BREVIPEDICELLUS and ERECTA control the expression of AtPRX17 to prevent Arabidopsis callus browning

Efficient in vitro callus generation is required for tissue culture propagation, a process that allows for plant regeneration and transgenic breeding for desired phenotypes. Identifying genes and regulatory elements that prevent impaired callus growth and callus browning is essential for the development of in vitro callus systems. Here, we show that the BREVIPEDICELLUS and ERECTA pathways in Arabidopsis calli converge to prevent callus browning, and positively regulate the expression of the isoperoxidase gene AtPRX17 in rapidly growing calli. Loss-of-function mutations in both BREVIPEDICELLUS and ERECTA resulted in markedly increased callus browning. Transgenic lines expressing 35S pro::AtPRX17 in the bp-5 er105 double mutant background fully rescued this phenotypic abnormality. Using in vivo (chromatin immunoprecipitation-PCR and transient expression) and in vitro (electrophoretic mobility shift assays) assays, we observed that the BREVIPEDICELLUS protein binds directly to the upstream sequence of AtPRX17 to promote its transcription during callus growth. ERECTA is a ubiquitous factor required for cell proliferation and growth. We show that ERECTA positively regulates the expression of the transcription factor WRKY6, which directly binds to a separate site on the AtPRX17 promoter, further increasing its expression. Our data reveal an important molecular mechanism involved in the regulation of peroxidase isozyme expression to reduce Arabidopsis callus browning.