The evolution of herbicide resistance in weeds is an ongoing problem in California agriculture. Resistance to glyphosate has become particularly widespread across the state. Normally, glyphosate kills weedy plants by inhibiting 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme that is necessary for the production of some important amino acids in plants. However, repeated applications of glyphosate can result in the selection of rare mutant plants that have a unique ability (mechanism) to “resist” death by glyphosate. The mutant plants survive glyphosate application and produce seeds, which give rise to more resistant plants the following year. Over time, repeated glyphosate applications will result in a field, orchard, or vineyard containing mostly, or only, resistant plants of the weed targeted by the glyphosate treatments.

To date, three major mechanisms of resistance to glyphosate have been identified in weeds around the world. The most common resistance mechanism involves mutations in the gene encoding the EPSPS enzyme. The mutations cause plants to produce an enzyme that is slightly different in chemical structure from the normal enzyme so that glyphosate cannot inhibit it. Plants that have the modified form of the enzyme will continue to produce the amino acids that are important to their survival and do not die after treatment with glyphosate.

For her Ph.D. research, graduate student Elizabeth (Libby) Karn worked in Marie Jasieniuk's lab at UC Davis to identify the gene mutations causing glyphosate resistance in Italian ryegrass she sampled from orchards and vineyards in Sonoma and Lake Counties. Libby identified seven different mutations (alleles) in glyphosate-resistant ryegrass plants (see Figure 1 in https://www.frontiersin.org/articles/10.3389/fpls.2017.00777/full). Four of these mutations (alleles) have been shown to cause glyphosate resistance in ryegrass from other parts of the U.S. and the world. The presence of four different known resistance mutations (alleles) indicates that resistance originated at least four separate times, not just once, in ryegrass from Lake and Sonoma Counties. It also indicates that management of Italian ryegrass in the region will only be successful if integrated weed management, which includes both non-herbicide practices as well as herbicides other than glyphosate, is implemented by growers in the area.