Light regulates SlCOP1-mediated degradation of SlJAF13, a transcription factor essential for anthocyanin biosynthesis in Aft tomato fruit.

When they are exposed to light, the fruit of tomato plants (Solanum lycopersicum) carrying the dominant gene Anthocyanin fruit (Aft) accumulate anthocyanins. As the regulatory mechanism underlying this accumulation remains unclear, the role played by light in the regulation of SlJAF13, a bHLH transcription factor responsible for anthocyanin biosynthesis in tomato fruit peel, was examined. Gene expression analysis, GUS staining, and immunoblotting assays revealed that light enhanced the stability of SlJAF13 protein at a post-transcriptional level rather than transcriptionally. Protein-protein interaction assays and in vitro ubiquitination analysis revealed that CONSTITUTIVE PHOTOMORPHOGENIC1 (SlCOP1), a RING E3 ubiquitin ligase, physically interacted with SlJAF13, resulting in the ubiquitination and subsequent degradation of SlJAF13. Additionally, reductions in the levels of both anthocyanins and SlJAF13 protein were observed in fruit from plants over-expressing SlCOP1, providing further evidence that the suppressive effect of SlCOP1 on anthocyanin accumulation facilitated SlJAF13 degradation. These findings confirm the role of light in the stabilization of SIJAF13 in tomato fruit and thus provide novel insight into anthocyanin regulation in an important horticultural crop species under light conditions.