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

Abstract

Summary: 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 PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA‐105 (COP 1/SPA) ubiquitin ligase, as cop1 and spa mutants produce anthocyanins also in the dark. Here, we show that COP 1 and SPA proteins interact with the myeloblastosis (MYB) transcription factors PRODUCTION OF ANTHOCYANIN PIGMENT 1 (PAP)1 and PAP 2, 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 PAP 1 gene family, indicating that COP 1 functions upstream of the PAP 1 gene family. PAP 1 and PAP 2 proteins are degraded in the dark and this degradation is dependent on the proteasome and on COP 1. Hence, the light requirement for anthocyanin biosynthesis results, at least in part, from the light‐mediated stabilization of PAP 1 and PAP 2. Consistent with this conclusion, moderate overexpression of PAP 1 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 PAP 1 and PAP 2 transcript levels in dark‐grown seedlings. Taken together, these results indicate that the COP 1/SPA complex affects PAP 1 and PAP 2 both transcriptionally and post‐translationally. Thus, our findings have identified mechanisms via which the COP 1/SPA complex controls anthocyanin levels in Arabidopsis that may be useful for applications in biotechnology directed towards increasing anthocyanin content in plants.