A Target-Site Mutation Is Present In A Glyphosate-Resistant Lolium Rigidum Population

A target-site mutation is present in a glyphosateresistant Lolium rigidum population
AM WAKELIN* & C PRESTON* 
*School of Agriculture, Food and Wine, University of Adelaide, Glen Osmond, SA, Australia, and  CRC for Australian Weed Management, Waite Campus, SA, Australia Received 29 January 2006 Revised version accepted 19 May 2006

Summary
Annual ryegrass (Lolium rigidum) is the only weedspecies to have evolved resistance to the broad-spectrum herbicide glyphosate in Australia. A population that had failed to be controlled by glyphosate was collected from a vineyard in the Adelaide Hills region of South Australia. Dose–response experiments on this population (SLR 77) showed that it was glyphosate resistant, with an LD50 that was 1.9–3.4 times higher than that of a susceptiblepopulation (VLR 1). The movement of radiolabelled glyphosate within SLR 77 plants showed that this population did not have the differential glyphosate translocation mechanism of resistance common to several other Australian glyphosate-resistant populations. Subsequent analysis of shikimic acid accumulation within the plant after glyphosate treatment showed that this population accumulated significantlyless shikimic acid than a susceptible population, but more than a

glyphosate-resistant population with the translocation mechanism, indicating the possible involvement of another mechanism of resistance. Sequencing of a portion of the SLR 77 5-enolpyruvylshikimate-3-phosphate synthase gene was carried out and a mutation causing an amino acid change at position 106 from proline to threonine wasidentified. This mutation is likely to be responsible for glyphosate resistance in this population, as mutations in this position have been found to be responsible for glyphosate resistance in goosegrass (Eleusine indica) from Malaysia. This paper represents the first report of target-site glyphosate resistance in L. rigidum and provides evidence that this species has at least two mechanisms ofglyphosate resistance present in Australia. Keywords: glyphosate resistance, Lolium rigidum, annual ryegrass, 5-enolpyruvylshikimate-3-phosphate synthase, shikimic acid, target site.

WAKELIN AM & PRESTON C (2006) A target-site mutation is present in a glyphosate-resistant Lolium rigidum population. Weed Research 46, 432–440.

Introduction
Lolium rigidum Gaudin (annual ryegrass) is widelydistributed across the southern region of Australia where it is both a valuable pasture grass and a serious economic weed of cropping systems (Gill, 1996). Herbicide-resistant L. rigidum was first confirmed in 1980 (Heap & Knight, 1982) and since then this species has evolved resistance to herbicides from seven different modes of action (MOA) (Preston, 1999), with the additional problem of some populationsexhibiting resistance to multiple herbicide chemistries (Preston et al., 1996; Preston, 2003). The widespread evolution

of herbicide resistance in L. rigidum to these different herbicide MOA groups has led to an increased reliance on the herbicide glyphosate for control of this species. Glyphosate is an herbicide that was considered to be at a low risk for resistance evolution (Bradshaw et al.,1997), yet by the mid-1990s glyphosate-resistant L. rigidum had been confirmed in two populations in New South Wales, Australia (Powles et al., 1998; Pratley et al., 1999). Currently, the number of Australian L. rigidum populations confirmed resistant to glyphosate stands at 54 (Preston, 2006) and is continually increasing, with more putative resistant populations tested every year.Correspondence: AM Wakelin, School of Agriculture, Food and Wine, University of Adelaide, PMB 1, Glen Osmond, SA 5064, Australia. Tel: (+61) 8 83037298; Fax: (+61) 8 83037109; E-mail: angela.wakelin@adelaide.edu.au Ó 2006 The Authors Journal compilation Ó 2006 European Weed Research Society. Weed Research 46, 432–440

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Glyphosate is the active...