Cyclic electron flow compensates loss of PGDH3 and concomitant stromal NADH reduction.

In nature plants constantly experience changes in light intensities. Low illumination limits photosynthesis and growth. However, also high light intensities are a threat to plants as the photosynthetic machinery gets damaged when the incoming energy surpasses the capacity of photochemistry. One limitation of photochemistry is the constant resupply of stromal electron (e^-) acceptors, mainly NADP. NADP is reduced at the acceptor-side of photosystem I. The resulting NADPH is utilized by the Calvin-Benson-Bassham cycle (CBBC) and the malate valve to ensure sufficient oxidized NADP ready to accept e^- from PSI. Lately, additional pathways, which function as stromal e^- sinks under abiotic stress conditions, were discovered. One such reaction in Arabidopsis thaliana is catalyzed by PHOSPHOGLYCERATE DEHYDROGENASE 3 (PGDH3), which diverts e^- from the CBBC into NADH. pgdh3 loss-of-function mutants exhibit elevated non-photochemical quenching (NPQ) and fluctuating light susceptibility. To optimize plant photosynthesis in challenging environments knowledge on PGDH3's metabolic integration is needed. We used the source of high NPQ in pgdh3 as a starting point. Our study reveals that increased NPQ originates from high cyclic electron flow (CEF). Interestingly, PGDH3 function seems very important when the CEF-generator PROTON GRADIENT REGULATION5 (PGR5) is lost. Consequently, pgr5pgdh3 double mutants are more sensitive to fluctuating light.