Functional analysis of homeodomain-leucine zipper transcription factors in Arabidopsis thaliana

Homeodomain-leucine zipper (HDZip) proteins constitute a large family of transcription factors apparently unique to plants. To elucidate the function of these factors, the biochemical properties in vitro as well as the effects on transgenic plants when expressed at high levels were studied. The conclusion is that HDZip proteins are very similar with respect to DNA-binding specificity in vitro but appear to be active in different aspects of plant development. Thus, functional specificity of HDZip proteins is most likely determined by other aspects of proteins function, e.g. their capacity to interact with other proteins. High-level expression of the HDZip gene ATHB5 in transgenic plants results in hypersensitivity to the inhibitory effect of the plant hormone abscisic acid (ABA) on seed germination and seedling root growth. Furthermore, the expression of ATHB5 in germinating seedlings is downregulated in the Arabidopsis ABA response mutants abi3 and abi5. Together, these data suggest that ATHB5 acts as a regulator of seed germination and postgerminative growth downstream of ABI3 and ABI5 in an ABA response-signaling pathway. Enhanced levels of the HDZip gene ATHB13 in transgenic Arabidopsis confer a sugar-dependent reduction of cotyledon width. In addition, a subset of known sugar-dependent genes was hyperinduced by sucrose in Arabidopsis seedlings overexpressing ATHB13. These data suggest that ATHB13 affects both cotyledon morphology and gene regulation as a component of a sucrose-signaling pathway. Loss-of-function mutations in ATHB5 and ATHB13 did not result in any discernable mutant phenotypes, suggesting that these genes are only required under specific physiological conditions or that they act in a redundant fashion in the plant