Characterization of a rice β-glucosidase highly expressed in flower and germinating shoot

Abstract

The cDNAs for two β-glucosidase (E.C. 3.2.1.21) isozymes from rice (Oryza sativa L.), designated bglu1 and bglu2, were cloned and sequenced. The cDNA sequences for bglu1 and bglu2 included open reading frames encoding 504 and 500 amino acid precursor proteins, respectively. Both of these enzymes appeared to enter the secretory pathway, as judged by their N-terminal signal sequences. Southern blots using gene-specific probes indicated that bglu1 and bglu2 were single copy genes. The bglu1 and bglu2 mRNAs were highly expressed in the shoot during germination, with a similar time course. However, differences were seen in expression in mature plants, where bglu1 was highly expressed in flowers, but bglu2 was not. A recombinant thioredoxin fusion protein produced from the bglu1 cDNA in redox-deficient Excherichia coli (BGlu1) hydrolyzed p-nitrophenol β-D-glucoside, β-D-fucoside, and other p-nitrophenol β-D-glycosides, and was strongly inhibited by glucono-1,5-lactone. It also hydrolyzed some natural glucosides, including the rice-derived pyridoxine-5′-O-β-D-glucoside, and hydrolyzed and transglycosylated short β-(1 → 3) and β-(1 → 4) linked gluco-oligosaccharides. Based on the results, possible functions of BGlu1 include hydrolysis and recycling of oligosaccharides generated from rapid cell wall expansion during seed germination and flower expansion, and release of the coenzyme pyridoxine from its glucose-conjugated storage form. © 2003 Elsevier Ireland Ltd. All rights reserved.