Translation of in Vitro Inhibition by Marine Natural Products of the C sub(4) Acid Cycle Enzyme Pyruvate P sub(i) Dikinase to in Vivo C sub(4) Plant Tissue Death

Marine organism derived extracts, previously identified as containing compounds that inhibited the C sub(4) acid cycle enzyme pyruvate P sub(i) dikinase (PPDK), were assessed for their ability to exhibit an effect on the C sub(4) plants Digitaria ciliaris and Echinochloa crus-galli. Oxygen electrode studies revealed that over half of these extracts inhibited C sub(4) acid driven photosynthesis in leaf slices. Seventeen extracts had a deleterious effect on C sub(4) plants in vivo within 24 h, whereas 36 caused an observable phytotoxic response in one or both of the C sub(4) 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.