A NAC transcription factor in Rosa multiflora regulates lignin accumulation by interacting with RmBZR1 during prickle hardening.

Prickles are specialized hardening outgrowths characterized by the lignin deposition, the primary component of secondary cell walls (SCWs). While NAC transcription factors are known to function as crucial regulators of SCW formation through hierarchical transcriptional networks, their specific roles in prickle lignification and hardening in Rosa species have yet to be fully elucidated. In this study, we identified a NAC transcription factor in R. multiflora, RmPPN1, which exhibits preferential expression in developing and mature prickles. Ectopic expression of RmPPN1 significantly enhanced lignin accumulation in transgenic Arabidopsis. Through yeast two-hybrid screening of a prickle cDNA library, we isolated RmBZR1 as an interactor partner of RmPPN1. Expression analysis revealed that RmBZR1 displays the highest expression in initial prickles, a pattern that is complementary to the expression profile of RmPPN1. Exogenous application of brassinolide (BL) and its inhibitor brassinazole (BRZ) exerted opposing effects on lignin accumulation in developing prickles during early morphogenesis. Functional characterization of RmBZR1 demonstrated its negative regulation on lignin biosynthesis. Exogenous treatment with BRZ effectively rescued the phenotype of reduction in lignin content. Furthermore, RmBZR1 binds to PAL1 promoter and represses its transcription, while RmPPN1 protein physically interacts with RmBZR1 to alleviate its suppression of the PAL1 promoter. Together, these findings suggest a novel feedback regulatory module in which RmPPN1 and RmBZR1 coordinately control lignin biosynthesis during prickle development.