Shade avoidance responses are mediated by the ATHB-2 HD-Zip protein, a negative regulator of gene expression

The ATHB-2 gene encoding an homeodomain-leucine zipper protein is rapidly a nd strongly induced by changes in the ratio of red to far-red light which n aturally occur during the daytime under the canopy and induce in many plant s the shade avoidance response. Here, we show that elevated ATHB-2 levels i nhibit cotyledon expansion by restricting cell elongation in the cotyledon- length and -width direction. We also show that elevated ATHB-2 levels enhan ce longitudinal cell expansion in the hypocotyl, Interestingly, we found th at ATHB-2-induced, as well as shade-induced, elongation of the hypocotyl is dependent on the auxin transport system. In the root and hypocotyl, elevat ed ATHB-2 levels also inhibit specific cell proliferation such as secondary growth of the vascular system and lateral root formation, Consistent with the key role of auxin in these processes, we found that auxin is able to re scue the ATHB-2 lateral root phenotype, We also show that reduced levels of ATHB-2 result in reciprocal phenotypes. Moreover, we demonstrate that ATHB -2 functions as a negative regulator of gene expression in a transient assa y. Remarkably, the expression in transgenic plants of a derivative of ATHB- 2 with the same DNA binding specificity but opposite regulatory properties results in a shift in the orientation of hypocotyl cell expansion toward ra dial expansion, and in an increase in hypocotyl secondary cell proliferatio n, A model of ATHB-2 function in the regulation of shade-induced growth res ponses is proposed.