Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2013
Publication Date: 5/31/2013
Citation: Hoagland, R.E., Jordan, R.H., Teaster, N.D. 2013. Bioassay and characterization of several palmer amaranth biotypes with varying tolerances to glyphosate. American Journal of Plant Sciences. 4(5)1029-1037.
Interpretive Summary: Palmer amaranth, a widely distributed and aggressive weed, has recently evolved resistance to glyphosate. Bioassays to rapidly determine resistance, and biochemical/immunological comparisons (inherent protein and pigment levels, electrophoretic protein profiles, and immunological responses to an EPSPS protein antibody) in a sensitive, a resistant, and two unclassified biotypes of this weed were studied. Slight differences were noted in extractable protein and chlorophyll content of greenhouse-grown plants, but one biotype lacked the red pigment, betalain. Seed germination bioassays at several glyphosate levels were unsuccessful due to lack of ample seeds, but measurement of seedling growth in this bioassay was a useful resistance detection tool. A leaf disc bioassay (visual ratings and chlorophyll analysis) and a shikimate accumulation assay were also effective. Both unclassified biotypes were glyphosate-resistant. Some differences were found in the electrophoresed protein profiles, and the western blots had weak labeling in the glyphosate-sensititve biotype, but strong labeling in the resistant plants. This supports results by others that resistance in this weed is due to increased EPSPS copy number. Results indicate the utility of bioassays for determination of resistance and provide important comparative biochemical information among weed biotypes.
Technical Abstract: The wide distribution of Palmer amaranth in the southern U.S. became a serious weed control problem prior to the extensive use of glyphosate-resistant crops. Currently glyphosate-resistant populations of Palmer amaranth occur in many areas of this geographic region creating an even more serious threat to crop production. Investigations were undertaken using four biotypes (one glyphosate-sensitive, one resistant and two of unknown tolerance) of Palmer amaranth in order to study bioassay techniques for the rapid detection and level of resistance in populations of this weed and to comparatively characterize these plants with respect to their inherent protein, chlorophyll and pigment levels, and their protein profiles and immunological responses to an EPSPS protein antibody after electrophoresis. Only slight differences were found in four biotypes grown under greenhouse conditions regarding extractable soluble protein and chlorophyll content, but one biotype was found to be devoid of the red pigment, betalain. Seed germination bioassays at various glyphosate concentrations were unsuccessful, due to the lack of sufficient seeds of each biotype to perform a valid bioassay, but measurement of early growth of seedlings in this bioassay was useful detection tool. A leaf disc bioassay (using visual ratings and/or chlorophyll analysis) and an assay for shikimate accumulation were effective methods for determining herbicide resistance levels. The two unknown biotypes were found to be resistant to glyphosate. Some differences were found in the electrophoresed protein profiles of the biotypes, and western blots demonstrated a weak labeling of antibody in the glyphosate-sensititve biotype, whereas strong labeling occurred in the resistant plants. This later point substantiates results by others that resistance in this species is due to increased copy number of EPSPS. Results indicate the utility of certain bioassays for the determination of resistance and provide useful comparative information on the levels of inherent constituents among closely related plants.