Biosynthesis of geranial, a potent aroma compound in ginger rhizome (Zingiber officinale): Molecular cloning and characterization of geraniol dehydrogenase

Abstract

Fresh ginger (Zingiber officinale) rhizome is characterized by a pleasant citrus aroma and pungent flavor. The majority of the aroma-contributing volatiles are monoterpenoids, especially geraniol derivatives such as geranial, geranyl acetate, geraniol and citronellol. In this study, we investigated the interconversion of geraniol derivatives by incorporation experiments using deuterium-labeled geraniol and geranyl acetate. GC-MS analysis revealed that the incorporated geraniol and geranyl acetate were transformed and detected as geranial, geraniol, geranyl acetate and citronellol; however, nerol and neral were hardly detected. Next, we isolated and characterized a cDNA encoding geraniol dehydrogenase (ZoGeDH) by expressed sequence tag database mining. Phylogenic analysis of ZoGeDH resulted in its categorization into the cinnamyl alcohol dehydrogenase (CAD) group, along with the previously reported GeDHs of sweet basil (Ocimum basilicum) and wild perilla (P. setoyensis, P. citriodora, and P. frutescens). The recombinant ZoGeDH catalyzed the NADP-dependent oxidation from geraniol to citral. Furthermore, its substrate specificity was the highest for geraniol and nerol, while that for cinnamyl alcohol was 32% of the activity observed for geraniol. The expression levels of ZoGeDH in various ginger plant tissues were in accordance with the accumulation of geranial, except in old rhizome.