SUMMARYAnthocyanins 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 PHOTOMORPH-OGENIC1/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.

ORCID ID: 0000-0002-8800-2400 (V.R.)CULLIN4-RING E3 ubiquitin ligases (CRL4s) regulate key developmental and stress responses in eukaryotes. Studies in both animals and plants have led to the identification of many CRL4 targets as well as specific regulatory mechanisms that modulate their function. The latter involve COP10-DET1-DDB1 (CDD)-related complexes, which have been proposed to facilitate target recognition by CRL4, although the molecular basis for this activity remains largely unknown. Here, we provide evidence that Arabidopsis thaliana DET1-, DDB1-ASSOCIATED1 (DDA1), as part of the CDD complex, provides substrate specificity for CRL4 by interacting with ubiquitination targets. Thus, we show that DDA1 binds to the abscisic acid (ABA) receptor PYL8, as well as PYL4 and PYL9, in vivo and facilitates its proteasomal degradation. Accordingly, we found that DDA1 negatively regulates ABAmediated developmental responses, including inhibition of seed germination, seedling establishment, and root growth. All other CDD components displayed a similar regulatory function, although they did not directly interact with PYL8. Interestingly, DDA1-mediated destabilization of PYL8 is counteracted by ABA, which protects PYL8 by limiting its polyubiquitination. Altogether, our data establish a function for DDA1 as a substrate receptor for CRL4-CDD complexes and uncover a mechanism for the desensitization of ABA signaling based on the regulation of ABA receptor stability.

SignificancePhenotypic adaptations of plants in response to changes in climate are well known to be mediated by molecular mechanisms, including activation or suppression of transcription factors that control target gene expression. However, the chromatin changes that are essential for the binding of transcription factors are much less understood. Gene derepression at the chromatin level is considered to be the starting point for gene transcription. We report a mechanism of gene derepression through which HOS15 promotes the degradation of histone deacetylase HD2C in a cold-dependent manner that correlates with increased levels of acetylated histones on COR gene chromatin. Moreover, HOS15 directly promotes COR gene transcription by association of CBF transcription factors with the “open” state of the target COR chromatin.