Investigations into the mechanism of glyphosate resistance in Lolium rigidum

Glyphosate is a widely used non-selective herbicide to which so far only three weed species have evolved resistance. Here, we report on the mechanism of glyphosate resistance in one resistant population of Lolium rigidum (Gaud.). Experiments demonstrate that glyphosate resistance in this population is directly correlated with increased transport of the herbicide to leaf tips. No significant differences in the level of expression of the herbicide target site, EPSP synthase, between resistant and susceptible plants were found and the enzyme is equally sensitive to inhibition by glyphosate in both populations. Similarly, plant metabolism of glyphosate does not contribute to resistance. Resistant and susceptible plants are equally capable of absorbing the applied herbicide. The most notable difference between resistant and susceptible populations is found in the translocation of glyphosate. Following treatment, an accumulation of glyphosate in the roots of susceptible plants is observed, whereas glyphosate accumulates in the leaf tips of resistant plants. Taken together, it seems likely that an alteration to the cellular transport of glyphosate confers resistance. (C) 2002 Elsevier Science (USA). All rights reserved.