Plant stomata open in response to blue light under a background of red light. Red light induces starch accumulation through guard cell photosynthesis, whereas blue light causes rapid starch degradation via the activation of plasma membrane H+-ATPase through phosphorylation of BLUS1 by phototropins, thus promoting stomatal opening. However, how phototropins mediate starch degradation remains unclear. In this study, we identified WD-repeat protein 48 (WDR48), which is essential for starch degradation. Phosphoproteomic analysis revealed that WDR48 was phosphorylated in response to blue light. Mutation of WDR48 impaired starch degradation and stomatal opening by blue light. Phototropins interacted with and phosphorylated WDR48. WDR48 and BLUS1 constituted separate signalling pathways required for starch degradation. We propose that the coordinated control of starch synthesis by red light and its degradation by blue light signalling through WDR48 and BLUS1 is a key mechanism for stomatal opening by red and blue light.
Yamauchi et al. (Fri,) studied this question.