were dramatically expressed in GhBASS5overexpressed lines,especially under salt stress conditions.Taken together,these results suggest that GhBASS5 is a plastid-localized Na+transporter,and high expression of GhBASS5 impairs salt tolerance of plants via increasing Na+ transportation and accumulation at both cell and tissue levels.Then,the involvement of BASS5 in plant drought tolerance was explored.Since BASS5 was reported as a GLS biosynthetic gene in Arabidopsis,we continued to study the roles of BASS5 in the response mechanisms of plants against drought stress.Glucosinolates(GLS)are health-promoting phytochemicals which encompass plant defense mechanisms,and Na+priming affects GLS production in Arabidopsis.However,nobody clearly explained the relationship of GLSs to Na+ and plant drought tolerance.Here,we demonstrated that BASS5 imported keto acids into plastids by the Na+-dependent manner so that keto acids were utilized in the phase of GLS side-chain elongation and biosynthesis.BASS5 involved in long-distance Na+transport from roots to shoots,which facilitates plants to modulate Na+ to GLS signaling in response to drought.The enhanced aliphatic GLSs induced stomatal closure to alleviate drought sensitivity of stressed plants,as well as the synergy of supplying mild Na+concentration and overexpressing GhBASS5 mounted GLS levels,which in turn boosted drought tolerance of plants.Hence,this is a novel finding that B ASS5 engages in the interaction chain of Na+ions availability,elevated GLS accumulation and plant drought tolerance.