Herbicide resistant Palmer amaranth weeds show fitness and persistence

Palmer amaranth is a weed native to the southwestern United States that has developed resistance to various herbicides since the late 1980s. Its resistance to glyphosate was first confirmed in 2006 in Macon County, Georgia, and has since spread to 13 states. Palmer amaranth, which is capable of producing more than 600,000 seeds per female plant, significantly affects crop yields throughout the southern United States. 

An article featured in the current issue of Weed Science offers results from a greenhouse experiment focused on how resistance might affect the continued fitness of Palmer amaranth plants by comparing glyphosate-resistant plants to glyphosate-susceptible plants.

The mechanism of resistance studied in this population was amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene. Palmer amaranth growth rate, height and volume, final biomass, photosynthetic rate, length of inflorescence, pollen variability, and seed set were evaluated. If measures of plant growth, such as height and volume, showed a positive correlation with the EPSPS gene, this would suggest good fitness of the resistant plant. Fecundity-related measures, such as shorter inflorescence and increased number of days to first flower, could indicate a cost in the fitness of the resistant plant. Metabolic overproduction of the enzyme and disruption to other genes could potentially weaken the plant.

Some resistant plants had more than 100 EPSPS genes, but this study did not identify a fitness cost for resistant Palmer amaranth. The amplified EPSPS gene did not cause the plant to work harder or to divert resources to fight the herbicide. This means that glyphosate resistance will probably persist, making it unlikely that susceptible Palmer amaranth would once again become dominant if glyphosate use was discontinued. Farmers will continue to be faced with finding different methods of removing Palmer amaranth from their fields.

Aspects of this study also show the importance of controlling for genetic background when estimating fitness costs. Great variation was found in fitness measures even between sibling plants from the same family. Without a diverse sample, it could be concluded that these differences were due to the amplified EPSPS gene. These study results could also be used to develop a resistance evolution simulation model to help predict and guide future weed management.