TITLE:
Bioassay and Characterization of Several Palmer Amaranth (Amaranthus palmeri) Biotypes with Varying Tolerances to Glyphosate
AUTHORS:
Robert E. Hoagland, Robin H. Jordan, Neal D. Teaster
KEYWORDS:
Amaranthus palmeri; Betalain; Chlorophyll; EPSPS; Glyphosate-Resistant Weeds; Palmer Amaranth; Pigweed; Western Blot
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.4 No.5,
May
29,
2013
ABSTRACT: The wide distribution of
Palmer amaranth (Amaranthus palmeri) in the southern US 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 from Georgia and two of unknown tolerance from Mississippi) of
Palmer amaranth to assess bioassay techniques for the rapid detection and level
of resistance in populations of this weed. These plants were characterized with respect to chlorophyll, betalain, and protein levels and
immunological responses to an antibody of 5-enolpyruvylshikimate-3-phosphate
synthase (EPSPS) the target site of glyphosate. 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. Measurement of
early growth (seedling shoot elongation) of seedlings was a useful detection
tool to determine glyphosate resistance. 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 this herbicide. Some differences
were found in the protein profiles of the biotypes, and western blots
demonstrated a weak labeling of antibody in the glyphosate-sensitive biotype,
whereas strong labeling occurred in the resistant plants. This latter point supports research by others, that increased copy
number of the EPSPS gene (and increased EPSPS protein levels) is the resistance
mechanism in this species. 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.