Central role of cyclic electron transport around photosystem I in the regulation of photosynthesis

Cyclic electron transport around photosystem I generates ATP without the accumulation of NADPH in chloroplasts. In angiosperms, electron transport consists of a PGR5-PGRL1 protein-dependent pathway and a chloroplast NADH dehydrogenase-like complex-dependent pathway. Most likely, the PGR5-PGRL1 pathway corresponds to the cyclic phosphorylation discovered by Arnon and contributes mainly to Delta pH formation in photosynthesis. ATP synthesis utilizes this Delta pH formed by both linear and PSI cyclic electron transport. Furthermore, acidification of the thylakoid lumen downregulates light energy utilization in photosystem II and also electron transport through the cytochrome b(6)f complex. In the absence of PGR5, chloroplast NDH compensates for the reduced Delta pH formation to some extent. Additionally, proton conductivity is upregulated, probably through ATPase, in pgr5 mutants. The photosynthetic machinery likely forms a complex network to maintain high photosynthesis activity under fluctuating light conditions.