High Concentrations Of CPPU Induce Gibberellin To Regulate The Synthesis Of Cucurbitin B In Oriental Melon (Cucumis Melo Var. Makuwa Makino) Fruit

Oriental melon（Cucumis melo var.makuwa Makinois）an important horticultural crop with its sweetness and fragrance.Bitterness often appears in ripe melon fruit,which greatly affects the commercial value of melon.The bitter substance in melon is cucurbitin B（Cu B）,which is a tetraicyclic triterpenoid compound,and is most easily accumulated in the pedicle pulp of melon.CPPU is widely used in melon production.Generally,the normal spray concentration is about 5mg·L^-1,but often due to high temperature,drought and multiple spraying factors in the greenhouse,the concentration becomes high after spray.When the CPPU concentration is high,melon fruits tend to become bitter.Generally,CPPU can regulate plant growth and development and various physiological and biochemical processes by affecting endogenous hormone levels.However,it is not clear whether the exogenous application of high CPPU can affect the related hormone pathways and thus affect the bitter taste of fruits.Therefore,to clarify whether high CPPU regulate the synthesis of bitter substances in melon fruits by mediating endogenous hormone pathway provides a theoretical basis for improving the quality of melon.In this study,’HDB’and’YMR’were used as materials.On the day of flowering,5,20 and 25mg·L^-1CPPU were applied to ovary respectively.Artificial pollination as control,and samples were taken every 5d.The content of Cu B was determined by HPLC,and the expression of genes related by q RT-PCR.In addition,the contents of 3d endogenous gibberellin（GA）and cytokinin（CTK）were analyzed after artificial pollination,5 and 20 mg·L^-1CPPU,and the relative gene expression levels of DELLA in gibberellin signaling and ARRs in the cytokinin signaling pathway were determined.Besides,20mg·L^-1CPPU was sprayed on the flowering day of’YMR’,and 1.5d after fruit setting,GA(50,100mg·L^-1GA3)and its inhibitors(100,200 mg·L^-1PAC)were sprayed respectively.Water as control,and samples were taken every day after treatment.The accumulation of Cu B and the expression patterns of genes related to cucurbitin synthesis were analyzed.The results are as follows.1.The accumulation of Cu B was the highest at 5d after flowering.the accumulation of Cu B was significant under CPPU treatment,and the expression of key genes Cm Bi and Cm ACT were significantly up-regulated,suggesting that high CPPU could significantly promote the accumulation of Cu B.2.After CPPU treatment,the content of endogenous CTK decreased significantly,and the main cytokinin active substances t Z,c Z and IP were significantly decreased after CPPU treatment.However,the expression of ARRs showed different levels.Suggesting that CPPU treatment inhibited the accumulation of endogenous CTK.But can activate endogenous cytokinin signal transduction;3.After high CPPU treatment,endogenous GA₃content was significantly accumulated in melon fruit,and gibberellin synthesis genes Cm GA3ox and Cm GA20ox were significantly up-regulated.And DELLA gene was down-regulated.Suggesting that high CPPU treatment induced endogenous GA accumulation and promoted gibberellin signal transduction.4.Exogenous GA and its inhibitors were also treated in the experiment.It was found that Cu B was significantly accumulated and related genes of cucurbitin synthesis were significantly up-regulated after1.5d of GA₃treatment.After 1.5d of treatment with PAC,the content of Cu B decreased significantly,and the expression of genes related to cucurbitin synthesis was inhibited.In conclusion.In conclusion,high CPPU can increase the content of cucurbitin B in melon fruits.CPPU treatment can significantly reduce the content of endogenous CTK in melon fruits,but can activate the signaling of CTK.Meanwhile,high concentrations of CPPU can significantly promote the synthesis and signal transduction of endogenous GA₃in fruits,and GA also plays a role in inducing the synthesis of Cu B.