Unravelling the role of phytosulfokine ( <scp>PSK</scp> ) in regulating rose petal growth through cell proliferation

Petal size significantly affects the ornamental quality of flowers, and cell proliferation plays a crucial regulatory role during the early stages of petal growth. Phytosulfokine (PSK) is a peptide hormone involved in regulating plant growth and development. However, its function in regulating petal size and the underlying mechanisms remain unclear. In this study, we isolated the PSK precursor gene RhPSK3;1, which is highly expressed during the early stages of petal growth in roses (Rosa hybrida). Silencing RhPSK3;1 shortened the duration of the cell proliferation phase in rose petals, reduced the number of abaxial sub-epidermal cells and ultimately decreased petal size, whereas exogenous PSK application produced the opposite effects. Rescue experiments confirmed that exogenous PSK application largely reversed the phenotypes induced by RhPSK3;1 silencing. Additionally, the homeodomain-leucine zipper II family transcription factor RhHAT9 directly binds to the RhPSK3;1 promoter and inhibits its transcription. In summary, our results elucidate the role of the peptide hormone PSK in rose petal growth, highlight its positive regulation of cell proliferation in petals, and enhance our understanding of petal size regulation from the perspective of peptide hormones.