Dissecting the shade avoidance response in Arabidopsis

The shade avoidance response is a strategy of major adaptive significance to plants in natural communities, and depends on the ability of the plant to perceive the presence of neighbours. Within a vegetation, the ratio of red to far-red (R/FR) is lowered by the absorption of R light by photosynthetic pigments. This change is perceived through the phytochrome system as a signal of the proximity of neighbours. Upon sensing a low R/FR ratio, a plant reacts very rapidly and enhances elongation growth even before it is directly shaded. If the plant succeeds in the attempt to overgrow its neighbours and the photosynthetic organs perceive daylight again, the shade avoidance response is rapidly reverted through phytochrome photoconversion. Recent work revealed that the same low R/FR signal that induces hypocotyl elongation also triggers a rapid arrest of leaf primordium growth ensuring that all plant resources are redirected into extension growth. Genetic and molecular studies demonstrated the existence of a previously unrecognized regulatory circuit underlying plant response to canopy shade, which involves both auxin and cytokinin. Consistent with the rapidity of the growth responses to low R/FR and its reversibility upon perception of high R/FR, the induction of positive regulators is very rapid and reversible. Strikingly, low R/FR also provokes a rapid induction of the HFR1/SICS1 gene to negatively regulate the downstream signaling initiated by the same signal. HFR1/SICS1 acting as a negative controller of the shade avoidance response ensures that an exaggerated reaction does not occur when the plant is unsuccessful in escaping canopy shade.