Jasmonate Regulates Rice Mesocotyl Elongation Through The Interaction Between OsMYC2 And Gibberellin

Rice（Oryza sativa L.）is one of the important food crops in the world.At present,the area of direct-seeded rice is increasing,and dirct seeding rice has the advantages of saving labor and cost.Shallow direct seeding is easy to lodging,while deep direct seeding is hard to emerge,which limits the development of direct seeding rice.The mesocotyl is a unique organ of monocots located between the scutellum and the coleoptile node.The elongation of the mesocotyl can send the seedlings out of the ground under the condition of deep direct seeding,and promote the emergence of rice in deep direct seeding.Therefore,The difficulty of seedling emergence of deep seeded rice can be improved by cultivating long mesocotyl rice varieties.Previous results have showen that the phytohormones Jasmonate and Gibberellin（GA）play important roles in regulating the elongation of rice mesocotyl,and there is an interaction between these two phytohormones.However,the mechanism of their interaction remains unclear.Although it is known that the transcription factor OsMYC2 is a positive regulator of jasmonate signaling in rice,and OsMYC2 mediates many physiological functions of jasmonate,the regulatory role and mechanism of OsMYC2 regulates rice mesocotyl elongation by the interaction of jasmonate and GA are remain unclear.This project explored the mechanism of OsMYC2 regulates rice mesocotyl elongation by the interaction between jasmonate and GA.The main research results are as follows:1.GA promotes mesocotyl elongation,and jasmonate inhibits mesocotyl elongation.In this study,the effects of exogenous GA₃（Gibberellin A3,an active GA）and JA（Jasmonic acid,a jasmonate）on mesocotyl elongation in the wild-type rice Dongjin（DJ）under dark conditions were analyzed.The results showed that GA and jasmonate promoted and inhibited mesocotyl elongationin in a concentration-dependent manner,respectively.2.Jasmonate may inhibit the elongation of rice mesocotyl by reducing the content of active GA.In this study,the effects of jasmonate on the expression of GA metabolism-related genes and GA content in rice mesocotyls were analyzed.The results showed that exogenous JA inhibited the expression of GA synthesis genes and promoted the expression of GA inactivation genes in mesocotyls,thereby reducing the contents of active GAs GA₁and GA₃in mesocotyls,and reducing the length of mesocotyls;while exogenous of JA synthesis inhibitor Ibuprofen（IBU）promotes the expression of GA synthesis genes and inhibits the expression of GA inactivation genes,thereby increasing the contents of active GAs GA₁and GA₃in mesocotyls,and increasing the length of mesocotyls.The above results indicated that jasmonate may reduce the content of active GA by inhibiting the expression of GA synthesis genes and promoting the expression of GA inactivation genes,thereby inhibiting the elongation of rice mesocotyls.3.Jasmonate reduces active GA content by regulating the expression of GA metabolism genes through its signaling element OsMYC2,thereby inhibiting the elongation of rice mesocotyls.In this study,wild-type DJ,over-expression line OsMYC2-OE and RNA interference expression line OsMYC2-RNAi were used as experimental materials to compare and analyze the differences of the expression levels of OsMYC2 gene,mesocotyl length,and the expression levels of GA metabolism-related genes and GA content in the period of mesocotyl rapid elongation under dark conditions.The results showed that mesocotyl length was negatively correlated with the expression of OsMYC2.Compared with the wild-type DJ,the expression of GA synthesis genes decreased,the expression of GA inactivation genes enhanced,the content of active GA decreased in OsMYC2-OE mesocotyls.However,OsMYC2-RNAi is the opposite.Morever,The content of GA and mesocotyl length were increased in OsMYC2-OE when treated with GA,and exogenous PP333（GA synthesis inhibitor）treatment decreased the content and length of OsMYC2-RNAi mesocotyl.In addition,yeast one-hybrid and dual luciferase activity experiments further demonstrated that OsMYC2 can inhibit the expression of GA synthesis genes OsKO1 and OsGA20ox1 by binding to the promoter of GA metabolism genes,and promote the expression of GA inactivation genes OsGA2ox5 and OsGA2ox6.The above results showed that jasmonate inhibitd the expression of GA synthesis genes and promoted the expression of GA inactivation genes through OsMYC2,thereby reducing the content of active GA and inhibiting the elongation of rice mesocotyls.4.Jasmonate negatively regulates cell division（CD）and cell elongation（CE）by reducing GA content through OsMYC2,thereby inhibiting the elongation of mesocotyl.In this study,the effects of DJ,OsMYC2-OE,OsMYC2-OE+GA,OsMYC2-RNAi and OsMYC2-RNAi+PP333 treatments on the number and length of rice mesocotyl cells and the expression of CD-related and CE-related genes were analyzed under dark conditions.The results showed that compared with DJ,the expression of CD-related and CE-related genes were down-regulated,the number of cells were decreased,and the cell length was shortened.However,OsMYC2-RNAi is the opposite.In addition,exogenous GA treatment could restore decreased CD-related and CE-related gene expression,cell number and cell length,mesocotyl length in the OsMYC2-OE,and exogenous PP333 could restore increased CD-related and CE-related gene expression,cell number and cell length,mesocotyl length in OsMYC2-RNAi.The results showed that jasmonate inhibited CD and CE by reducing the content of GA through OsMYC2,and then inhibiting the elongation of mesocotyl.5.Reducing the expression of OsMYC2 can promote the elongation of rice mesocotyls and improve the seedling emergence rate of direct seeding rice.In this study,DJ,OsMYC2-OE and OsMYC2-RNAi were used as experimental materials to analyze the effects of OsMYC2 on rice mesocotyl elongation and seeding emergence under deep mulching conditions.The results showed that compared with DJ,the mesocotyl length and emergence rate of OsMYC2-OE were significantly decreased;while the mesocotyl length and emergence rate of OsMYC2-RNAi were significantly increased.The results showed that reducing the expression of OsMYC2 can promote the elongation of rice mesocotyls and improve the emergence rate of rice deep seeding.In conclusion,jasmonate reduces active GA content by inhibiting the expression of GA synthesis genes and promoting the expression of GA inactivation genes through OsMYC2,and then inhibits mesocotyl elongation by inhibiting mesocotyl CD and CE.Reducing the expression of OsMYC2 increases the seedling emergence rate of driect deep seeding rice by promoting mesocotyl elongation.