Time-dependent bistability leads to critical slowing down during floral transition in Arabidopsis.

Developmental transitions occur in the life cycles of all multicellular organisms. Despite their fundamental relevance, the underlying dynamics remain poorly understood. In plants, floral transition is a key developmental process whereby the shoot apical meristem changes from producing leaves to forming flowers. Using quantitative imaging, developmental genetics, and dynamical systems theory, we show that a time-dependent bistable switch between expression of APETALA2, a key floral inhibitor, and the floral activators SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 and FRUITFULL, can explain the dynamics of floral transition in Arabidopsis. Notably, we detect a slowing down of the inhibitor dynamics, consistent with the system crossing a critical point of a bistable switch and transiently experiencing a ghost attractor. We demonstrate that this time-dependent bistability is essential to generate the range of dynamical behaviours measured across genotypes, including oscillations in the inhibitor, and that it also confers robustness in the transition. Collectively, our work provides quantitative evidence of time-dependent bistability underlying floral transition, which introduces a new timescale to this developmental process.