Molecular Cloning and Characterization of a Brassinosteroid - Regulated Gene f rom Elongating Soybean ( GIycine max 1 . ) Epicotyls ' Daniel

Brassinosteroids promote elongation and regulate gene expression in soybean (Clycine max 1.) stems. We construded a cDNA library from brassinosteroid-treated soybean epicotyls and used differential hybridization to isolate a cDNA (pBRU1) corresponding to a transcript whose abundance is increased by brassinosteroid treatment. Sequence analysis of pBRU1 revealed an open reading frame of 283 amino acids with a putative signal peptide of 29 amino acids. The sequence had extensive homology (77% identity, 89% similarity) over 114 contiguous amino acids to the meri-5 gene of Arabidopsis thaliana (1.1. Medford, J.S. Elmer, H.J. Klee [1991] Plant Cell 3: 359-370), and significant homology (48% identity, 62% similarity) to a xyloglucan endotransglycosylase localized in the cell walls of nasturtium (1. de Silva, C.D. Jarman, D.A. Arrowsmith, M.S. Stronach, S. Chengappa, C. Sidebottom, J.S. Reid [1993] Plant J 3: 701-711). RNase protedion studies showed that BRUl transcript levels are not increased by 1.0 p~ auxins, cytokinins, abscisic acid, or gibberellic acid and that BRUl expression i s highest in stem tissue. Findings from studies with run-on transcripts from isolated soybean nuclei most likely indicate that the regulation of BRUl by brassinosteroids i s largely posttranscriptional. The elevated levels of BRUl transcripts in elongating tissue and the homology with a xyloglucan endotransglycosylase suggest a possible role for the BRUl protein in brassinosteroid-stimulated elongation.