The thioredoxin system protects PSI from photoinhibition in coordination with PSI cyclic electron transport under fluctuating light conditions.

Under natural growth conditions, plants are constantly exposed to changes in light intensity. PROTON GRADIENT REGULATION 5 (PGR5)-dependent photosystem I cyclic electron transport (PSI-CET) is known to be required to protect PSI from photoinhibition during such fluctuating light. Moreover, the Arabidopsis thaliana (Arabidopsis) pgr5 mutant cannot survive under fluctuating light conditions due to severe PSI photoinhibition. Recently, we demonstrated that the thioredoxin (Trx) system also supports PSI photoprotection under fluctuating light conditions. Of the five Trx types in Arabidopsis chloroplasts, the x- and y-type Trxs prevent over-reduction of the PSI acceptor side. In the mutant lacking both types (trx x trx y1y2), strong acceptor-side limitation and PSI photoinhibition occurred. To further clarify the roles of PSI-CET and the Trx system in PSI photoprotection, we analyzed multiple mutants. In the pgr5-5 trx x trx y1y2 quadruple mutant, the PSI acceptor side was more reduced than in either the pgr5-5 or trx x trx y1y2 mutant. After exposure to fluctuating light, the pgr5-5 trx x trx y1y2 mutant also showed more severe PSI photoinhibition. Furthermore, when plants were transferred from constant light to fluctuating light conditions for growth, the pgr5-5 trx x trx y1y2 mutant displayed pronounced photoinhibition, and its leaves bleached and died. These results indicate that the Trx system acts cooperatively with PSI-CET to protect PSI from photoinhibition under fluctuating light conditions.