The NPR7-TGA6-MYB1 module promotes anthocyanin biosynthesis and integrates salicylic acid and strigolactone signaling...

The salicylic acid (SA) signal transduction pathway and its association with anthocyanin biosynthesis have been reported, but the molecular mechanisms underlying SA-mediated anthocyanin biosynthesis remain unclear. In this study, we elucidated the SA signaling pathway associated with anthocyanin biosynthesis and the crosstalk between SA and strigolactone (SL) in apple (Malus × domestica). MYELOBLASTOSIS 1 (MdMYB1) played a pivotal role in SA-induced anthocyanin biosynthesis, and the SA-responsive factor TGACG-MOTIF-BINDING FACTOR 6 (MdTGA6) facilitated anthocyanin accumulation by directly activating MdMYB1 expression. The key regulatory component of SA signaling, NONEXPRESSER OF PR GENES 7 (MdNPR7), potentiated MdTGA6-mediated transcriptional activation of MdMYB1, thereby sustaining MdTGA6-promoted anthocyanin biosynthesis. Furthermore, MdNPR7 physically interacted with MdMYB1 to enhance transcriptional activation of downstream anthocyanin biosynthetic genes. Additionally, SA-suppressed E3 ubiquitin ligase RING-H2 FINGER PROTEIN A2a (MdRHA2a) mediated ubiquitination and degradation of MdNPR7 in the absence of SA. The SL signaling repressor SUPPRESSOR OF MAX2 1-LIKE 8 (MdSMXL8) antagonized MdNPR7-promoted anthocyanin biosynthesis by dissociating the MdNPR7-MdTGA6 and MdNPR7-MdMYB1 protein complexes, thereby mediating SA-SL signaling crosstalk. Taken together, our results delineate the molecular mechanism of SA-induced anthocyanin biosynthesis and reveal a regulatory mode that may integrate the SA and SL signaling pathways to regulate anthocyanin production.