Control of Non-Photochemical Exciton Quenching by the Proton Circuit of Photosynthesis

Abstract

This chapter discusses our current understanding of the chloroplast proton circuit, i.e., those reactions that involve the storage and utilization of light energy in the transfer of protons, and its importance for regulating photosynthetic light-capture/electron-transfer reactions. The photosynthetic machinery of plants is finely tuned to balance the needs for efficient light capture with an avoidance of photodamage by regulating the capture of light energy, via thermal dissipation of excess excitation energy (assessed from non-photochemical quenching, NPQ, of chlorophyll fluorescence) by regulating light-driven electron transfer processes. In addition to driving ATP synthesis at the chloroplast ATP synthase, the thylakoid electrochemical gradient of protons or proton-motive force (pmf) plays a central role in regulating NPQ. The transthylakoid proton concentration gradient (ΔpH) component of pmf triggers the “energy-dependent”, or qE component of NPQ, which protects photosystem II from photodamage and regulates electron transfer through the cytochrome b 6 f complex, thereby preventing damage to photosystem I. The extent and mode of storage in ΔpH and ΔΨ of pmf are regulated by several processes that respond to the metabolic, or physiological, state of the organism. The extent of pmf is determined by proton influx (via linear and alternative electron flows) into the thylakoid lumen, and proton efflux through the chloroplast ATP synthase. Both processes are modulated by, or responsive to, environmental conditions and resulting metabolic fluctuations. Proton influx is controlled by linear electron flow and a series of alternative electron flow pathways, possibly including cyclic electron flow around photosystem I, the Mehler peroxidase reaction (or water-water cycle), and oxidation of plastoquinol by the plastid terminal oxidase. The fraction of pmf stored as ΔpH is also regulated by plastidic ionic strength or luminal buffering capacity, altering the sensitivity of pH-dependent processes to pmf. The integrated regulation of these processes is an open, active area of research.