Flavonoids as antioxidants in plants under abiotic stresses

Flavonoids make a relevant contribution to the response mechanisms of higher plants to a plethora of abiotic stresses. In addition to the long-reported functions as screeners of damaging short-wave solar radiation, flavonoids have been suggested as playing key functions as antioxidants in stressed plants, by inhibiting the generation and reducing reactive oxygen species (ROS) once formed. The ROS-scavenging properties of flavonoids are restricted to few structures, namely, the dihydroxy B-ring-substituted flavonoid glycosides. This structure activity relationship conforms to the well-known stress-induced preferential biosynthesis of dihydroxy B-ring-substituted both flavones and flavonols. These flavonoids, especially the derivatives of quercetin, have been shown to greatly affect the movement of auxin at intra-and intercellular levels, and hence to tightly regulate the development of individual organs and the whole plant. The effectiveness of flavonoids to inhibit the activity of the auxin efflux facilitator proteins tightly depends on the chemical features that confer the antioxidant potential. In this review article, we discuss about (1) the effect of different abiotic stresses on the accumulation of individual flavonoids, (2) the potential role served by antioxidant flavonoids in the antioxidant machinery of plants exposed to severe stress conditions, and (3) the function of flavonoids as developmental regulators.