Molecular and biochemical characterization of BNST4, an ethanol-inducible steroid sulfotransferase from Brassica napus, and regulation of BNST genes by chemical stress and during development

We previously characterized a steroid sulfotransferase from Brassica napus, designated BNST3, having a substrate preference for metabolic precursors of 24-epibrassinolide. A function in brassinosteroid inactivation was hypothesized for the sulfotransferase, based on the lack of biological activity of 24-epibrassinolide sulfate, and the induction of BNST genes in response to salicylic acid. In this paper, we demonstrate that BNST genes are inducible by ethanol and other xenobiotics, as well as low oxygen stress. A cDNA coding for a new member of the BNST gene family, designated BNST4, was cloned from ethanol-treated B. napus seedlings. BNST4 displays a similar specificity towards 24-epibrassinosteroids as compared with BNST3. However, the specificity observed with steroids in vitro is relatively broad, which may be compatible with a function in detoxication under stress conditions. Furthermore, the induction of BNST proteins by the herbicide safener naphthalic anhydride suggests a possible contribution of sulfotransferases to herbicide metabolism. The developmental pattern of BNST3 expression was determined in transgenic Arabidopsis transformed with a promoter-GUS fusion. In seedlings, early BNST3 expression is observed in the transition zone between the root and hypocotyl, and at the tip of cotyledons. In adult plants, BNST3 is expressed in pollen grains, prior to dehiscence. This pattern of expression is suggestive of a metabolic function associated with specific processes of cell differentiation and tissue maturation during normal development.