ECTOPIC EXPRESSION OF THE ARABIDOPSIS TRANSCRIPTIONAL ACTIVATOR ATHB-1 ALTERS LEAF CELL FATE IN TOBACCO

The Arabidopsis thaliana Athb-1 is a homeobox gene of unknown function . By analogy with homeobox genes of other organisms, its gene product, Athb-1, is most likely a transcription factor involved in development al processes, We constructed a series of Athb-1-derived genes to exami ne the roles of Athb-1 in transcriptional regulation and plant develop ment, Athb-1 was found to transactivate a promoter linked to a specifi c DNA binding site by transient expression assays, In transgenic tobac co plants, overexpression of Athb-1 or its chimeric derivatives with h eterologous transactivating domains of the yeast transcription factor GAL4 or herpes simplex virus transcription factor VP16 conferred deeti olated phenotypes in the dark, including cotyledon expansion, true lea f development, and an inhibition of hypocotyl elongation, Expression o f Athb-1 or the two chimeric derivatives also affected the development of palisade parenchyma under normal growth conditions, resulting in l ight green sectors in leaves and cotyledons, whereas other organs in t he transgenic plants remained normal, Both developmental phenotypes we re induced by glucocorticoid in transgenic plants expressing a chimeri c transcription factor comprising the Athb-1 DNA binding domain, the V P16 transactivating domain, and the glucocorticoid receptor domain, Pl ants with severe inducible phenotypes showed additional abnormality in cotyledon expansion. Our results suggest that Athb-1 is a transcripti on activator involved in leaf development.