Light and the E3 ubiquitin ligase COP1/SPA control the protein stability of the MYB transcription factors PAP1 and PAP2 involved in anthocyanin accumulation in Arabidopsis

Abstract

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.

MYB75 Phosphorylation by MPK4 Is Required for Light-Induced Anthocyanin Accumulation in Arabidopsis

“…In Arabidopsis, anthocyanins only accumulate in the light (Maier et al, 2013). In agreement with its function in regulating anthocyanin biosynthesis, MYB75/PAP1 is regulated by light at both the transcriptional and posttranslational levels.…”

“…Consistent with its key role in light-induced anthocyanin biosynthesis, MYB75/PAP1 gene expression is induced by light and precedes the expression of MYB90/PAP2 and the structural genes (CHS, DFR, F3H, and LDOX) (Cominelli et al, 2008). However, overexpression of MYB75/PAP1 is insufficient on its own to stimulate anthocyanin accumulation in the dark (Maier et al, 2013;Li et al, 2014). In the dark, MYB75/PAP1 is targeted for degradation by the CONSTITUTIVE PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYTOCHROME A (COP1/SPA) E3 ubiquitin ligase.…”

“…In the dark, MYB75/PAP1 is targeted for degradation by the CONSTITUTIVE PHOTOMORPHOGENIC1/SUPPRESSOR OF PHYTOCHROME A (COP1/SPA) E3 ubiquitin ligase. When plants are exposed to light, COP1/SPA is inactivated and MYB75/PAP1 is stabilized and able to enhance anthocyanin biosynthesis (Maier et al, 2013). Accordingly, anthocyanins accumulate even in the dark in cop1 and spa mutants (Deng et al, 1991;Hoecker et al, 1998;Laubinger et al, 2004).…”

“…Overexpressing the MYB TFs increases anthocyanin levels in Arabidopsis (Borevitz et al, 2000;Gonzalez et al, 2008). Therefore, it seems that these MYB TFs are limiting factors regulating anthocyanin biosynthesis (Maier et al, 2013). In Arabidopsis seedlings, the expression of MYB75 predominates over the other three MYBs, indicating that MYB75, also known as PRODUCTION OF ANTHOCYANIN PIGMENT1 (PAP1), is a key TF in activating anthocyanin biosynthetic genes (Gonzalez et al, 2008).…”

MYB-related transcription factors function as regulators of the circadian clock and anthocyanin biosynthesis in Arabidopsis

“…[18][19][20][21][22][23] In Arabidopsis, anthocyanin is only accumulated in the presence of light and this accumulation is promoted by different abiotic and biotic stress conditions such as cold, drought, and pathogen attack. 24,25 Almost 2 decades ago, the expressions of the genes involved in anthocyanin biosynthesis, such as CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and DIHYDROFLA-VONOL 4-REDUCTASE (DFR), were found to be regulated by a circadian rhythm. 26 A number MYB transcription factors have been found to act as regulators of the flavonoid biosynthesis pathway.…”