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. In this work, we identified an auxin response factor 3 (CsARF3) that plays an essential role in carpel development in cucumber. Deletion of CsARF3 resulted in an androecious phenotype with only male flowers, whereas overexpression of CsARF3 led to an increased number of female flowers. CsARF3 promotes femaleness by directly stimulating the expression of the meristem maintenance gene CsSTM and repressing the activity of the 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. Editor's summary: Cucumber plants selectively arrest the development of either male or female organs to generate unisexual flowers. Han et al. found that a cucumber auxin response factor, ARF3, promotes female carpel development by up-regulating STM, a transcription factor that helps to maintain stem cell pools. Additionally, ARF3 activates ethylene synthesis in male organs to prevent stamen formation. This work provides mechanistic insight into how auxin interplays with ethylene to facilitate floral sex determination. —Madeleine Seale [ABSTRACT FROM AUTHOR]