The unexpected flavone synthase-like activity of polyphenol oxidase in tomato

The biosynthesis of flavones has drawn considerable attention. However, the presence of flavones and their biosynthesis in tomato (Solanum lycopersicum) remain unclear. Here, we confirmed that flavones are present in MicroTom tomato and unexpectedly found that a tomato polyphenol oxidase (SlPPO F) possesses a flavone synthase-like activity and catalyzes the conversion of eriodictyol to luteolin without the need for any cofactor. SlPPO F showed a similar K_m value to that of other polyphenol oxidases, and could be inhibited by ascorbic acid. The flavone synthase-like activity of SlPPO F exhibited strict substrate specificity and only accepted flavanones with two hydroxyl groups (3' and 4') on the B ring as substrates. SlPPO F showed higher catalytic efficiency and better thermostability than type I flavone synthase from Apium graveolens, suggesting its possible application in enzyme engineering. In summary, we identified flavones in tomato and unraveled a polyphenol oxidase exhibiting flavone synthase-like activity.