Response of Selected Hard Red Wheat Lines to Imazamox as Affected by Number and Location of Resistance Genes, Parental Background, and Growth Habit

Imidazolinone-resistant (IR) wheat (Triticum aestivum L.) was released for commercial production in portions of the USA in 2002 and has provided growers with a new technology to selectively control winter annual grass weeds. Imidazolinone herbicides inhibit acetolactate synthase (ALS) in susceptible plants; however, IR wheat has an altered target site which confers resistance to these herbicides. The mutation-derived resistance trait of most commercially available IR winter wheat cultivars is located on the D-genome; however, winter and spring wheat cultivars with the resistance trait on the A, B, or D genome or on multiple genomes are currently under development. Four groups of near-isoline wheat with spring or winter growth habit and resistance genes on the B, D, or both B and D genomes were compared for whole plant and ALS enzyme response to imazamox. Biomass accumulation after treatment was similar among Band Dgenome resistant winter wheat biotypes and was always higher than Band D-genome resistant spring wheat biotypes. D-genome resistant spring wheat was more resistant than B-genome resistant spring wheat and the two-gene resistant spring wheat had an additive level of tolerance to imazamox compared with single-gene resistant spring wheat. Growth habit (spring vs. winter) did not affect in vitro ALS activity among Bor among D-genome resistant cultivars; however, D-genome resistant cultivars had significantly higher in vitro ALS activity in the presence of imazamox compared with B-genome resistant cultivars regardless of growth habit. D-genome resistance appears to provide greater tolerance to imazamox compared with B-genome resistance; however, multiple-genome resistance likely will be required to consistently avoid crop injury in spring wheat from labeled U.S. rates. Although ALS extracted from winter wheat and spring wheat responded similarly to imazamox, whole plant responses demonstrates that tolerance is affected by factors other than resistance gene location. HERBICIDE RESISTANT (HR) crop cultivars exist for most major world crops and several minor crops and ornamentals (Devine, 2005). Growers in North America, in general, quickly accepted HR crop technology with 70 to 80% market share in some crops (Duke, 2005). Imidazolinone-resistant (Clearfield) winter wheat was made commercially available in 2002 in the Great Plains and in 2003 in the Pacific Northwest regions of the USA. The imidazolinone herbicides inhibit acetolactate synthase (ALS, also known as acetohydroxyacid synthase, EC 4.1.3.18), the first enzyme unique to the biosynthesis of the branched-chain amino acids (Shaner et al., 1984). Imazamox (Beyond) was registered for use on IR-wheat because of the herbicides weed spectrum, limited soil persistence, and toxicological properties (Anonymous, 2004; Shaner et al., 1996). This technology has provided new opportunities for selective control of winter annual grass weeds including jointed goatgrass (Aegilops cylindrica Host), feral rye (Secale cereale L.), Italian ryegrass (Lolium multiflorum Lam.), and the brome complex (Bromus spp.) in wheat (Geier et al., 2004; Zemetra et al., 1998). The IR trait (FS4) was isolated after wheat-seed mutagenesis and screening with imazethapyr {1-[(2chloro-5-pyridyl)methyl]-N-nitro-imidazolidin-2-imine}. Controlled crossing followed by progeny testing with herbicide treatment indicated that one gene provided tolerance to the imidazolinone herbicides (Newhouse et al., 1992). The original wheat mutant and most currently commercialized wheat cultivars have the resistant allele on the long arm of chromosome six in the D genome (Anderson et al., 2004). Because modern wheat is an allohexaploid (2n 5 42) consisting of three diploid genomes: A, B, and D (Poehlman and Sleper, 1995) and ALS in wheat is produced by a multigene family, the enzyme produced by the A and B genome homoeologues remains susceptible to imidazolinone herbicides (Pozniak et al., 2004a). New IR-wheat cultivars are under development using conventional breeding techniques and existing mutations or a combination of seed mutagenesis and conventional breeding (Tan et al., 2005). These efforts have resulted in mutation-derived wheat lines with the imidazolinone resistance trait on the long arm of chromosomes 6D, 6B, and 6A (AhasL-D1,AhasLB1, andAhasL-A1) (Pozniak et al., 2004a). Conventional backcrossing programs are being used to create wheat lines with multiple-genome imidazolinone resistance. While IR-winter wheat generally has adequate safety to labeled rates of imazamox [(RS)-2-(4-isopropyl-4methyl-5-oxo-2-imidazolin-2-yl)-5-methoxymethylnicotinic acid], crop injury occasionally occurs (Geier et al., 2004). IR-spring wheat appears to be more sensitive to imazamox than winter wheat and single-gene resistant cultivars do not have adequate tolerance to U.S. labeled rates of the herbicide (Pozniak et al., 2004b, Tan et al., 2005). Wheat injury consists of minor chlorosis, dark green color of developed leaves, or stunting and yield loss can occur (Pozniak et al., 2004b). Crop injury can be influenced by application parameters such as high imazamox rates, surfactants, and application timing (Frihauf et al., 2005; Geier et al., 2004; Stougaard et al., 2004). Anecdotal evidence also suggests that B.D. Hanson, USDA-ARS Water Management Unit, 9611 S. Riverbend Ave., Parlier CA, 93648; D.L. Shaner, USDA-ARS Water Management Unit, 2150 Centre Ave, Building D, Suite 320, Fort Collins, CO 80526; P. Westra and S.J. Nissen, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523. This article is a U.S. government work and is in the public domain in the USA. Received 28 Oct. 2005. *Corresponding author ([email protected]). Published in Crop Sci. 46:1206–1211 (2006). Crop Breeding & Genetics doi:10.2135/cropsci2005.10-0392 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ALS, acetolactate synthase; HR, herbicide resistant; IR, imidazolinone resistant. R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 1206 Published online March 27, 2006