Integration of light quality signals regulates ABA abundance and stomatal movements during seedling establishment

Summary Obtaining sufficient light for photosynthesis and avoiding desiccation are two key challenges faced by seedlings during early establishment. Perception of light quality via specialised photoreceptors signals the availability of sunlight for photosynthesis. Canopy shade is depleted in red (R) and enriched in far‐red (FR) light, lowering R : FR ratio, while direct sunlight and sunflecks contain UV‐B. The balance between these wavelengths can determine the developmental strategy adopted by seedlings to either avoid shade, via stem elongation, or promote the expansion of photosynthetic organs. How seedlings regulate stomatal movements in different light environments is poorly understood. Using FR and UV‐B supplementation to mimic aspects of canopy shade and sunlight, respectively, we monitored stomatal apertures in Arabidopsis thaliana cotyledons and gas exchange in the cotyledons of Chinese kale ( Brassica oleracea var alboglabra). We show that low R : FR inhibits stomatal opening via a mechanism involving PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and increased abscisic acid (ABA). UV‐B perceived by the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor acts antagonistically, promoting stomatal opening in a response that requires phototropin photoreceptors. The convergence of phytochrome and UVR8 signalling to control ABA abundance enables plants to coordinate stem elongation and water use, potentially facilitating seedling establishment in dynamic light environments.