Cytokinin-responsive P-type cyclins control Arabidopsis radial style morphology

The biological mechanisms responsible for correct shape acquisition at the apex of the female reproductive organ, the gynoecium, remain poorly understood, despite its fundamental importance for successful plant reproduction and seed production. This process involves a rare bilateral-to-radial symmetry transition in Arabidopsis thaliana, orchestrated in part by the transcription factor SPATULA (SPT). Here, we show that SPT negatively regulates cell cycle regulators CYCLIN-P3;1 (CYCP3;1) and CYCP3;2, which are antagonistically promoted by the hormone cytokinin (CK), to control the radial style morphology by orchestrating a coherent feed-forward loop that converges on the spatial regulation of CYCP3;1 and CYCP3;2 expression. Overexpression of CYCP3s disrupts style radial symmetry, causing the split-style phenotype and hypersensitivity to CK observed in the spt mutant. Finally, we demonstrate a genetic link connecting the machinery of cell division orientation, controlled by auxin, with the cell proliferation input induced by CK, which adds robustness to the apical fusion of the carpels.