Plants growing in close proximìty exhibit the shade avoidance syndrome in response to reflected far-red (FR) radiation from neighbors. The shade avoidance response essentially redirects resources and growth potential from the leaf and storage organs into increased extension growth to optimize light capture by plants. Molecurar-genetic studies implicated ATHB2, a homeodomain-leucine zippper (HD- Zip) transcription factor, as a major regutator of the shade avoidance response in Arabidopsis. To further characterize the function of ATHB2 and related HD-Zip proteins (HD-Zip II) Arabidopsis transgenic plants expressing a derivative of ATHB2 containing a single aminoacid substitution in the HD domain (ATHB2N51A), whích is known to abolish DNA binding of homodimeric as well as heterodimeric complexes, were generated. This mutation is expected to produce a dominant-negative phenotype. Indeed, at the seedling stage, the phenotype of the ATHB2N51A lines is the same as that shown by antiiense ATHB2 lines, but more severe. At later stages of development, the dominant-negative plants produce more leaves and an higher number of lateral inflorescences than the controls. Furthermore, expression of ATHB2N51A in Arabidopsis seedlings attenuates the elongation response induced by shade. We then took advantage of the Arabidopsis dominant negative athb2 mutant to investigate the function of the tomato orthologues. The HD-Zip domain within the HD-Zip II family has been highly conserved during evolution, and thus it is likely that overexpression of the Arabidopsis ATHB2N5IA gene in tomato may result in the formation of heterodimeric complexes with the endogenous HD-ZIP II proteins preventing that of functional dimers. Transgenic tomato expressing costitutively the Arabilopsis ATHB2N51A gene showed strong alteration of phenotype. Plants had shorter internodes and high proliferation of lateral branches. Furthermore flowering was delayed. We can speculate that the Arabidopsis ATHB2N51A forms heterodimers with the tomato ortholog, so that the transcription of the target gene is highly affected.