Dancing to the tune of the light regime: from perception to secondary metabolite biosynthesis in plants

The orchestra of ambient light intensity and wavelengths are pivotal in regulating the biosynthesis of secondary metabolites. Hence, the implications of light on plants' secondary metabolites are an active area of research, and new information is slowly being accrued. Plants have inbuilt light-sensitive molecules, such as: phytochromes, cryptochromes, phototropins, UVR8, and zeitlupe family proteins. In particular, blue light absorbed by cryptochrome has positive effects on antioxidants and other bioactive compounds. However, far-red light perceived by phytochrome reduces anthocyanin biosynthesis in plants, whereas mixed light in varying proportions is more beneficial for secondary metabolite synthesis. This article reviews the current information on plants' perception of light, their downstream signaling, and the role of different light regimes on secondary metabolite biosynthesis and accumulation. It also examines the epigenetic regulation of secondary metabolite biosynthesis under various light regimes. Additionally the review examines the economic and medicinal aspects of manipulating/controlling/optimizing secondary metabolite synthesis by light modulation. Consequently, this review is expected to benefit any researchers involved in fundamental and applied research.