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The development of unisexual flowers through floral organ suppression has been critical for promoting genetic diversity within angiosperms. About 20-30% of angiosperms have unisexual flowers, and many sexual systems exist and have been independently derived through convergent evolution. Unisexual flowers are especially prevalent in the Poaceae (grass family). The grasses contain several important agricultural crops for humans such as wheat, rice, barley, and maize. Some of the genetics of unisexual flower development in maize and barley have been studied, but the precise mechanisms controlling grass unisexual flower development remain elusive. In maize, unisexual flowers develop in two separate inflorescences, the male tassel which has only male stamens and the female ear which contains only female carpels. Carpel suppression occurs in the tassel and lower flower of the ear through a programmed cell death pathway. Previously identified genes in this pathway have multiple roles in flower development making it difficult to dissect the molecular underpinnings of carpel suppression in maize. One more specific regulator of carpel suppression in the tassel is the class 1 HD-ZIP transcription factor GRASSY TILLERS1 (GT1). gt1 mutants fail to suppress carpels in the tassel and also fail to maintain dormant axillary buds. Here, I identify a novel carpel suppression gene, RAMOSA3 (RA3), encoding a trehalose-6-phosphate phosphatase, that has a genetic interaction with GT1 to suppress carpel and tiller bud growth. Trehalose-6-phosphate (T6P) is a signaling molecule that regulates carbon status in the cell and can negatively impact bud dormancy. In addition to the genetic interaction between GT1 and RA3, I found that exogenous trehalose, which can increase T6P to toxic levels, can suppress carpels in gt1 ra3 double mutants, suggesting that sugar signaling is important for carpel suppression. RNA-seq profiling of gt1, ra3, and gt1 ra3 mutant tassels revealed misexpression of sugar signaling and characterized eudicot dormancy genes. Rising sucrose and trehalose-6-phosphate levels are strongly correlated with outgrowth of axillary buds in eudicots. Axillary bud suppression is also regulated by the environment and several hormone pathways, and can greatly differ among species and within species in different environments. Because of the great plasticity in pathways regulating bud dormancy, I propose a dormancy pathway was recruited to suppress maize carpels. |
