ARF3-mediated auxin signaling is essential for sex determination in cucumber

Sex determination underpins genesis of male and female flowers with particularly important implications in plant breeding. Auxin and ethylene regulate femaleness in cucurbits. Here, we identified an auxin response factor 3 ( CsARF3 ) that plays an essential role in carpel development in cucumber. Knockout of CsARF3 resulted in an androecious phenotype with only male flowers, whereas overexpression of CsARF3 led to increased number of female flowers. CsARF3 promotes femaleness by directly stimulating the expression of meristem maintenance gene CsSTM , and repressing the activity of gynoecious gene CsWIP1 . Auxin and ethylene exhibit a reciprocal relationship during sex determination, in which ethylene promotes carpel formation through auxin at the early stage of flower development. The auxin signaling in carpels then enhances ethylene biosynthesis to inhibit stamen development.