Flavonol catabolism in Arabidopsis thaliana: subcellular localization of a flavonol 3-O-glucoside beta-glucosidase and biochemical evidence for a flavonol rhamnoside 7-O-alpha-rhamnosidase

Flavonols are specialized plant metabolites with numerous human health promoting properties. Flavonol glycosides accumulate in Arabidopsis in response to abiotic stresses, but their concentrations decrease with stress recovery. Moreover, the catabolism of flavonol 3-O-β-glucosides is dependent upon activity of the β-glucosidase BGLU15. Transient expression of a BGLU15-mCherry fusion protein in onion epithelial cells revealed that it is localized to the apoplast; this implies flavonol glucoside catabolism is an extracellular process. In Arabidopsis, the predominant glycosyl modification of flavonols is rhamnosylation at their 7-hydroxy position. High-performance liquid chromatography analysis of enzyme assays containing cell-free Arabidopsis leaf extracts revealed the presence of flavonol 7-O-α-rhamnosidase activity. This activity increased in Arabidopsis leaves as they aged. Minor levels of this activity were present in roots, whereas no activity was detected in stems. The identification of this novel flavonol 7-O-α-rhamnosidase activity may have future implications for biotechnological strategies aimed at enhancing flavonol levels in plants.