Transcriptomic Analysis Implies That GA Regulates Sex Expression via Ethylene-Dependent and Ethylene-Independent Pathways in Cucumber (Cucumis sativus L.)

Abstract

Sex differentiation of flower buds is an important developmental process that directly affects fruit yield of cucumber (<i>Cucumis sativus</i> L.). Plant hormones, such as gibberellins (GAs) and ethylene can promote development of male and female flowers, respectively, however, the regulatory mechanisms of GA-induced male flower formation and potential involvement of ethylene in this process still remain unknown. In this study, to unravel the genes and gene networks involved in GA-regulated cucumber sexual development, we performed high throughout RNA-Seq analyses that compared the transcriptomes of shoot tips between GA₃ treated and untreated gynoecious cucumber plants. Results showed that GA₃ application markedly induced male flowers but decreased ethylene production in shoot tips. Furthermore, the transcript levels of <i>M</i> (<i>CsACS2</i>) gene, ethylene receptor <i>CsETR1</i> and some ethylene-responsive transcription factors were dramatically changed after GA₃ treatment, suggesting a potential involvement of ethylene in GA-regulated sex expression of cucumber. Interestingly, GA₃ down-regulated transcript of a C-class floral homeotic gene, <i>CAG2</i>, indicating that GA may also influence cucumber sex determination through an ethylene-independent process. These results suggest a novel model for hormone-mediated sex differentiation and provide a theoretical basis for further dissection of the regulatory mechanism of male flower formation in cucumber. <b>Statement:</b> We reveal that GA can regulate sex expression of cucumber via an ethylene-dependent manner, and the <i>M</i> (<i>CsACS2</i>), <i>CsETR1</i>, and <i>ERFs</i> are probably involved in this process. Moreover, <i>CAG2</i>, a C-class floral homeotic gene, may also participate in GA-modulated cucumber sex determination, but this pathway is ethylene-independent.