Target-Site Point Mutations Conferring Resistance to ACCase-Inhibiting Herbicides in Smooth Barley (Hordeum glaucum) and Hare Barley (Hordeum leporinum)

Abstract

Acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides affect fatty acid biosynthesis in plants and are widely used to control smooth and hare barley in dicot crops in Australia. Recently, growers have experienced difficulty in controlling smooth and hare barley with herbicides from this mode of action. Dose–response experiments conducted on five suspected resistant populations confirmed varying levels of resistance to quizalofop and haloxyfop. The level of resistance in these populations was greater than 27-fold to quizalofop and greater than 15-fold to haloxyfop. The quizalofop dose required to reduce shoot biomass by 50% (GR50) for the resistant populations varied from 52.6 to 111.9 g ha−1, and for haloxyfop from 26.5 to 71.3 g ha−1. Sequencing the CT domain of the ACCase gene from resistant plants of different populations confirmed the presence of previously known mutations Ile1781Leu and Gly2096Ala. Amino acid substitution at the 2096 position conferred a greater level of resistance to haloxyfop than the substitution at the 1781 position. This study documents the first known case of field-evolved target-site resistance to ACCase-inhibiting herbicides in Australian populations of smooth barley.

Basis of ACCase and ALS inhibitor resistance in Hordeum glaucum Steud

“…Although haloxyfop was also ineffective in controlling R populations at the field rate (39 g ha −1 ), it exhibited greater activity than quizalofop at higher herbicide rates. This is consistent with a previous study conducted on H. glaucum . Similarly to quizalofop, haloxyfop was ineffective in controlling the UN15 population even at the maximum herbicide dose used (eightfold the recommended field rate) (Figs C and D), and therefore the LD 50 or GR 50 could not be estimated (Tables and ).…”

“…As per manufacturer's instructions, ISOLATE Plant DNA Mini kit (Bioline, Alexandria, NSW) was used to extract DNA from 50–100 mg of plant tissue under liquid nitrogen. For the ACCase gene, polymerase chain reactions (PCRs) were performed using two sets of previously described primers, which amplified nearly the entire carboxyl transferase (CT) domain of the plastidic ACCase gene. For the ALS gene, PCRs were performed using two sets of primers designed on the basis of homologous regions of ALS gene sequences of Arabidopsis thaliana (AY042819) and Raphanus raphanistrum (AJ344986).…”

“…ACCase inhibitors from APP and CHD subgroups and ALS inhibitors from SU and IMI subgroups are commonly used to control H. glaucum in Australia. Currently, this species has evolved resistance to three different groups of herbicides: ACCase inhibitors, ALS inhibitors and bipyridiliums . Previously, 1781‐Leu and 2096‐Ala mutant alleles in the CT domain of the ACCase gene have been reported to confer resistance to APP herbicides in populations of H. glaucum .…”

“…Currently, this species has evolved resistance to three different groups of herbicides: ACCase inhibitors, ALS inhibitors and bipyridiliums . Previously, 1781‐Leu and 2096‐Ala mutant alleles in the CT domain of the ACCase gene have been reported to confer resistance to APP herbicides in populations of H. glaucum . So far the mechanism of resistance to ALS inhibitors has not been reported; however, Pro‐197‐Thr/Ser mutant alleles in the ALS gene of a closely related species H. leporinum have been previously reported to confer resistance to ALS inhibitors …”

“…Research by Fleet and Gill has shown that weed management practices used in cropping systems of SA have increased seed dormancy in H. glaucum , which may have contributed to its increased abundance in field crops. Incidence of resistance to APP ACCase inhibitors and ALS inhibitors in H. glaucum populations has already been reported . The combination of herbicide resistance and high seed dormancy would make it difficult for Australian farmers effectively to control such weed populations in their fields.…”