The continuous use of glyphosate has resulted in the selection of glyphosate-resistant (GR) biotypes in 13 weed species. Decreased translocation of glyphosate to the meristematic tissue is the primary mechanism of resistance in horseweed, hairy fleabane, rigid ryegrass, and Italian ryegrass, and the resistance is inherited as a single, semidominant nuclear trait. The question is: What role does decreased translocation play in glyphosate resistance, and what is the actual mechanism(s)? The enzyme 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS), the target site of glyphosate, preferentially accumulates in the active meristems of plants. Inhibition of EPSPS results in the accumulation of shikimate. Leaf disc assays across a number of species show that the maximum accumulation of shikimate occurs in young, rapidly expanding tissue. Gene expression studies have also shown that the gene encoding EPSPS is maximally expressed in meristems. Thus, glyphosate needs to translocate to the growing points of plants to be effective. In some GR weed biotypes, glyphosate moves in the treated leaf via the transpiration stream; but instead of being loaded into the phloem, it is trapped in the distal portion of the leaf. These results suggest that there is some type of inhibition of glyphosate-loading into the phloem in GR plants. However, this mechanism may involve uptake of glyphosate at the cellular level. Shikimate accumulation in isolated leaf discs occurs at high glyphosate concentrations in both susceptible and GR biotypes of horseweed and Italian ryegrass; but at low concentrations, shikimate accumulation occurs only in susceptible biotypes. Decreased cellular uptake of glyphosate might occur by one of four mechanisms: (1) the active uptake system no longer recognizes glyphosate, (2) an active efflux system pumps glyphosate out of the cell into the apoplast, (3) an active efflux system pumps glyphosate out of the chloroplast into the cytoplasm, or (4) glyphosate is pumped into the vacuole and sequestered in the cell.
Nomenclature: Glyphosate; hairy fleabane, Conyza bonariensis L.; horseweed, Conyza canadensis L.; Italian ryegrass, Lolium multiflorum Lam.; rigid ryegrass, Lolium rigidum Gaud