Evolution and spread of glyphosate resistant barnyard grass (Echinochloa colona (L.) Link) from Australia

Abstract

Echinochloa colona is an important summer-growing weed species in northern Australian cropping regions. The intensive use of glyphosate in summer fallow operations has led to the appearance of glyphosate resistant E. colona populations at a large number of sites. Studies of the genetic diversity, resistance mechanisms, inheritance and spread of resistance were undertaken to better understand the evolution of glyphosate resistance in this species. A survey of 65 barnyard grass populations collected from Queensland and New South Wales determined 34 populations were resistant to glyphosate with resistance levels ranging from 2 to 11-fold. High genetic diversity within three populations and between 62 populations was identified by the AFLP technique. A total of 99.2% of alleles identified within populations were polymorphic with a higher percentage of polymorphic alleles within the two resistant populations compared to the susceptible population. The level of glyphosate resistance in populations was dependent on the ambient temperature. Resistant populations showed a noticeably higher level of resistance at 30°C compared to 20°C whereas there was no effect of temperature on the response of the susceptible population. Experiments were carried out on glyphosate absorption and translocation in resistant and susceptible plants to identify the reason for these differences and the results showed a considerable decrease in glyphosate absorption into leaves at 30°C. Differences were also identified in glyphosate translocation between the treated leaves and the other sections of plants at the different temperatures. There were no differences in glyphosate absorption or translocation between the susceptible population and the resistant populations suggesting that differences in absorption and translocation of the herbicide are not the mechanism of resistance in the studied populations. Studies of EPSPS gene copy number showed gene amplification was not the resistance mechanism either. A mutation was detected at codon 106 (proline substituted by serine) of the EPSPS gene of the most resistant population, A533.1, indicating the presence of target-site resistance in this population. Gene flow by pollen exchange between the glyphosate resistant population A533.1 and the susceptible population Echi S occurred at a frequency of 1.38% when progeny from the susceptible parent was tested at 240 g a.e. ha⁻¹ of glyphosate. The mutation in the EPSPS gene was detected in 24 F₁ progenies of this population pair. Segregation of resistance in the gene flow experiment between resistant and susceptible individuals occurred at a 3:1 resistance : susceptibility ratio in the F₂ generation indicating the trait of glyphosate resistance is a single dominant trait of E. colona. Sequencing the EPSPS cDNA of five parental and F₂ filial individuals revealed at least two EPSPS genes present in E. colona. Shikimate accumulation of the F₁ hybrid and the glyphosate response of F₂ progenies were intermediate between the two parental populations.