421 Fourteenth Australian Weeds Conference Summary The molecular basis of resistance to AC- Case-inhibiting herbicides was investigated for wild oat (Avena spp.) biotypes in the northern grain-grow- ing region of Australia. Of 14 wild oat populations assessed, two resistant populations (UQT and UQM) were identified. UQT was resistant to the aryloxyphe- noxypropionate, fenoxaprop-p-ethyl, but not to the cyclohexanedione, sethoxydim. By comparison, UQM was resistant to both herbicides. This indicates that the molecular basis of resistance to ACCase-inhibit- ing herbicides differs for the two populations. DNA sequencing of the genes encoding plastidic ACCase of UQT plants revealed a single nucleotide mutation resulting in an Ile-Asn substitution homologous to that recently found in Alopecurus myosuroides Hudson (black grass) and Lolium rigidum Gaudin (annual ryegrass) populations which are also resistant to ary- loxyphenoxypropionate but not cyclohexanedione herbicides. Our study is the first to detect this mutation in wild oat. We have developed a PCR-based molecular diagnostic test for this mutation. Future work will involve determining the genetic basis of resistance in the UQM population. Keywords Wild oat, ACCase-inhibitors, ACCase, herbicide resistance. INTRODUCTION The aryloxyphenoxypropionates (APPs) and cyclohex- anediones (CHDs) are two chemically dissimilar classes of herbicide that are used to selectively control grass weeds. Normally, both herbicides inhibit the plastidic acetyl-CoA carboxylase (ACCase) enzyme of grasses, thereby inhibiting de novo lipid biosyn- thesis and killing the plant. However, several weed biotypes resistant to these herbicides have evolved, including major grassy weed species such as annual ryegrass (L. rigidum), blackgrass (A. myosuroides), green foxtail (Setaria viridis (L.) Beauv.) and wild oat (Avena fatua L.). In grasses, plastidic ACCase is a multidomain enzyme. While the exact binding site(s) of APPs and CHDs to plastidic ACCase is unclear, it is known that both herbicides interact with the carboxyl-transferase (CT) domain (Nikolskaya et al. 1999). Recently, it has been shown that several amino acid substitutions in the CT domain can confer resistance to APPs and/or CHDs. An Ile-1781-Leu substitution in the CT-β domain conferred resistance to both APPs and CHDs in L. rigidum (Zagnitko et al. 2001), A. myosuroides (Brown et al. 2002), S. viridis (Delye et al. 2002) and A. fatua. (Christoffers et al. 2002). This mutation is homologous in all four weeds, and can be conferred by a single point mutation (A-to-T or A-to-C) in the first nucleotide of this isoleucine codon. More recently, an Ile-2041-Asn substitution in the CT-α domain was reported to correlate with resistance to APPs but not CHDs in A. myosuroides and L. rigidum (Delye et al. 2003). The aims of this study were to determine the molecular basis of ACCase-inhibiting herbicide resistance in wild oat in the northern grain-growing region of Australia, and to develop a molecular diag- nostic test for the rapid detection of these ‘resistance’ mutations. MATERIALS AND METHODS Plant material Fourteen wild oat (Avena sterilis L. ssp. ludoviciana Durieu) populations from the northern grain-growing region of Australia were assessed for resistance to APP and CHD herbicide. This included 12 randomly collected populations and two suspected resistant populations (UQM and UQT) supplied by farmers. A known susceptible population was used as a control. For each population, 60 plants were grown in pots (5 plants per pot) containing composted bark, hardwood sawdust and sand (2:2:1) with slow-release fertiliser, under outdoor conditions in the winter months of July to August 2003 at Gatton Queensland, Australia. Plants were watered as needed. Herbicide treatment At the 3 to 4 leaf stage, 30 plants of each population were sprayed with the recommended rate of APP (Wildcat ® , fenoxaprop-P- ethyl, 110 g a.i. L -1 ) and CHD (Sertin ® , sethoxydim 186 g a.i. L -1 ) respectively. Herbicide was applied using a traversing boom spray in the wind tunnel laboratory at the Centre for Pesticide Application and Safety, The University of Queensland (UQ), Gatton. Molecular characterisation of resistance to ACCase-inhibiting herbicides in wild oat in the northern grain-growing region of Australia Wenjie Liu 1 , Dion Harrison 1 , Chris O’Donnell 2 , Steve Adkins 2 and Richard Williams 1 1 Agricultural Molecular Biotechnology Laboratory, School of Agronomy and Horticulture, University of Queensland, Gatton, Queensland 4343, Australia. 2 School of Land and Food Sciences, University of Queensland, Brisbane, Queensland 4072, Australia