Rapid clearance of cytokinin controls transition from cell division to expansion during petal development in rose (Rosa hybrida)

Abstract The size of lateral organs is determined by well-coordinated cell proliferation and cell expansion. The transition from cell proliferation to expansion remains a largely unknown question in plant biology. Here, we report that miR159, an evolutionarily conserved microRNA, plays a crucial role in the transition from cell proliferation to expansion in rose ( Rosa hybrida ) petals through governing rapid cytokinin catabolism. We uncovered that Cytokinin Oxidase/Dehydrogenase 6 ( CKX6 ) is an authentic target of miR159 in petals. Knocking down miR159 levels resulted in the accumulation of CKX6 transcripts and precocious cytokinin clearance, consequently leading to an earlier transition to cell expansion and smaller petals. Conversely, knockdown of CKX6 caused excess cytokinin and delayed cell expansion, mimicking the effects of exogenous cytokinin application. MYB73, a R2R3-type MYB transcription repressor, recruited a co-repressor (TOPLESS) and a histone deacetylase (HDA19) to form a suppression complex, which governed the expression of MIR159 by modulating H3K9 acetylation levels at the MIR159 promoter. This work thus provides insights for ensuring correct timing of cell expansion and organ size via control of cytokinin catabolism.