Light and the E3 ubiquitin ligase <scp>COP</scp>1/<scp>SPA</scp> control the protein stability of the <scp>MYB</scp> transcription factors <scp>PAP</scp>1 and <scp>PAP</scp>2 involved in anthocyanin accumulation in Arabidopsis

TLDR

Anthocyanins accumulate only in light-grown Arabidopsis, and darkness repression depends on the COP1/SPA ubiquitin ligase, as cop1 and spa mutants produce anthocyanins in the dark. The study aims to demonstrate that COP1/SPA proteins interact with the MYB transcription factors PAP1 and PAP2 and regulate their transcript and protein levels to control anthocyanin biosynthesis. COP1/SPA ubiquitin ligase targets PAP1 and PAP2 for proteasomal degradation in darkness and suppresses their transcription, while light stabilizes these proteins. COP1/SPA-mediated degradation and transcriptional repression of PAP1/2 in darkness, coupled with light-dependent stabilization, explains why anthocyanin accumulation requires light and why cop1 mutants accumulate anthocyanins in the dark, highlighting the complex’s dual regulatory role and its potential for biotechnological enhancement of anthocyanin content.

Abstract

Anthocyanins are natural pigments that accumulate only in light-grown and not in dark-grown Arabidopsis plants. Repression of anthocyanin accumulation in darkness requires the CONSTITUTIVELY PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYA-105 (COP1/SPA) ubiquitin ligase, as cop1 and spa mutants produce anthocyanins also in the dark. Here, we show that COP1 and SPA proteins interact with the myeloblastosis (MYB) transcription factors PRODUCTION OF ANTHOCYANIN PIGMENT1 (PAP)1 and PAP2, two members of a small protein family that is required for anthocyanin accumulation and for the expression of structural genes in the anthocyanin biosynthesis pathway. The increased anthocyanin levels in cop1 mutants requires the PAP1 gene family, indicating that COP1 functions upstream of the PAP1 gene family. PAP1 and PAP2 proteins are degraded in the dark and this degradation is dependent on the proteasome and on COP1. Hence, the light requirement for anthocyanin biosynthesis results, at least in part, from the light-mediated stabilization of PAP1 and PAP2. Consistent with this conclusion, moderate overexpression of PAP1 leads to an increase in anthocyanin levels only in the light and not in darkness. Here we show that SPA genes are also required for reducing PAP1 and PAP2 transcript levels in dark-grown seedlings. Taken together, these results indicate that the COP1/SPA complex affects PAP1 and PAP2 both transcriptionally and post-translationally. Thus, our findings have identified mechanisms via which the COP1/SPA complex controls anthocyanin levels in Arabidopsis that may be useful for applications in biotechnology directed towards increasing anthocyanin content in plants.