Tolerance to acetolactate synthase and acetyl-coenzyme A carboxylase inhibiting herbicides in Vulpia bromoides is conferred by two co-existing resistance mechanisms Qin Yu, L.J. Shane Friesen, Xiao Qi Zhang, and Stephen B. Powles * Western Australian Herbicide Resistance Initiative, School of Plant Biology, University of Western Australia, Crawley, WA 6009, Australia Received 25 April 2003; accepted 16 July 2003 Abstract Vulpia bromoides is a grass species naturally tolerant to acetolactate synthase (ALS) and acetyl-coenzyme A car- boxylase (ACCase) inhibiting herbicides. The mechanism of tolerance to ALS herbicides was determined as cytochrome P450-monooxygenase mediated metabolic detoxification. The ALS enzyme extract partially purified from V. bromoides shoot tissue was found to be as sensitive as that of herbicide susceptible Lolium rigidum to ALS-inhibiting sulfonylurea (SU), triazolopyrimidine (TP), and imidazolinone (IM) herbicides. Furthermore, phytotoxicity of the wheat-selective SU herbicide chlorsulfuron was significantly enhanced in vivo in the presence of the known P450 inhibitor malathion. In contract, the biochemical basis of tolerance to ACCase inhibiting herbicides was established as an insensitive AC- Case. In vitro ACCase inhibition assays showed that, compared to a herbicide susceptible L. rigidum, the V. bromoides ACCase was moderately (4.5- to 9.5-fold) insensitive to the aryloxyphenoxypropionate (APP) herbicides diclofop, fluazifop, and haloxyfop and highly insensitive (20- to >71-fold) to the cyclohexanedione (CHD) herbicides sethoxydim and tralkoxydim. No differential absorption or de-esterification of fluazifop-P-butyl was observed between the two species at 48 h after herbicide application, and furthermore V. bromoides did not detoxify fluazifop acid as rapidly as susceptible L. rigidum. It is concluded that two co-existing resistance mechanisms, i.e., an enhanced metabolism of ALS herbicides and an insensitive target ACCase, endow natural tolerance to ALS and ACCase inhibiting herbicides in V. bromoides. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Acetolactate synthase; Acetyl-coenzyme A carboxylase; 1-Aminobenzotriazole; Cytochrome P450 monooxygenase; Fluazifop-P-butyl metabolism; Malathion; Vulpia bromoides * Corresponding author. Fax: +61-8-9380-7834. E-mail address: spowels@agric.uwa.edu.au (S.B. Powles). 0048-3575/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.pestbp.2003.07.004 Pesticide Biochemistry and Physiology 78 (2004) 21–30 www.elsevier.com/locate/ypest PESTICIDE Biochemistry & Physiology