Regulatory Components of Shade Avoidance Syndrome

Competition for light has an important impact on plant development. Plants sense the presence of nearby competitor vegetation as a change in the light quality, i.e. a reduced red to far-red ratio. The responses to shade are generally referred to as the shade avoidance syndrome (SAS), and involve various developmental changes aimed to outgrow the neighbouring plants, and are characterized by enhanced elongation, reduced leaf expansion, decreased branching and ultimately early flowering. These responses can be detrimental in agriculture, because they induce reallocation of resources into elongation growth at the expense of harvestable organs, hence lowering the crop yield. Genetic analyses performed on the SAS response of seedlings have shown the involvement of several transcription factors in the regulation of this response. At least in a few cases, it has been shown that phytochrome rapidly regulates the expression levels of several modulators of hormone responsiveness, rapidly linking shade perception, massive changes in gene expression and modification of hormone sensitivity of the responsive tissues. Here we develop our view on how shade-modulated changes in the transcriptional profiles result in complex SAS responses.