Involved in the regulation of the cyclic electron flow (CEF) around Photosystem I. Essential for the reduction of PGRL1A by ferredoxin and for photoprotection. Interacts with PGRL1A and PGRL1B. Up-regulated by drought stress. Disulfide bonds; Cys-11 and Cys-105 are probably involved in the formation of disulfide bridges with 'Cys-300' and 'Cys-303' of PGRL1A. 'Cys-272' and 'Cys-275' of PGRL1A may also be used to form the disulfide bridges, but in this case the cyclic electron flow is lost. Belongs to the PGR5 family.