|
Sterol Biosynthesis and Signaling in Plant Development
Kathrin Schrick The characterization of Arabidopsis patterning mutants first led to the hypothesis that sterols have signaling roles in plants. The mutants exhibit defects such as multiple meristems in both the embryo and seedling. The developmental defects are not rescued by exogenous application of brassinosteroids, which are the only steroid hormones identified from plants thus far. Moreover, while mutants affecting the brassinosteroid biosynthesis pathway show characteristic dwarf phenotypes, they do not exhibit patterning defects in embryogenesis. These observations indicate that steroid molecules in addition to brassinosteroids have roles in embryonic development. Candidate sterol-binding proteins in plants include transcription factors of the homeodomain class termed HD-START. Proteins of this family contain a DNA-binding domain, the homeodomain (HD), associated with a leucine zipper (ZLZ or ZIP) dimerization domain, and a lipid/sterol binding domain (START). The START domain from human StAR has been shown to bind sitosterol as well as cholesterol in vitro, consistent with the possibility that the START domains from plants also bind sterols. The presence of START domains in transcription factors reveals a potential mechanism by which lipid/sterol ligands regulate gene transcription in plants. HD-START transcription factors are implicated cell differentiation during development: Several correspond to striking mutant phenotypes in Arabidopsis and have layer-specific expression patterns in both Arabidopsis and rice. It is postulated that the binding of ligands to the START domain of HD-START transcription factors functions to control cell differentiation in plants. Molecular, genetic, and proteomics approaches are being applied to investigate the role of START domains in plant development.
|
