The cotton transcription factor YP positively regulates petal size by enhancing brassinosteroid-dependent cell expansion.

Petal size is crucial for both ornamental appeal and reproductive success in flowering plants, yet the molecular basis of its variation in many non-model species, including cotton, remains poorly understood. Here, we defined the period from -1 DPA to 0 DPA (10:00) as the rapid elongation phase for cotton (Gossypium hirsutum) petals. Disruption of R2R3-MYB gene GhYP in cotton prematurely arrested petal development (by 06:00 on 0 DPA) and reduced petal size through restraining epidermal cell expansion, whereas ectopic expression of its G. arboreum homolog GaYP in tobacco enlarged petals by promoting cell expansion. Transcriptome and metabolome profiling of cotton petals across developmental stages revealed down-regulation of brassinosteroid (BR) biosynthesis genes in ghyp petals, coinciding with markedly reduced levels of bioactive castasterone (CS) and its upstream intermediates. Exogenous application of 24-epibrassinolide (24-EBL), a bioactive BR compound, rescued the small-petal phenotype of the ghyp mutant by significantly enlarging its petal epidermal cells. Furthermore, dual-luciferase (Dual-LUC) and yeast one-hybrid (Y1H) assays demonstrated that GhYP modulates the expression of multiple BR biosynthesis genes without detectable direct promoter binding. Collectively, these findings establish that YP transcription factors (GhYP and GaYP) promote epidermal cell expansion by enhancing BR biosynthesis, thereby increasing petal size. Our work will advance understanding of the genetic basis of cotton petal development.