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 intactleaves. In ruptured chloroplasts, AALs are superior
to AA as inhibitors of cyclic electron transport.