PlMYB51 and PlMYB1R1 switch PlABI5-PlCesA6 function to regulate seed dormancy and germination in Paeonia lactiflora

Seed dormancy is a key strategy for plants to adapt to fluctuating environments and ensure population propagation. Paeonia lactiflora seeds exhibit typical double dormancy, characterized by a prolonged germination cycle and low emergence rate that severely restrict their large-scale propagation. Although ABI5, the core transcription factor (TF) in the abscisic acid (ABA) signaling pathway, acts as a critical hub in maintaining seed dormancy, its upstream dynamic regulatory mechanism and downstream cell-wall-related target genes remain largely unknown. In this study, two upstream regulators of PlABI5, PlMYB51 and PlMYB1R1, were identified via yeast one-hybrid screening, and their regulatory mechanism was characterized. The results showed that both PlMYB51 and PlMYB1R1 belonged to the R2R3-MYB family. When acting alone, they directly bound to and activated the PlABI5 promoter, inhibiting seed germination. Upon forming a protein complex in the nucleus, their regulatory function was reversed and they repressed PlABI5 transcription to promote germination, supporting a proposed monomer activation-complex inhibition switch model. Furthermore, PlABI5 specifically targeted the PlCesA6 promoter and inhibited its expression without affecting the homolog PlCesA3. PlCesA6 positively regulated seed germination by promoting hypocotyl cell elongation, and its overexpression rescued the inhibition caused by PlMYB51/PlMYB1R1 overexpression. Collectively, this study proposes a working model for the PlMYB51/PlMYB1R1–PlABI5–PlCesA6 cascade in controlling seed dormancy and germination, and identifies a putative functional switching mode of MYB TFs mediated by protein-protein interactions. These findings provide key gene resources and a theoretical basis for artificial dormancy breaking and molecular breeding in P. lactiflora.