| Description: | Many plants respond to low R:FR ratio specifically with enhancement of cell elongation, and anticipation of flowering to the detriment of leaf production.The shade avoidance response is mediated by the phytochromes, but only recently a candidate, ATHB-2, for downstream factors participating in this response has been identified.ATHB-2 belongs to the HD-Zip class of transcription factors, peculiar to plants. ATHB-2 is rapidly and strongly induced by changes in the R:FR ratio.The phenotypes of plants overexpressing ATHB-2 are reminiscent of those displayed by wild-type plants germinated and grown in FR-rich light (longer hypocotyls and petioles, smaller and fewer leaves, and a thinner root mass).Seedlings with reduced levels of ATHB-2 show reciprocal phenotypes. Together with the tight regulation of the ATHB-2 gene by the phytochrome system, these data imply a major role for this HD-Zip protein in the regulation of the shade avoidance responses.Remarkably, anatomical studies in the hypocotyl of ATHB-2 transgenic plants indicated that the alteration of elongation growth was the result of major changes in both the orientation of cell expansion and the production of the secondary vascular tissue. Plants with reduced levels of ATHB-2 showed shorter epidermal and cortical cells while the proliferation of secondary vascular tissue was found to be strongly increased compared to wild-type plants.On the contrary, the elongated phenotype in the ATHB-2 overexpressing plants was found to be the consequence of the same two events but in opposite direction: a change in the orientation of cell expansion toward elongation in cells that do not divide and the inhibition of secondary cell proliferation. Similar changes have been observed in wild-type seedlings grown in environmental light conditions simulating canopy shade. Finally, our recent findings suggest that there is at some point a convergence between the phytochrome pathway that regulates the shade avoidance responses (through the action of ATHB-2) and auxin pathway(s) (http://www.ncbi.nlm.nih.gov/htbin-post/Entrez/query?uid=10477292&form=6&db=m&Dopt=b). Here, we propose the identification of downstream components in the shade avoidance responses, some of which might be direct targets of ATHB-2. To this end, RNA samples will be prepared from 2 week-old plants grown in high R:FR ratios (-) and in high R:FR ratios followed by 4h of FR-rich light treatment (+). The RNAs will be prepared from the aerial part of the plants. |
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