Negative autoregulation of the Arabidopsis homeobox gene ATHB-2

The Arabidopsis homeobox gene ATHB-2 is tightly regulated by light signals, and is thought to direct morphological changes during shade avoidance resp onses. To understand how ATHB-2 mediates light signals in plant morphogenes is, we investigated its transcriptional network. We constructed a gene enco ding a chimeric transcription factor (HD-Zip-2-V-G) that is expected to act ivate target genes of ATHB-2 in a glucocorticoid-dependent manner. In trans genic Arabidopsis plants expressing HD-Zip-2-V-G, glucocorticoid treatment activates the ATHB-2 gene itself, independent of de novo protein synthesis. An in vitro DNase I-footprinting experiment showed that recombinant ATHB-2 protein specifically bound to an ATHB-2 promoter region. These complementa ry results indicate that ATHB-2 recognizes its own promoter. Consistent wit h the fact that ATHB-2 itself has been shown to act as a repressor, express ion of the endogenous ATHB-2 gene was repressed in transgenic plants overex pressing an ATHB-2 transgene. Moreover, target-gene analysis using the HD-Z ip-2-V-G suggested that ATHB-2 recognizes other HD-Zip II subfamily genes. We conclude that ATHB-2 has a negative autoregulatory loop and may be invol ved in a complicated transcriptional network involving paralogous genes, as is the case with animal homeobox genes.