Sugar-dependent alterations in cotyledon and leaf development in transgenic plants expressing the HDZhdip gene ATHB13

ATHB13 is a new member of the homeodomain leucine zipper (HDZip) transcript ion factor family of Arabidopsis thaliana. Constitutive high-level expressi on of the ATHB13 cDNA in transgenic plants results in altered development o f cotyledons and leaves, specifically in plants grown on media containing m etabolizable sugars. Cotyledons and leaves of sugar-grown transgenic plants are more narrow and the junction between the petiole and the leaf blade le ss distinct, as compared to the wild type. High-level expression of ATHB13 affects cotyledon shape by inhibiting lateral expansion of epidermal cells in sugar-treated seedlings. Experiments with non-metabolizable sugars indic ate that the alteration in leaf shape in the ATHB13 transgenics is mediated by sucrose sensing. ATHB13 further affects a subset of the gene expression responses of the wild-type plant to sugars. The expression of genes encodi ng beta -amylase and vegetative storage protein is induced to higher levels in response to sucrose in the transgenic plants as compared to the wild ty pe. The expression of other sugar-regulated genes examined is unaffected by ATHB13. These data suggest that ATHB13 may be a component of the sucrose-s ignalling pathway, active close to the targets of the signal transduction.