MOLECULAR-CLONING AND CHARACTERIZATION OF A BRASSINOSTEROID-REGULATEDGENE FROM ELONGATING SOYBEAN (GLYCINE-MAX L.) EPICOTYLS

Brassinosteroids promote elongation and regulate gene expression in so ybean (Glycine max L.) stems. We constructed a cDNA library from brass inosteroid-treated soybean epicotyls and used differential hybridizati on to isolate a cDNA (pBRU1) corresponding to a transcript whose abund ance is increased by brassinosteroid treatment. Sequence analysis of p BRU1 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 (J.I. Medford, J.S. Elmer, H.J. K lee [1991] Plant Cell 3: 359-370), and significant homology (48% ident ity, 62% similarity) to a xyloglucan endotransglycosylase localized in the cell walls of nasturtium (J. de Silva, C.D. Jarman, D.A. Arrowsmi th, M.S. Stronach, S. Chengappa, C. Sidebottom, J.S. Reid [1993] Plant J 3: 701-711). RNase protection studies showed that BRU1 transcript l evels are not increased by 1.0 mu M auxins, cytokinins, abscisic acid, or gibberellic acid and that BRU1 expression is highest in stem tissu e. Findings from studies with run-on transcripts from isolated soybean nuclei most likely indicate that the regulation of BRU1 by brassinost eroids is largely posttranscriptional. The elevated levels of BRU1 tra nscripts in elongating tissue and the homology with a xyloglucan endot ransglycosylase suggest a possible role for the BRU1 protein in brassi nosteroid-stimulated elongation.