Flower size, a primary agronomic trait in breeding of ornamental plants, is largely determined by petal expansion. Generally, ethylene acts as an inhibitor of petal expansion, but its effect is restricted by unknown developmental cues. In this study, we found that the critical node of ethylene-inhibited petal expansion is between stages 1 and 2 of rose flower opening. To uncover the underlying regulatory mechanism, we carried out a comparative RNA-seq analysis. Differentially expressed genes (DEGs) involved in auxin-signaling pathways were enriched. Therefore, we identified an auxin/indole-3-acetic acid (Aux/IAA) family gene, RhIAA14, whose expression was development-specifically repressed by ethylene. The silencing of RhIAA14 reduced cell expansion, resulting in diminished petal expansion and flower size. In addition, the expressions of cell-expansion-related genes, including RhXTH6, RhCesA2, RhPIP2;1, and RhEXPA8, were significantly downregulated following RhIAA14 silencing. Our results reveal an Aux/IAA that serves as a key player in orchestrating petal expansion and ultimately contributes to flower size, which provides new insights into ethylene-modulated flower opening and the function of the Aux/IAA transcription regulator.

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.