Submitted to: Weed Science
Publication Type: Review Article
Publication Acceptance Date: 4/21/2008
Publication Date: 4/22/2008
Citation: Shaner, D.L. 2008. The Role of Translocation as a Mechanism of Resistance to Glyphosate. Weed Science. DOI 10.1614/WS-08-050.1 Interpretive Summary: Glyphosate is the most widely used herbicide in the world. Glyphosate resistant populations have been selected in a number of weed species. The predominant mechanism of resistance in these glyphosate resistant biotypes is reduced translocation of the herbicide to the growing points of the plant. This paper reviews the literature on the mechanisms of glyphosate uptake into cells and translocation in the phloem to the growing points. It also speculates on potential mechanisms that could explain reduced translocation in glyphosate resistant plants.
Technical Abstract: The continuous use of glyphosate has resulted in the selection of resistant biotypes in 13 different weed species. Three different mechanisms of resistance have been proposed for these biotypes: 1) Decreased translocation to meristems; 2) Mutation of target site 5-enol-pyruvylshikimate-3-phosphate synthase (EPSPS) and 3) Increased expression of EPSPS. Decreased translocation of glyphosate to the meristematic tissue has been documented in horseweed, hairy fleabane, rigid ryegrass and Italian ryegrass and the resistance trait 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)? EPSPS, the target site of glyphosate, is primarily located in the active meristems of plants. 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 EPSPS mRNA is maximally expressed in meristems. Thus, glyphosate needs to translocate to the growing points of the plant to be effective. Studies on the movement of glyphosate out of leaves show that the herbicide first moves via the transpiration stream (apoplasmic system) to the tips of the leaves, and then is subsequently loaded into the phloem (symplasmic system) which carries the herbicide out of the leaf. In resistant 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 resistant plants. However, this mechanism may involve the uptake of glyphosate at the cellular level. Studies with isolated leaf discs show that shikimate accumulation occurs at high concentrations of glyphosate in both susceptible and resistant biotypes of horseweed and Italian ryegrass; but at low concentrations, shikimate accumulation only occurs in susceptible biotypes. Glyphosate uptake at the cellular level appears to occur via two pathways. At low concentrations (<100 uM) glyphosate is taken up via an active mechanism that may involve phosphate transporters. At high concentrations (>100 uM) glyphosate appears to enter cells via diffusion. Decreased cellular uptake of glyphosate could occur by at least one of four mechanisms. 1) The high affinity uptake system no longer recognized glyphosate as a substrate; 2) An active efflux system is present which pumps glyphosate out of the cell into the apoplast; 3) An active efflux system is present which pumps glyphosate out of the chloroplast into the cytoplasm; or 4) Glyphosate is pumped into the vacuole and sequestered in the cell. Further research is needed to elucidate which, if any, of these proposed mechanisms are responsible for decreased translocation of glyphosate in resistant biotypes.