Genomic Brassinosteroid Effects

Detailed analysis of brassinosteroid (BR)-regulated genes can provide evidence of the molecular basis of BR effects. Classical techniques (such as subtractive cDNA cloning) as well as cDNA and oligonucleotide microarrays have been applied to identify genes which are upregulated or downregulated after BR treatment or are differently expressed in BR-deficient or -insensitive mutants compared with wild type plants. Genes encoding cell-wall-modifying enzymes, enzymes of the BR biosynthetic pathway, auxin response factors, and transcription factors are subject to BR regulation. Effects on several other metabolic pathways and interactions with other phytohormones have been reported as well, although some of these effects may depend on certain environmental conditions (for example, light/dark or stress), the developmental stage of the plants, and tissue types. The identification of components of the BR signal transduction pathway revealed different modes of transcriptional control in animals and plants. Steroid signaling in plants comprises the plasma membrane receptor kinases BRI1 and BAK1 and intracellular protein phosphorylations. Thus, BR signaling in plants is reminiscent of growth factor and TGF-beta signal transduction in animals. The phosphorylation cascade could be a basis of extensive signaling cross-talk and thereby explain the complexity of BR responses.