Glyphosate-Resistant Italian Ryegrass (Lolium multiflorum) in California: Distribution, Response to Glyphosate, and Molecular Evidence for an Altered Target Enzyme

Abstract

Selection by herbicides has resulted in widespread evolution of herbicide resistance in agricultural weeds. In California, resistance to glyphosate was first confirmed in rigid ryegrass in 1998. Objectives of this study were to determine the current distribution and level of glyphosate resistance in Italian ryegrass, and to assess whether resistance could be due to an altered target site. Seeds were sampled from 118 populations and seedlings were treated with glyphosate at 866 g ae ha−1. Percentage of survivors ranged from 5 to 95% in 54 populations. All plants from 64 populations died. One susceptible (S) population, four putatively resistant (R) populations, and one S accession from Oregon were used for pot dose–response experiments, shikimic acid analyses, and DNA sequencing. Seedlings were treated with glyphosate at eight rates, ranging from 108 to 13,856 g ae ha−1. Shoot biomass was evaluated 3 wk after treatment and fit to a log-logistic regression equation. On the basis of GR50(herbicide rate required to reduce growth by 50%) values, seedlings from putatively R populations were roughly two to 15 times more resistant to glyphosate than S plants. Shikimic acid accumulation was similar in all plants before glyphosate treatment, but at 4 and 7 DAT, S plants from California and Oregon accumulated approximately two and three times more shikimic acid, respectively, than R plants. Sequencing of a cDNA fragment of the EPSPS coding region revealed two different codons, both of which encode proline at amino acid position 106 in S individuals. In contrast, all R plants sequenced exhibited missense mutations at site 106. Plants from one population revealed a mutation resulting in a proline to serine substitution. Plants from three R populations exhibited a mutation corresponding to replacement of proline with alanine. Our results indicate that glyphosate resistance is widespread in Italian ryegrass populations of California, and that resistance is likely due to an altered target enzyme.

Verification of the Mechanism of Glyphosate Resistance in Italian Ryegrass Biotypes

“…These variations were also found on a paper conducted by Jasieniuk et al (2008), on glyphosate resistant and susceptible Lolium multiflorum biotypes. The sequencing results provide evidence that the resistance to glyphosate reported by the studied populations has several origins.…”

“…The amino acids were numbered based on the Arabidopsis thaliana sequence, adapted from Jasieniuk et al (2008). Partial sequence of the gene of the EPSPs enzyme of susceptible (S) and resistant (R) Lolium rigidum to glyphosate (Simarmata and Penner, 2008).…”

Glyphosate influence on the physiological parameters of Conyza bonariensis biotypes

“…Glyphosate influence on the physiological parameters of ... In Lolium multiflorum biotypes the substitution of proline for alanine has resistance levels up to 15 times, while substitutions of proline for serine confers resistance levels of up to five times (Jasieniuk et al 2008). In the Lolium rigidum population, the substitution of proline for threonine confers a resistance level of up to 3.4 times (Wakelin & Preston, 2006).…”

A Modified phosphate-carrier protein theory is proposed as a non-target site mechanism For glyphosate resistance in weeds

“…The resistance factor associated with the P 106 T mutation was found to be 3.4 in resistant biotypes of Lolium rigidum from Australia (Wakelin & Preston, 2006). However, another amino acid mutation found in the same position P 106 A in Lolium multiflorum from California, gave a resistance factor of 15 (Jasieniuk et al, 2008). These results suggest that different amino acid substitutions on EPSPs gene at position 106 replacing the amino acid proline may led to conformational differences in the enzyme, which causes different resistance levels according to the substitution.…”