A pgr5 suppressor screen uncovers a distinct mechanism safeguarding the cytochrome b6f complex from damage through PGR5

PROTON GRADIENT REGULATION5 (PGR5) is thought to promote cyclic electron flow (CEF) and its deficiency causes increased photosensitivity of photosystem I (PSI), leading to lethality under fluctuating light (FL). By screening for suppressor mutations that rescue FL lethality of pgr5 plants, we identified a portfolio of mutations affecting 12 photosynthesis-related proteins. Six are required for proper PSII function, one (CcdA) promotes cytochrome (cyt) b6f assembly, and another (PAA1) provides plastocyanin with its copper cofactor. Two other mutations are associated with the chloroplast FBPase cFBP1. This, together with targeted knockout of other genes in the pgr5 background, suggests three pathways to restore FL viability: (i) reduced electron flow to PSI due to defects in PSII, cyt b6f or plastocyanin but not PSI, (ii) increased electron flow from PSI due to inactivation of ACHT2, a regulator of cFBP1 activity, and (iii) hyperactivity of the NDH-dependent CEF due to inactivation of cFBP1. The remaining two suppressor mutations affected the cyt b6f complex. PFSC1 controls cyt b6f accumulation at early developmental stages. DEIP1/NTA1, previously suggested to be essential for cyt b6f assembly, appears to protect cyt b6f from deleterious effects of PGR5, since plants lacking both DEIP1/NTA1 and PGR5 are viable and accumulate cyt b6f.