PHYSIOLOGICAL-EFFECTS OF CORNEXISTIN ON LEMNA-PAUSICOSTATA

Cornexistin, a microbial product isolated from the fungus Paecilomyces variotii SANK 21086, has postemergence herbicidal activity on several important weeds at rates (0.5 kg/ha) that do not affect maize (Zea ma ys L.). In a duckweed (Lemna pausicostata Hegelm. 6746) bioassay, corn existin at 333 nM caused pronounced inhibition of growth within 72 hr. At the ultrastructural level, the first symptom was shrinkage and the n disappearance of chloroplast starch grains, followed by plasma membr ane and tonoplast disruption. Carbon dioxide fixation, as measured by infrared gas analysis, was reduced in a linear fashion by 10 muM corne xistin, resulting in complete inhibition after 24 hr. Photosynthetic p arameters more directly associated with light reactions, P-515 electro chromic transient and variable chlorophyll fluorescence, were more slo wly affected by cornexistin. Five hundred micromolars each of L-aspart ate, L-glutamate, or the TCA cycle intermediates, succinate, citrate, malate, oxaloacetate, and alpha-ketoglutarate prevented cornexistin ph ytotoxicity when these compounds were incorporated into the culture me dium at the time of first exposure to the herbicide. D-Aspartate and D -glutamate had little or no effect on phytotoxicity. Sucrose at 15 mM was also very effective in reducing the phytotoxicity of cornexistin, whereas concentrations of 1 mM or lower were ineffective. Aminooxyacet ic acid (AOA) at 3.33 muM caused phytotoxicity effects similar to thos e of cornexistin. AOA effects were overcome by the same compounds that prevented cornexistin phytotoxicity. Cornexistin inhibited aspartate amino transferase (AAT) in vitro by only 20-30% at high concentrations , only after incubation in a cell extract. In vitro incubation of thes e L. pausicostata protein extracts also caused changes in AAT activity stain patterns on polyacrylamide gels. Cornexistin may be metabolized to an inhibitor of one or more AAT isozymes in vivo. (C) 1994 Academi c Press, Inc.