Isoprenoids and phenylpropanoids are key components of the antioxidant defense system of plants facing severe excess light stress

•Primary antioxidants may decline under a severe excess of radiant energy.•Secondary metabolites are key components of the antioxidant machinery of plants.•Isoprenoids may function as chloroplast antioxidants in drought- and heat-stressed leaves.•Flavonoids may serve peculiar antioxidant functions in severely stressed plants.

Plants face excess light stress on daily as well as on seasonal basis. The excess of excitation energy on cellular organelles prone to reactive oxygen species (ROS) generation is further enhanced when plants growing in full sun concurrently experience drought and heat stress. These are the very conditions that promote the biosynthesis of a wide range of secondary metabolites. Plants display a highly integrated arsenal of ROS-detoxifying agents to keep ROS concentration under control for efficient signalling, while avoiding cell death. There is evidence that primary antioxidants, i.e., antioxidant enzymes and low molecular-weight antioxidants, such as ascorbic acid and glutathione, are depleted under a severe excess of radiant energy. Here we discuss about how relevant secondary metabolites, namely isoprene, carotenoids, and flavonoids may complement the function of primary antioxidants to avoid irreversible oxidative damage, when plants experience intense, even transient stress events. We offer evidence of how plants orchestrate daily the antioxidant machinery, when challenged against multiple environmental stresses. It is indeed conceivable that daily variations in sunlight irradiance and air temperature may greatly alter the effectiveness of primary and secondary ROS-detoxifying agents. Finally, we discuss about the possible inter-relation between isoprenoid and flavonoid metabolism in plants facing high light coupled with drought and heat stress, as a consequence of severe stress-induced redox imbalance.

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