Regulatory networks in shade avoidance response

The success of competitive interactions between plants determines the chance of survival of individuals and eventually of whole plant species. Shade-tolerant plants have adapted their photosynthesis to function optimally under low-light conditions. These plants are therefore capable of long-term survival under a canopy shade. In contrast, shade avoiding plants adapt their growth to perceive maximum sunlight and therefore rapidly dominate gaps in a canopy. Daylight contains roughly equal proportions of red (R) and far-red (FR) light, but within vegetation that ratio is lowered as a result of the R absorption by photosynthetic pigments. This light quality change is perceived through the phytochrome (phy) system as an unambiguous signal of the proximity of neighbours resulting in the shade avoidance response (SAR). This adaptive reaction is achieved by a set of responses including enhanced internode and petiole extension growth, increased apical dominance, retarded leaf development, and an acceleration of flowering. However, if a plant succeeds in the attempt to overgrow its neighbours and the photosynthetic organs perceive daylight again, the SAR is rapidly reverted through phytochrome photoconversion. The adaptive responses result in changes in the distribution of assimilates between leaves, stems, and roots. Since SAR results in allocation of resources to parts of the plant both harvestable and non-harvestable, any benefit for farmers from manipulation of the SAR in a crop will depend on the plant concerned and the part of plant to be harvested. Significant improvement in crop yield might be achieved by a better understanding of the adaptation mechanisms operating in the plant on the whole and at the level of single organs. Using Arabidopsis thaliana as a model system, a network of transcription factors acting as positive and negative regulators of shade avoidance response has been described. Low R/FR perceived by phyB and other type II phys significantly increases the stability of several PIF proteins which in turn induce the expression of genes coding for factors promoting shade avoidance responses (genes early induced by low R/FR). PIFs also induce the HFR1/SICS1 gene coding for a transcription factor that switches off the transcriptional cascade early induced by low R/FR. Our recent data demonstrate that HFR1/SICS1 functions in the phyB signal transduction pathway and acts in concert with another transcription factor modulated through phyA, a type I phytochrome, in the adaptation of the plant to canopy shade.