TCP3 is a substrate of the COP1/SPA ubiquitin ligase to regulate anthocyanin accumulation and flowering time in Arabidopsis

COP1 is a conserved ubiquitin ligase found in plants and animals. In plants, COP1
acts together with SPA proteins to suppress light signaling in darkness by promoting
the degradation of transcription factors involved in photomorphogenesis. Substrates of
this ubiquitin ligase share a Valine-Proline (VP) motif that interacts with the WD-repeat
domain of COP1 in plants and humans. Here, we have identified the transcription
factor TCP3 as a noncanonical substrate of COP1/SPA that lacks a VP motif. The
TCP domain of TCP3 directly interacts with the WD-repeat domains of COP1 and
SPA1. TCP3 requires the VP-binding cleft of COP1 for protein–protein interaction.
We further show that the TCP3 protein is degraded in darkness and preferentially in
short day through a COP1-dependent manner, while TCP3 is stabilized by red, far-red,
blue light, and long day conditions. COP1/SPA-mediated degradation of TCP3 inhib-
its anthocyanin accumulation by reducing the expression of anthocyanin biosynthesis
genes. COP1/SPA-mediated degradation of TCP3 is also important in regulating
flowering time. Taken together, our results have identified a noncanonical substrate of
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the COP1/SPA ubiquitin ligase, thereby also uncovering TCPs as a transcription factor
family that is targeted by COP1/SPA. Since the COP1/SPA-interacting TCP domain is
conserved among TCPs, it is possible that other members of the TCP family—having
divergent functions including cell fate determination and hormone signaling—are
targets of COP1/SPA as well.