SnRK1 acts upstream of PAP1, contributing to fine-tuning flavonoid biosynthesis in high-light.

In land plants, one of the processes vital to cope with environmental changes is the accumulation of photoprotective flavonoids such as flavonols and anthocyanins. The inactivation of SUCROSE NON-FERMENTING1 RELATED PROTEIN KINASE1 (SnRK1), which acts in a chloroplast-derived sugar signalling pathway, permits the activation of flavonoid biosynthesis in high-light. The present study provides genetic evidence that SnRK1 acts upstream of PRODUCTION OF ANTHOCYANIN PIGMENT1 (PAP1), encoding a crucial transcription factor that activates the anthocyanin branch of flavonoid biosynthesis during high-light acclimation. A time-resolved expression analysis indicates a two-step suppression of MYB LIKE2 (MYBL2), a repressor of anthocyanin production, involving SnRK1 inactivation for stable anthocyanin accumulation during prolonged high-light exposure. Furthermore, overexpression of PAP1 resulted in the marked suppression of MYB11, MYB12 and MYB111 and FLAVONOL SYNTHASE1, initiating the flavonol branch of the pathway. Analysis of a flavonoid-deficient CHALCONE SYNTHASE mutant overexpressing PAP1 suggests that regulation of flavonoid biosynthesis is independent of flavonoid intermediates and end products but PAP1-dependent. It is proposed that PAP1 suppresses the flavonol branch by an as yet unknown mechanism, thereby promoting the consumption of carbon building blocks for anthocyanin production to permit the fine-tuning of the pathway.