MICROBIAL PHYTOTOXINS AS POTENTIAL HERBICIDES

Microbes are sources of a diverse array of phytotoxic compounds. These compounds are generally structurally different from commercial herbic ides, targeting different molecular sites of action within the plant. These novel structures and sites can be excellent leads for the discov ery and development of safer synthetic herbicides. Microbial phytotoxi ns are often more environmentally benign than synthetic herbicides. Ex amples of phytotoxins from fungi (AAL-toxin, cornexistin, cyperin, and tentoxin) with novel structures and sites of action are discussed. AA L-toxin is toxic to a wide variety of weeds at very low dose rates. AA L-toxin and many of its analogues kill plants by inhibiting a ceramide synthase-like enzyme, causing rapid accumulation of free sphingoid ba ses that disrupt membranes. Cornexistin appears to be metabolically cn verted to an inhibitor of certain aspartate amino transferase isoenzym es. Its activity can be reversed by feeding aspartate and glutamate or with tricarboxylic acid cycle intermediates. Its activity is much lik e that of (aminooxy)acetate. Cyperin is a diphenylether phytotoxin tha t inhibits protoporphyrinogen oxidase, but does not kill plants by thi s mechanism. It appears to have other effects on porphyrin metabolism. Tentoxin is toxic by two mechanisms. It disrupts chloroplast developm ent by inhibiting the processing of a nuclear-coded plastid protein, a nd it also inhibits photophosphorylation by acting as an energy transf er inhibitor of coupling factor 1 ATPase. Other examples of phytotoxin s from microbes with promise as herbicides will be mentioned.