Auxin and cytokinin regulate growth dynamics underlying carpel initiation in Arabidopsis.

Plant organ initiation requires precise spatial and temporal coordination of cellular behaviors. In Arabidopsis, the gynoecium, the female reproductive structure formed by two fused carpels, is initiated after the termination of the floral meristem. Proper initiation of the carpels is crucial for ovule protection, successful fertilization, and the formation of diverse fruit structures. While the phytohormones auxin and cytokinin are known to regulate organogenesis, their interplay during the earliest stages of carpel development is not fully understood. In particular, how auxin-cytokinin crosstalk influences growth patterns that shape the emerging gynoecium remains unknown. Here, we combined confocal live imaging, hormone treatments, and hormone reporters to capture at cellular resolution the dynamic processes of cell expansion and division that drive the onset of the carpel developmental program. We show that carpel primordia initiation is driven by growth differences between the fast-expanding peripheral and slow-growing central regions, transforming the dome-shaped floral meristem into two fused carpels connected by a continuous ring of cells. Ectopic cytokinin treatments increase cell growth and division to promote an increase in the size of the carpel primordia, whereas inhibition of auxin transport has the opposite effect. Our results suggest that the interplay between auxin and cytokinin is indispensable for establishing the correct organ geometry during carpel initiation.