Glyphosate, paraquat and ACCase multiple herbicide resistance evolved in a Lolium rigidum biotype

Abstract

Glyphosate is the world's most widely used herbicide. A potential substitute for glyphosate in some use patterns is the herbicide paraquat. Following many years of successful use, neither glyphosate nor paraquat could control a biotype of the widespread annual ryegrass (Lolium rigidum), and here the world's first case of multiple resistance to glyphosate and paraquat is confirmed. Dose-response experiments established that the glyphosate rate causing 50% mortality (LD(50)) for the resistant (R) biotype is 14 times greater than for the susceptible (S) biotype. Similarly, the paraquat LD(50 )for the R biotype is 32 times greater than for the S biotype. Thus, based on the LD(50 )R/S ratio, this R biotype of L. rigidum is 14-fold resistant to glyphosate and 32-fold resistant to paraquat. This R biotype also has evolved resistance to the acetyl-coenzyme A carboxylase (ACCase) inhibiting herbicides. The mechanism of paraquat resistance in this biotype was determined as restricted paraquat translocation. Resistance to ACCase-inhibiting herbicides was determined as due to an insensitive ACCase. Two mechanisms endowing glyphosate resistance were established: firstly, a point mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene, resulting in an amino acid substitution of proline to alanine at position 106; secondly, reduced glyphosate translocation was found in this R biotype, indicating a co-occurrence of two distinct glyphosate resistance mechanisms within the R population. In total, this R biotype displays at least four co-existing resistance mechanisms, endowing multiple resistance to glyphosate, paraquat and ACCase herbicides. This alarming case in the history of herbicide resistance evolution represents a serious challenge for the sustainable use of the precious agrochemical resources such as glyphosate and paraquat.

Evolution of glyphosate resistance in a Lolium rigidum population by glyphosate selection at sublethal doses

“…Here, we show that with an initially glyphosate-susceptible L. rigidum population, three to four generations of recurrent selection at sublethal glyphosate doses (in two different environments) are sufficient to cause a shift toward glyphosate resistance (Tables 2-5, Figures 1 and 2, Photo Supplementary Figure S1). It is emphasized that this recurrent glyphosate selection study commenced with a population (VLR1) highly characterized as susceptible to a wide range of herbicides, including glyphosate (Yu et al, 2007). The results we obtained with glyphosate selection are similar to earlier experiments in which the same L. rigidum population evolved resistance from recurrent selection at sublethal doses of diclofop-methyl (Neve and Powles, 2005b).…”

“…The level of resistance to glyphosate after three or four generations of recurrent selection at sublethal glyphosate doses (Figures 1 and 2) was much lower than the level of diclofop-methyl resistance evident after three cycles of recurrent selection with diclofop-methyl (Neve and Powles, 2005b). Glyphosate resistance from recurrent selection at a sublethal glyphosate dose is moderate when compared with LD 50 values obtained in fieldevolved glyphosate-resistant populations of L. rigidum (Powles et al, 1998;Yu et al, 2007) or glyphosate-resistant biotypes in other species (reviewed in Gustafson, 2008;Vila-Aiub et al, 2008).…”

“…In L. rigidum, a reduced glyphosate translocation resistance mechanism and a mutation in the EPSPS target gene have been documented (Yu et al, 2007;Preston and Wakelin, 2008). These two mechanisms are each single gene traits (Powles and Preston, 2006;Preston, 2006a, 2006b).…”

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

“…This multiple resistant weed had two glyphosate resistance mechanisms, the P 106 A mutation and reduced translocation. The level of resistance in this biotype was found to be additive when compared to biotypes where resistance was due to only one mechanism of resistance (Yu et al, 2007).…”

“…The F1 population showed a superior resistance factor when compared to that of both parents, demonstrating that different glyphosate resistance mechanisms can be additive (Preston et al, 2009). The intersection of population with different resistance mechanisms leading to their addition may increase the overall weed resistance problem and may make it more difficult to manage weeds in particular crops (Yu et al, 2007;Preston et al, 2009). …”