Molecular mechanisms of plant adaptation to canopy shade

A plant growing in the field has the unique ability to sense the presence of other plants growing nearby and adjust its growth rate accordingly. The early perception of neighbour proximity depends on the detection of light quality changes. Within a vegetation community, the ratio of red (R) to far-red (FR) light is lowered by the absorption of R light by photosynthetic pigments. This light quality change is perceived through the phytochrome system as an unambiguous signal of the proximity of neighbours, and induces a suite of developmental responses (termed the shade avoidance response) that, when successful, result in the overgrowth of those neighbours. The shade avoidance response is a strategy of major adaptive significance to plants in natural communities. However, since elongation is often achieved at the expense of leaf and root growth, and, in the long term, low R/FR exposure leads to early flowering with a reduced seed set, shade avoidance can significantly impact yield in high-density crop plantings. Work performed by several laboratories, including ours, identified several key transcription factor genes that underpin shade avoidance and uncovered complex molecular interactions between low R/FR light and hormone signalling pathways. Among hormones, auxin seems to have a central role in several aspects of shade avoidance response. Many lines of evidence connect this hormone to the rapid elongation response provoked by light quality changes. Furthermore, our recent work establishes that auxin plays a pivotal role in leaf and root responses to low R/FR as well, suggesting that this hormone may act as a coordinator of plant growth responses to environmental light quality changes. Shade avoidance is a highly dynamic response. 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. On the other hand, persistent low R/FR signalling triggers mechanisms that prevent an exaggerated reaction when the plant is unsuccessful in escaping canopy shade. HFR1/SICS1, a bHLH transcription factor, plays a central role in the attenuation of virtually all plant responses to canopy shade. Our recent data demonstrate that HFR1/SICS1 functions in the phyB signal transduction pathway, and acts in concert with other transcription factors modulated through phyA in the adaptation of the plant to low R/FR environments.