Stress-induced flavonoid biosynthesis and the antioxidant machinery of plants

There is a growing body of evidence that flavonoids do not primarily function as UV-B screening pigments in photoprotection. Recent findings support the idea that excess light stress, irrespective of the relative proportions of the solar wavebands reaching the leaf surface, upregulates the biosynthesis of dihydroxy B-ring-substituted flavonoid glycosides, as a consequence of and aimed at countering the generation of ROS. Intriguingly, the very conditions that lead to the inactivation of antioxidant enzymes can also upregulate the biosynthesis of antioxidant flavonoids,which suggests flavonoids constituting a secondary ROS-scavenging system in plants exposed to severe/prolonged stress conditions. H2O2 may diffuse out of the chloroplast at considerable rates and be transported to the vacuole, the storing site for flavonoids, by tonoplast intrinsic proteins, under severe excess lightconditions. We suggest that the unanticipated key role of the vacuole in the ROS homeostasis might be mediated byflavonoids.