The ATHB-2 gene encoding an homeodomain-leucine zipper protein is rapidly and strongly induced by changes in the ratio of red to far-red light which naturally occur during the daytime under the canopy and induce in many plants the shade avoidance response. Here, we show that elevated ATHB-2 levels inhibit cotyledon expansion by restricting cell elongation in the cotyledon-length and -width direction. We also show that elevated ATHB-2 levels enhance longitudinal cell expansion in the hypocotyl. Interestingly, we found that ATHB-2-induced, as well as shade-induced, elongation of the hypocotyl is dependent on the auxin transport system. In the root and hypocotyl, elevated 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 rescue 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 assay. 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 radial expansion, and in an increase in hypocotyl secondary cell proliferation. A model of ATHB-2 function in the regulation of shade-induced growth responses is proposed.

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