ELONGATED HYPOCOTYL 5 mediates blue light-induced starch degradation in tomato.

Starch is the major storage carbohydrate in plants, and its metabolism in chloroplasts depends mainly on light. However, the mechanism through which photoreceptors regulate starch metabolism in chloroplasts is unclear. In this study, we found that the cryptochrome 1a (CRY1a)-mediated blue light signal is critical for regulating starch accumulation by inducing starch degradation through the HY5 transcription factor in the chloroplasts in tomato. cry1a mutants and HY5-RNAi plants accumulated more starch and presented lower transcript levels of starch degradation-related genes in their leaves than did the wild-type (WT) plants. Blue light significantly induced the transcription of starch degradation-related genes in the wild-type and CRY1a- or HY5-overexpressing plants but had little effect in the cry1a and HY5-RNAi plants. Dual-luciferase assays, electrophoretic mobility shift assays (EMSA) and chromatin immunoprecipitation (ChIP)-qPCR revealed that HY5 could activate the starch degradation-related genes PWD, BAM1, BAM3, BAM8, MEX1 and DPE1 by directly binding to their promoters. Silencing of HY5 and these starch degradation-related genes in CRY1a-overexpressing plants led to increased accumulation of starch and decreased accumulation of soluble sugars. These findings presented here not only deepen our understanding of how light control starch degradation and sugar accumulation but also allow us to explore potential targets for improving crop quality.