Study On The Mechanism Of Gibberellin-induced Fruit Set In Jujube(Ziziphus Jujuba Mill.)

Jujube（Ziziphus jujuba Mill.）is an important cultivated fruit tree and ecological tree species in China.Xinjiang is the largest dry jujube producing area in China,‘Huizao’is the main variety with the largest cultivation area,but the natural fruit setting rate is very low.Spraying GA₃ at flowering stage has become an important measure to improve the fruit set rate of jujube trees in Xinjiang.However,the regulation mechanism of GA₃-induced fruit set was not clear,and the application of GA₃ was blind.In this study,jujube flowers and young fruits in different development stages of fruit set were studied,the effects of GA₃ inducing on flower morphology and fruit set rate were studied.On the basis of clarifying the time point of phenotypic difference,LC-MS/MS and transcriptome sequencing were further used to analyze the differences of endogenous hormone content and gene expression in jujube flowers of three stages and young fruits at physiological fruit dropping stage,the key differential genes and metabolic pathways involved in jujube floral organ development,GA anabolism and signal transduction were identified,three key genes Zj EXPA8,Zj LAX3 and Zj CYP78A7 were cloned and analyzed.The aim was to analyze the physiological and molecular mechanism of GA₃-induced fruit setting in jujube from the levels of morphology,hormone and gene.The main contents and results are as follows:（1）It took 4-5 days for jujube flowers to blossom from bud broken stage to ovary expansion stage,and it took 2-3 days for the flowers to turn into fruit during the ovary expansion stage,that was,it took 6-8 days from flowering to fruit setting.Sepal flattening stage,filament wilting stage and ovary expansion stage could be regarded as the key stage of fruit set.Compared with the natural fruit setting,the difference in phenotype induced by GA₃was mainly reflected in the initial stage after the transformation from jujube flower to young fruit.Fruit set rate was significantly accelerated in GA₃-treated fruits,especially in the first week of young fruit development,fruit weight per was significantly increased.In addition,GA₃ treatment accelerated the decomposition of ovules.Spraying GA₃ at flowering stage could significantly increase the fruit set rate of jujube.After entering the ovary expansion stage,the number of fruit set of jujube decreased gently in the early stage,but decreased significantly in the late stage,and an obvious fruit drop peak could be observed about 10 days after the young fruit began to develop,while GA₃ treatment delayed the peak of fruit drop.（2）Spraying GA₃ at flowering stage promoted the anabolism of GA in jujube flower during fruit set stage,significantly increased the contents of GA₃,GA₁ and GA₁₅.Spraying GA₃ could significantly increase the contents of IAA,ICAld and c Z in jujube flowers,but had no obvious effect on the content of ABA in each period.Therefore,GA₃ induction shows that it can increase the content of GA,auxin and c Z,form a hormone level conducive to fruit set,and start the transformation from flower to young fruit in the fruiting process.Exogenous GA₃ plays a role through GA signal transduction pathway with DELLA protein as the core element in fruit setting stage.A total of 21 genes related to the development of pollen,ovary and ovule,1 gene related to GA biosynthesis pathway and 3 genes related to GA signal transduction pathway were identified by by RNA-seq.（3）A total of 2391 differentially expressed genes were identified by transcriptome sequencing in the treatment and control group at the stage of physiological young fruit drop,including 1677 down-regulated genes and 714 up-regulated genes.Differential genes were mainly concentrated in biological processes such as material metabolic activities,nutrient absorption and GA anabolism,as well as secondary metabolite anabolism,carbohydrate,lipid,amino acid,inorganic ion transport and metabolism,cell signal and development,indicating that young fruit abscission was closely related to nutrient absorption at this stage.GA₃treatment increased the contents of GA and CTK in young fruits,decreased the contents of IAA,ICAld,ABA and ABA-GE,enhanced GA and CTK signal and weakened ABA signal.The growth and development of young fruits were promoted and abscission was inhibited.（4）Zj EXPA8,Zj LAX3 and Zj CYP78A7 contained the open reading frames of 762bp,1398bp and 1614bp,encoding 253,465 and 537 amino acids,with molecular weights of26.91k Da,52.47k Da and 60.37k Da,respectively.The isoelectric points were 8.42,8.64 and7.73,respectively,and the Gen Bank numbers were NC＿029681.1,XP＿015885608.1 and XP＿015890966.1,respectively.Phylogenetic tree results showed that Zj EXPA8 was most closely related to sweet orange,Zj LAX3 was most closely related to cherry,peach,melon and bamboo melon,and Zj CYP78A7 was most closely related to large leaf oak,mulberry and cork oak.These three genes may play a role mainly in the bud yellow stage and sepal flattening stage.GA₃treatment at flowering stage induced the up-regulated expression of three genes.