LAUSR

Male and female unisexual flowers evolved from hermaphroditic ancestors, and control of flower sex is useful for plant breeding. We isolated a female-to-male sex transition mutant in melon and identified the causal gene as the carpel identity gene CRABS CLAW (CRC). We show that the master regulator of sex determination in cucurbits, the transcription factor WIP1 whose expression orchestrates male flower development, recruits the corepressor TOPLESS to the CRC promoter to suppress its expression through histone deacetylation. Impairing TOPLESS-WIP1 physical interaction leads to CRC expression, carpel determination, and consequently the expression of the stamina inhibitor, the aminocyclopropane-1-carboxylic acid synthase 7 (CmACS7), leading to female flower development. Our findings suggest that sex genes evolved to interfere with flower meristematic function, leading to unisexual flower development. Description Floral sex determination In melons, flowers initially develop bisexually, but further development brings arrest of either carpel or stamen development, refining mature flowers into male or female. Only the female flowers produce melons. Zhang et al. have identified the genes involved in turning the bisexual primordial flower into either a male or female flower. The zinc finger transcription factor WIP1 interferes with carpel development, allowing male flower development to proceed. Conversely, expression of an enzyme involved in producing the hormone ethylene, perhaps supported by auxin signaling, supports and promotes female flower development. —PJH In melon and cucumber plants, sex determination genes manipulate the flower meristem to develop male or female unisexual flowers.