Effect of Phosphinothricin on Nitrogen Metabolism of Trichoderma Species and Its Implications for Their Control of Phytopathogenic Fungi

Abstract

The antagonistic control of the phytopathogen Fusarium oxysporum by Trichoderma species is impaired in the presence of phosphinothricin, a microbial toxin commercialized for chemical weed control under trade names Basta, Ignite, and Herbiace. The influence of phosphinothricin on growth parameters and nitrogen metabolism in nine strains of Trichoderma and a closely related fungus Nectria ochroleuca was investigated. The presence of 1 m M phosphinothricin was lethal to the Trichoderma anamorph of Hypocrea gelatinosa and almost equally inhibitory to Trichoderma polysporum. In other species it caused marked reductions in the hyphal protein content, accompanied by lower biomass yields in all except N. ochroleuca. Under normal growth conditions, these rapidly growing fungi maintain highly active levels of the two key enzymes of ammonium assimilation, glutamine synthetase, and NADPH-dependent glutamate dehydrogenase. Phosphinothricin, by causing an inhibition in glutamine synthetase activity, diminishes the enzymic potential for the combined operation of these two alternative pathways. The level of NADPH-glutamate dehydrogenase activity was exceptionally high in N. ochroleuca, and although declining in the presence of phosphinothricin, remained much higher than those in nine strains of Trichoderma. The levels of NADPH-glutamate dehydrogenase in T. harzianum, T. citrinoviride, and T. viride were higher in the presence than in the absence of phosphinothricin. In the presence of phosphinothricin, the levels of both aspartate aminotransferase and alanine aminotransferase activities were stimulated in T. harzianum, T. atroviride, T. citrinoviride, and T. viride and that of alanine aminotransferase in T. koningii. Trichoderma harzianum and T. atroviride contain single anionic molecular forms of glutamine synthetase, NADPH-glutamate dehydrogenase, and alanine aminotransferase and two isoforms of aspartate aminotransferase both in the presence and in the absence of phosphinothricin.