CDNA cloning of radish (Raphanus sativus) myrosinase and tissue-specific expression in root.

Two cDNA clones of myrosinase (thioglucoside glucohydrolase, EC 3.2.3.1) were isolated from radish (Raphanus sativus) seedlings. Both clones were identified as MB (B type myrosinase) from their sequence homology at the amino acid level to MBs cloned from other Brassicaceae species. The tissue distribution of gene expression and enzyme activity of myrosinase corresponded well to the site of glucosinolate accumulation in different tissues of radish. The myrosinase-glucosinolate system was localized in the cotyledons in the seedlings and in the peel of the root in the mature plant. Tissue printing analysis showed that myrosinase mRNA and activity were localized in the epidermis and vascular cambium that were present in the peripheral part of the root but few signals were detected in the parenchyma inside of the vascular cambium. Since the myrosinase-glucosinolate system is known to be a defense system in higher plants, the localization of the myrosinase-glucosinolate system in the peel of the root may act to protect the sink organ from the attack of herbivores or pathogens in soil.