1a.Objectives (from AD-416):
The objective of this cooperative research project is to develop a field assay to test for glyphosate resistance in weeds.
1b.Approach (from AD-416):
The availability of water for irrigated agriculture is decreasing as population growth continues in the western U.S. and water is being diverted from farms to urban communities. The Water Management Research Unit of USDA-ARS is beginning a multi-year research project on the effects of limiting irrigation on growth and yield of corn, sunflower, dry beans and wheat. Effective weed management is critical in maximizing yield in crops, particularly under water deficit conditions. Glyphosate has become an essential tool in managing weeds in both conventional and glyphosate-resistant crops. The evolution of glyphosate resistant weed populations in the last 10 years threatens the effectiveness of glyphosate. One key to preventing and/or managing the evolution of glyphosate resistant weed populations is the early detection of glyphosate resistant populations before they become widespread. Assays have been developed which can detect glyphosate resistance, but most of these assays require either several weeks to complete or easy access to a laboratory. An assay that could be conducted in the field within 24-48 hours is highly desirable. Most glyphosate resistance assays measure the accumulation of shikimate in plant tissue after treatment with glyphosate. Susceptible plants accumulate high levels of shikimate whereas resistant plants do not. However, the present method for detecting shikimate requires access to a UV/Vis spectrophotometer. Monsanto scientists have developed a method to detect shikimate colorimetrically and can be seen with the naked eye. This assay has the potential to be used in the field to measure the relative levels of shikimate in plant tissue without needing access to a spectrophotometer. The approach in this research will be to determine the utility of the shikimate detection system developed by Monsanto to be used in a field based assay. The new assay will be compared to the current spectrophotometric assay and the feasibility of using this assay procedure to develop a robust field assay will be determined and tested under field conditions on a wide number of weed species.
Significant progress has been made on this project. The project has focused on which plant part to assay for maximum shikimate accumulation and sensitivity to glyphosate. We have found that the young expanding tissue is the best source material. The shikimate dehydrogenase coupled reaction with diaphorase works best with the dye MTT which is a good substrate for all sources of diaphorase and produces a blue color. A field kit has been developed and will be tested by outside collaborators to fine tune the assay.