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Project description

Characterization of the expression, spatial distribution, and spread of glyphosate resistance in exotic ryegrasses (Lolium spp.) of California. (05XA012)
Program Exotic Pests and Diseases Research Program
M.A. Jasieniuk, Plant Sciences, UC Davis
Host/habitat Unspecified
Pest Ryegrass Lolium spp.
Discipline Plant Sciences
Agricultural Systems
Start year (duration)  2005 (Three Years)
Objectives Quantify the incidence, expression, and spatial distribution of glyphosate resistance in exotic ryegrasses (Lolium rigidum, L. multiflorum, L. perenne) of California.

Identify populations and/or plants in which resistance is conferred by an EPSPS-related mechanism (alteration or overproduction of 5-enol-pyruvylshikimate-3-phosphate synthase).

Sequence the gene encoding EPSPS and identify mutations unique to resistant plants for a subset of populations and/or plants where resistance is identified as EPSPS-related.

Determine the degree to which the three exotic Lolium species hybridize, and potentially transfer resistance genes, among themselves.

Exotic pests Lolium rigidum and L. multiflorum cause significant economic losses in orchards, vineyards, and winter cereal crops of California. Although once effectively controlled by glyphosate, the recent evolution of resistance in L. rigidum threatens new invasions and the loss of conventional weed management practices. We propose to characterize the expression and spatial distribution of glyphosate resistance, quantify the degree of hybridization and potential for resistance gene transfer between species, and initiate studies on the underlying genetic and molecular basis of glyphosate resistance in Lolium. Knowledge of factors favoring the selection and spread of herbicide resistance is essential for the development of sustainable strategies of managing invasive weeds in agricultural systems of California.

Our results indicate that resistance to glyphosate in weedy annual ryegrass is widespread in California in a diversity of cropping systems, including fruit and nut orchards, vineyards, and wheat fields. Despite its wide distribution, however, a large proportion of plants and populations also remain susceptible to glyphosate. The variation probably reflects the diversity of crops, weed management systems, and intensities of glyphosate selection in the state.

Levels of resistance to glyphosate also varied among resistant populations. Dose response experiments showed that resistant plants were two to 15 times more resistant to glyphosate than susceptible plants depending on the population and location of seed collection. Sequencing of the gene encoding the enzyme EPSP synthase, which glyphosate binds to and inactivates, revealed two mutations in resistant plants that cause amino acid alterations to the target enzyme. Thus, an altered target site is likely to be the underlying cause of resistance to glyphosate in Italian ryegrass within California.

Our results indicate that resistance to glyphosate is widespread in weedy ryegrass populations of California in a diversity of cropping systems, including fruit and nut orchards, vineyards, and wheat fields. Dose response experiments using four resistant and two susceptible populations showed that the levels of resistance were similar or higher than found in rigid ryegrass in Australia, Italian ryegrass in Oregon and Chile, and goosegrass in Malaysia. Shik imicacid bioassays showed that the two susceptible populations accumulated significantly more shikimate than did the four resistant populations, which indicates that resistance is due to an EPSPS-related mechanism. Preliminary sequencing of the gene encoding EPSP synthase to which glyphosate binds revealed two mutations causing amino acid alterations to the target enzyme. The mutations are likely to be responsible for glyphosate resistance in California ryegrass, as mutations in this position have previously been found to cause resistance to glyphosate in goosegrass in Malaysia and rigid ryegrass in Australia. Knowledge of the mechanism of glyphosate resistance is necessary to propose management practices that reduce glyphosate-resistant ryegrass populations. Populations with different mechanisms of resistance may respond differently to methods used to manage resistance and prevent spread of resistance genes. Based on preliminary results using molecular markers, rigid ryegrass, Italian ryegrass, and perennial ryegrass readily hybridize and thus are likely to have transferred glyphosate resistance genes among themselves.

To characterize the occurrence, expression, and spatial distribution of glyphosate resistance in annual ryegrasses (Lolium rigidum, L. multiflorum), we collected seeds from plants in orchards, vineyards, winter cereals and roadsides in northern California. Then, we tested 40 seedlings from each of 119 locations for resistance or susceptibility to glyphosate at the label rate for ryegrasses equal to 866 g ae/ha. This initial screening revealed a wide range of frequencies of resistance (from 0% to 95%). We are currently conducting dose response experiments using seed from four resistant and two susceptible accessions to quantify levels of resistance. We are also beginning studies to characterize the genetic mechanism(s) of resistance.

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