The Function Of Salicylic Acid In Regulating Bud Growth In Litchi And Its Mechanism

Bud dormancy is a characteristic growth in perennial plants for surviving environmental conditions that are adversive for growth.Bud dormancy is regulated by multiple factors and inovlves multiple genes.So far,studies of bud dormancy have focused on deciduous trees.Studies on bud dormancy of tropical evergreen tree species have been rare.The mechanisms of bud dormancy development and removal in the evergreen trees remains to be studied.Litchi(Litchi chinensis Sonn.),a member in the Sapindaceae family is an evergreen tree and an important tropical fruit crop in south China.According to changes in morphological characteristics of buds,their growth—rest cycle can be divided into four stages: dormant stage(S1),breaking stage(S2),rapid growth stage(S3)and growth cessation stage(S4).Salicylic acid(SA)is an endogenous hormone involved in the regulation of various physiological and biochemical processes in plants,such as heat generation,flowering,stomatal closure,lateral bud breaking,seed germination,sex differentiation and other growth and development processes.SA also induces resistance against undesirable biological and abiotic stresses.Transcriptomic study of litchi bud at different stages showed that SA might be involved in the regulation of dormancy release of litchi buds.The roles of SA in the regulation of bud development remain unclear.In this sudy,we examined the effects of SA on litchi bud dormancy release and analyzed the mechanims of its effects.'Feizixiao' litchi trees grown in the experimental orchard in South China Agricultural University were used as the experimental material in this study conducted from 2015 to 2017.Effects of exogenous SA on bud dormancy removal and on the expression of meristem activity realted genes LcWUS and LcCLV were analyzed.Changes in endogenous SA and IAA in buds were studied at different stages.Roles of SA responding WRKY members in bud dormancy regulation were analyzed.The main results are as follows:1.The content of SA in litchi buds at different stages was detected.It was found that SAwas lowest in dormant buds,increased significantly during budbreak.It decreased as bud entered rapid growth stage and increased to a highest level during growth cessation stage.Exogenous SA treatment at dormant stage advanced budbreak and promoted the expression of LcWUS and LcCLV in buds.The exogenous SA treatment also increased the content of IAA,thereby promoting the sprouting of litchi apex buds.Treatement of SA at the rapid growth stage showed no significant effect on the bud development.During the dormant stage,Silencing SA synthetic gene LcICS will delay the termination of lychee dormancy.Otherwise,SA will promote the dormancy of litchi buds.Therefore,SA has a positive effect on bud dormancy removal in litchi.2.WRKY is an important family of transcription factor found in plants in recent years.It has 75 members in Arabidopsis thaliana.They are characterized by the highly conserved core amino acid sequence WRKYGQK in C-terminal and a zinc finger structure.WRKYs are affected by pathogens,damage,SA and other factors and participates in many physiological processes such as disease resistance,injury,growth and development as well as senescence.Nighteen LcWRKYs were screened and seven LcWRKYs showed significant changes in expression during dormancy—growth phase changes in litchi buds.They were LcWRKY7,LcWRKY17,LcWRKY33,LcWRKY40,LcWRKY41,LcWRKY46 and LcWRKY62.Litchi protoplasts were used as validation materials.SA at different concentrations promoted the expression of LcWRKY7,LcWRKY17 and LcWRKY40 in litchi protoplasts,and their expression levels were significantly positively correlated with SA concentration.LcWRKY7,LcWRKY17 and LcWRKY40 were all found located in the nucleus.3.To further demonstrate the function of WRKY transcription factor,virus induced silencing of the LcWRKY40 delayed the budbreak of litchi,indicating that LcWRKY40 had a positive role in dormancy removal of litchi buds.