Antimycin A-like molecules inhibit cyclic electron transport around photosystem I in ruptured chloroplasts

• Antimycin A3 (AA) inhibits PSI cyclic electron transport.
• AA-like compounds inhibit PSI cyclic electron transport at lower concentrations than AA.
• AAL1 targets the same site as AA to inhibit PSI cyclic electron transport.
• V3K alteration in PGR5 does not confer resistance to AAL2.

Antimycin A3 (AA) is used as an inhibitor of cyclic electron transport around photosystem I. However, the high concentrations of AA that are needed for inhibition have secondary effects, even in chloroplasts. Here, we screened for chemicals that inhibited ferredoxin-dependent plastoquinone reduction in ruptured chloroplasts at lower concentrations than those required for AA. We identified two AA-like compounds: AAL1 and AAL2. AAL1 likely shares an inhibitory site with AA, most probably in the PGR5–PGRL1 protein complex, and enhances O2 evolution in photosystem II, most likely via an uncoupler-like effect. AAL1 and AAL2 are unlikely to penetrate intact leaves. In ruptured chloroplasts, AALs are superior to AA as inhibitors of cyclic electron transport.