RIGID RYEGRASS (Lolium rigidum) with GROUP G/9 resistance: (INHIBITION OF ENOLPYRUVYL SHIKIMATE PHOSPHATE SYNTHASE ) Inhibition of EPSP synthase
MUTATION: PROLINE 106 to ALANINE
Rigid Ryegrass (Lolium rigidum) is a monocot plant in the poaceae family. A single amino acid substitution from Proline 106 to Alanine has led to resistance to Inhibition of Enolpyruvyl Shikimate Phosphate Synthase as indicated in the table below.		Rigid Ryegrass

Chemical Family	Example Herbicide	Resistance Level
Glycines	glyphosate	Intermediate Resistance < 10 fold

REFERENCES

Yu Qin ; Cairns, A. ; Powles, S.. 2007. Glyphosate, paraquat and ACCase multiple herbicide resistance evolved in a Lolium rigidum biotype. Planta 225 : 499 - 513.

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 (LD₅₀) for the resistant (R) biotype is 14 times greater than for the susceptible (S) biotype. Similarly, the paraquat LD₅₀ for the R biotype is 32 times greater than for the S biotype. Thus, based on the LD₅₀ 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..

This case was entered by Todd Gaines Email: todd.gaines@colostate.edu

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Cite this site as: Heap, I.  The International Survey of Herbicide Resistant Weeds.  Online.  Internet.  Saturday, November 23, 2024 .  Available  www.weedscience.org
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