Marine organism derived extracts, previously identified as containing compounds that inhibited the C4 acid cycle enzyme pyruvate Pi dikinase (PPDK), were assessed for their ability to exhibit an effect on the C4 plants Digitaria ciliaris and Echinochloa crus-galli. Oxygen electrode studies revealed that over half of these extracts inhibited C4 acid driven photosynthesis in leaf slices. Seventeen extracts had a deleterious effect on C4 plants in vivo within 24 h, whereas 36 caused an observable phytotoxic response in one or both of the C4 plants used for in vivo testing. None of the extracts inhibited PPDK metabolism of pyruvate via a directly competitive mechanism, instead hindering the enzyme by either mixed or uncompetitive means. This screening strategy, using a suite of assays, led to the isolation and identification of the herbicidal marine natural product ilimaquinone
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