Antisense Arabidopsis plants indicate that an apoplastic α-xylosidase and α-glucosidase are encoded by the same gene

Arabidopsis thaliana (L.) contains two genes, AtAglu-1 and AtXYL1, that are homologous to eukaryotic and bacterial acid α-glucosidases [EC 3.2.1.20]. Products of these Arabidopsis genes are secreted and are more abundant in growing tissues or near plant surfaces. The AtXYL1 gene product is a 96–kDa glycoprotein active with 4-methylumbelliferyl-α-D-glucopyranoside. The lack of appropriate substrates for α-glucosidase in the apoplast has made it difficult to predict the function of these enzymes. AtXYL1 was also reported to encode an 89–kDa α-xylosidase [EC 3.2.1.–] active with xyloglucan oligosaccharides. We sought to determine whether these enzymes were the same product of AtXYL1, and hoped to shed light on the function of this gene in Arabidopsis by constructing transgenic plants expressing a portion of AtXYL1 in the antisense orientation. Despite having no obvious visible phenotype, some transgenic lines possessed up to 70% less acid α-glucosidase activity per mg protein than wild type plants. The neutral, non-glycosylated α-glucosidase was not affected by the transgene. In several antisense lines, α-xylosidase activity with XGO as the substrate and acid α-glucosidase activity was reduced proportionally. In wild type plants, the acid α-glucosidase and α-xylosidase activities were both associated with glycoproteins and were similarly sensitive to the α-glucosidase inhibitors castanospermine and 1–deoxynojirimycin, consistent with their being encoded by a single gene.