Mechanisms underpinning natural variation in non-photochemical quenching kinetics.

Plants use light as an energy source to reduce carbon dioxide into carbohydrates during photosynthesis. However, when the incident light exceeds the photosynthesis rate, the excess energy must be dispersed, or it can result in the unregulated formation of harmful reactive oxygen species, especially in plants exposed to very high light or abiotic stress conditions that compromise photosynthetic efficiency. The excess energy is typically dispersed harmlessly as heat, which can be measured as non-photochemical quenching (NPQ) of chlorophyll fluorescence. NPQ kinetics vary within plant populations, and understanding the basis of this variation will contribute to improving resiliency to abiotic stresses, including high light, in crops. Here it is reviewed how three key NPQ genes, Photosystem II subunit S (PsbS), Violaxanthin de-epoxidase (VDE), and Zeaxanthin epoxidase (ZEP), contribute to natural variation in NPQ kinetics. PsbS expression level is an important determinant of NPQ variation, whereas VDE and ZEP contribute to NPQ variation via post-translational regulation related to natural variation in many genes affecting these enzymes' activity. Post-translational mechanisms that influence NPQ, including redox regulation via thioredoxins and regulation of ascorbate availability, thylakoid lumen pH, and violaxanthin accessibility are discussed. There are also addressed NPQ regulatory mechanisms beyond PsbS, ZEP, and VDE, including natural regulation of light accessibility, modulation of light harvesting, and feedback from the steps following light harvesting. Finally, how this knowledge can be harnessed to engineer more resilient crops is briefly summarized.