Molecular Mechanism Of Auxin-related Genes CsIVP And CsYUC Regulating Organ Development And Stress Resistance In Cucumber

The vascular system is responsible for delivering water and nutrients throughout the plant.Vasculature formation and configuration play crucial roles in plant organ development and stress resistance.So far,studies about vascular development mainly focus on model plant Arabidopsis,while the underlying molecular mechanisms of vascular development in horticulture crops remain elusive.Cucumber(Cucumis sativus L.)is a world cultivated vegetable crop with great economical and nutritional value,and possesses well-developed bicollateral vasculature.Thus,we utilized the methods and tools in phytophysiology,developmental biology and biochemistry to explore the function of the vascular regulator Cucumis sativa Irregular Vasculature Patterning(CsIVP)in cucumber.Phylogenetic analysis showed that IVP arised prior to the emergence of angiosperms,and existed in the primitive plants.CsIVP,as a bHLH transcription factor,is highly expressed in cucumber vascular tissues.Knock-down of CsIVP by RNAi causes severe vasculature disorganization and abnormal morphogenesis in cucumber,inculding dwarfism,abnormal morphogenesis,and reduced fertility.Furthermore,CsIVP-RNAi plants were tolerant to low-nitrogen and downy mildew.Biochemical studies indicated that CsIVP functions in vascular organization development by interacting with CsAUX4 in the auxin signaling pathway,and physically interacting with homologs of well-known vasculature regulators including CsMYB116,CsBP,CsYAB5 and CsKAN2.In the CsIVP-RNAi plants,more nitrate is remobilized from old leaves to young leaves,illustrating that the increased tolerant to low-nitrogen stress due to optimized nitrate allocation and enhanced nitrate transport.Disease-resistant mechanism analyses demonstated that CsIVP regulates plant disease resistance through mediating JA content and directly interacting with the JAZ8 in the jasmonic acid signal pathway.Thus,CsIVP regulates bicollateral vasculature patterning and integrates innate programming of plant development,nutrient transport,and stress resistance.The phytohormone auxin is essential for plant growth and development,and YUCCA(YUC)proteins catalyze a rate-limiting step for endogenous auxin biosynthesis.Despite YUC family genes have been isolated from several species,little is known about the function of YUC homologs in agricultural crops with respect to stress reponse and developmental regulation.In this study,we isolated 10 YUC family genes(CsYUCs)from cucumber and explored their expression pattern under four types of stress treatments.Our data showed that CsYUC8 and CsYUC9 were specifically upregulated to elevate the auxi level under high temperature.CsYUC10b was dramatically increased but CsYUC4 was repressed in response to low temperature.CsYUC10a and CsYUC11 act against the upregulation of CsYUC10b under salinity stress,suggesting that distinct CsYUC members participate in different stress response,and may even antagonize each other to maintain the proper auxin levels in cucumber.Further,CsYUC11 was specifically expressed in the male flower in cucumber,and enhanced tolerance to salinity stress,and regulated pedicel and stamen development through auxin biosynthesis in Arabidopsis.As such,this study dissected the biological function and regulatory mechansim of vasculature regulator CsIVP,and explored the function of auxin biosynthesis genes CsYUCs in response to abiotic stresses and during reproductive organ development,thus built the molecular link between organ development and plant stress resistance.The results obtained in this study provide the theory basis and gene resources for molecular breeding for cucumber cultivars with high nutrient efficiency and stress resistance.