Dark-induced inactivation of the carbon assimilation process requires a water water cycle driven oxidative burst

Chloroplast metabolism is constantly fine-tuned by light availability through the perception and transmission of reductive and oxidative signals that activate or deactivate distinct metabolic enzymes. The reducing power originating from the photosynthetic electron transport chain has been shown to fuel the redox regulatory network, linking electron transport to the reductive activation of photosynthetic enzymes. However, the source of the oxidizing equivalents required to reverse photosynthetic enzyme activation and drive them toward an oxidized, inactive state has not yet been experimentally demonstrated. Here, we resolve redox dynamics associated with carbon assimilation inactivation by combining time-resolved redox imaging during the light-to-dark transition (LDT) with gas-exchange-based measurements. Dark-induced inactivation of carbon assimilation proved oxygen-dependent and coincided with an oxygen-dependent oxidative burst triggered during the LDT. This oxidative burst was suppressed under conditions that blocked electron transport to PSI or in plants in which PSI was photoinactivated. Notably, pgr5 and pgrl1ab mutants exhibited attenuated oxidative bursts and suppressed LDT-associated carbon assimilation inactivation, demonstrating that PGR5/PGRL1-dependent activity is required to generate the oxidative burst that drives CBC inactivation during LDT. These results establish a direct mechanistic link between oxygen- and PSI-dependent oxidative bursts and the inhibition of photosynthesis and mark the water-water cycle (WWC) as the primary source of the transient accumulation of oxidative equivalents that drive inactivation of Calvin-Benson cycle enzymes in darkness.