Regulatory networks for the shade avoidance response

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 as an unambiguous signal of the proximity of neighbours through the phytochrome system. Upon sensing a low R/FR ratio, a shade avoiding plant reacts very rapidly and enhances elongation growth at the expenses of leaf and root development. If the plant succeeds in the attempt to overgrow its neighbours, the shade avoidance response is rapidly reverted through phytochrome photoconversion. Many lines of evidence connect the phytohormone auxin to the rapid elongation response provoked by light quality changes. Our recent work establishes that auxinplays a pivotal role in leaf and root responses to low R/FR aswell, suggesting that this phytohormone may act as a coordinator of plant growth responses to environmental light quality changes. Persistency of a low R/FR signal results in attenuation of the shade avoidance response, which is controlled in part through the action of the bHLH HFR1/SICS1 transcription factor gene. 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 this unfavourable environmental condition.