PlSIRP1 Activates PlABI5 via Dual Mechanisms to Repress PlPIP2;1 and Maintain ABA-Mediated Seed Dormancy in Paeonia lactiflora.

Seed dormancy is a critical adaptive trait that ensures seedling establishment under favourable conditions. Paeonia lactiflora seeds exhibit dual hypocotyl and epicotyl dormancy, severely limiting large-scale propagation. Although abscisic acid (ABA) is the central hormone governing dormancy, the molecular mechanisms linking ABA signalling to seed water status remain elusive. Here, we identified that the RING-H2 E3 ubiquitin ligase PlSIRP1 acts as a positive regulator of ABA-mediated seed dormancy in P. lactiflora. PlSIRP1 expression was induced by ABA and peaked during deep dormancy. Mechanistically, PlSIRP1 promoted dormancy through dual nondegradative mechanisms: it directly bound and activated the PlABI5 promoter, while concurrently interacting with PlABI5 in the nucleus to enhance its DNA-binding and transcriptional repression activities without triggering PlABI5 ubiquitination or altering its stability. We further demonstrated the aquaporin PlPIP2;1 as a direct downstream target; PlABI5 repressed PlPIP2;1 transcription to restrict seed water uptake and hypocotyl elongation. Genetic and transgenic analyses established that PlSIRP1 functions upstream of PlABI5 to maintain dormancy, a regulatory hierarchy conserved in Arabidopsis thaliana. Collectively, our findings establish the PlSIRP1-PlABI5-PlPIP2;1 signalling module and reveal a nondegradative paradigm by which an E3 ubiquitin ligase couples ABA signalling to aquaporin-mediated water transport. These results provide molecular insights into ABA-dependent dormancy regulation in perennial plants and suggest potential strategies for alleviating seed dormancy to improve propagation efficiency.