Phytochrome A elevates plant circadian-clock components to suppress shade avoidance in deep-canopy shade

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Plants can detect the proximity and density of neighboring vegetation using phytochrome photoreceptors. In shade-sensitive species, canopy cover drives stem elongation, facilitating light foraging. In deep shade, where resources are severely limited, excessive elongation growth can be detrimental to plant survival. In these conditions, phytochrome-A signaling suppresses shade avoidance. Here, we provide a molecular mechanism controlling this response. We show that phytochrome A promotes early-evening increases in the expression of the circadian-clock components TIMING OF CAB EXPRESSION 1(TOC1), PSEUDO RESPONSE REGULATOR 7 (PRR7), EARLY FLOWERING 3 (ELF3), and ELF4, encoding proteins that collectively limit hypocotyl elongation. This indicates that light signals are integrated with the circadian clock to control plant development in stressful environments.