PRR proteins of the circadian clock call time on plant shade avoidance

Light limitation represents a significant threat to plant survival. Shade-intolerant species have, therefore, evolved mechanisms to detect and avoid shading by neighbors. Plants detect the proximity and density of neighboring vegetation by monitoring alterations in light quality (1). Phytochrome photoreceptors detect changes in the ratio of red (R) to far-red light (FR), with phytochrome B performing a dominant role. R is absorbed by living vegetation and used for photosynthesis, whereas the majority of FR is transmitted through and reflected within canopies. R:FR is, therefore, reduced proportionally with increasing depth of canopy (1). Early perception of encroaching shade enables plants to rapidly elongate stems and elevate leaves to overtop competitors and avoid light limitation. Such responses are termed shade avoidance and can promote survival in mixed stands (1). Shade avoidance is regulated by a group of transcription factors named PHYTOCHROME INTERACTING FACTORS (PIFs), with PIF4, PIF5, and PIF7 performing dominant roles (2⇓–4). These PIFs collectively promote synthesis of the growth-promoting hormone auxin (4, 5). In sunlight (high R:FR), phytochrome B becomes activated …