INTERACTION OF SOIL MICROFLORA WITH THE BIOHERBICIDE PHOSPHINOTHRICIN

Phosphinothricin, a microbial toxin synthesized industrially for chemical weed control and currently under development as a selective weed killer in cultivation of transgenic plants engineered to resist its presence, is investigated for its effects on the distribution of microorganisms in 15 agricultural and non-agricultural soils. In agricultural soils, the presence of 1 mM phosphinothricin reduced the number of fungi isolated by about 20% and bacteria by about 40%. Under these conditions the isolation of bacteria from boreal forest soils was also suppressed by about 20%. Differences in herbicide resistance were confirmed when a random selection of fungi, bacteria and actinomycetes isolated in the absence of herbicide was grown with 1 mM phosphinothricin. Soil isolates growing in the presence of 1 mM phosphinothricin exhibited a wide spectrum of tolerance to increasing herbicide concentration over the range of 0-50 mM phosphinothricin. Of fungal isolates, the plant pathogen Verticilium alboatrum was among the most resistant, while the mycoparasitic species Trichoderma harzianum and T. longipilus were among the most sensitive to the presence of phosphinothricin.