Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: September 10, 2007
Publication Date: February 5, 2008
Citation: Shaner, D.L. 2008. The role of absorption and translocation as a mechanism of resistance to glyphosate. Weed Science Society of America Meeting Abstracts.Weed Science Society of America,Chicago,Illinois, Feb 4-7, 2008. Technical Abstract: The continuous use of glyphosate has resulted in the selection of resistant biotypes in 13 different species. Three different mechanisms of resistance have been proposed for these biotypes: 1) Decreased translocation to meristems; 2) Mutation of target site (EPSPS) and 3) Increased expression of EPSPS. Decreased translocation of glyphosate to the meristematic tissue has been documented in Conyza canadensis, Lolium rigidum, and Lolium multiflorum and the resistance trait is inherited as a single, semidominant nuclear trait. The question is: What role does decreased translocation play as a mechanism of resistance to glyphosate and what is the actual mechanism? EPSPS, the target site of glyphosate, is primarily located in the growing points of plants. Leaf discs assays across a number of species show that the maximum accumulation shikimate occurs in young, rapidly expanding tissue. Gene expression studies have also shown that EPSPS mRNA is maximally expressed in meristematic tissue. 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 to the tips of the leaves, and then is subsequently loaded into the phloem which carries the herbicide out of the leaf. In resistant Lolium, 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 must 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 Conyza canadensis and Lolium multiflorum, 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 (<50 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 three 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; or 3) Glyphosate is translocated into the vacuole and trapped in the cell. Further research is needed to elucidate which, if any, of these mechanisms are responsible for decreased translocation of glyphosate in resistant biotypes.