Photoprotective role of plant secondary metabolites: Biosynthesis, photoregulation, and prospects of metabolic engineering for enhanced protection under excessive light

Light is essential for plants, but excessive light is damaging to plant health. Photoprotection is defined as the prevention against damaging effects of intense solar radiation. Plants have evolved morpho-physiological and biochemical adaptations to shield themselves from harmful radiations. Secondary metabolites are low molecular weight organic compounds that play manifold roles in plants including defense and interactions with the environment. Flavonoids are phenolic compounds while carotenoids are tetraterpenes that play crucial role in photoprotection. Several studies indicate that light regulates the accumulation of these metabolites. The underlying mechanistic details about the light signaling factors regulating the synthesis of these secondary metabolites have expanded over the last few years. These include the photoreceptors PHY, CRY and UVR8, light-regulated transcription factors like HY5, PIFs, MYBs and BBXs and their downstream targets PSY, CHI, CHS, FLS, that modulate carotenoid and flavonoid biosynthesis. Here we discuss these molecular mechanisms and how metabolic engineering is now being used to modulate these pathways, to enhance the production of beneficial metabolites. This can be useful for the generation of climate-resilient plants resistant to excessive radiation in the future. Additionally, optimum artificial lighting can enhance the production of these metabolites which often have nutritional and medicinal benefits.