Ferredoxin-plastoquinone reductase involved in cyclic electron flow (CEF) around photosystem I. The homodimer is probably not involved in CEF. Inhibited by antimycin A. Homodimer and heterodimer with PGR5 (Probable). Interacts with PGR5, FD2, psaD1, LFNR1 and LFNR2. Interacts also with petC and a Fe-containing cofactor (FCC) (By similarity). A number of isoforms are produced. According to EST sequences. Up-regulated by drought stress. The C-terminal loop (247-313) is required for ferredoxin binding. Disulfide bonds; Cys-289 and Cys-292 are probably involved in the formation of disulfide bridges with 'Cys-11' and 'Cys-105' of PGR5 while Cys-261 and Cys-264 are probably involved in the binding of a Fe-containing cofactor (FCC). No visible phenotype; due to the redundancy with PGRL1A. Pgrl1a and pgrl1b double mutant grows slowly and has pale green leaves. Thioredoxins prevent homodimerization. Belongs to the PGR5 family.