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Summary: The evolution of herbicide resistance in weeds is an example of parallel evolution, through which genes encoding herbicide target proteins are repeatedly represented as evolutionary targets. The number of herbicide target‐site genes differs among species, and little is known regarding the effects of duplicate gene copies on the evolution of herbicide resistance.We investigated the evolution of herbicide resistance in Monochoria vaginalis, which carries five copies of sulfonylurea target‐site acetolactate synthase (ALS) genes. Suspected resistant populations collected across Japan were investigated for herbicide sensitivity and ALS gene sequences, followed by functional characterization and ALS gene expression analysis.We identified over 60 resistant populations, all of which carried resistance‐conferring amino acid substitutions exclusively in MvALS1 or MvALS3. All MvALS4 alleles carried a loss‐of‐function mutation. Although the enzymatic properties of ALS encoded by these genes were not markedly different, the expression of MvALS1 and MvALS3 was prominently higher among all ALS genes.The higher expression of MvALS1 and MvALS3 is the driving force of the biased representation of genes during the evolution of herbicide resistance in M. vaginalis. Our findings highlight that gene expression is a key factor in creating evolutionary hotspots. [ABSTRACT FROM AUTHOR]

Sagittaria trifolia is a noxious paddy weed in East Asia. This weed propagates both sexually and asexually. Information on the role of sexual reproduction in the spread of herbicide‐resistant gene(s) and control of S. trifolia is important. Therefore, effects of oxygen, light, and temperature on the germination; germination percentages of fresh and stored seeds; and seed dormancy cycles were clarified. Seeds of S. trifolia require hypoxia and light with temperatures above 30/20°C for germination. Germination percentages increased after 1 month of storage on the soil surface. Germination percentages at 35/25°C for seeds submerged in the summer (simulating a well‐drained paddy during winter) and year round (simulating a poorly drained paddy) for 5 months and exhumed on April 7 were 84.7 and 94.4%, respectively, and decreased gradually to 41.3 and 40.0%, respectively, on June 2 (7 months burial). Germination percentages increased again and reached 63.7 and 94.0%, respectively, on July 25 (9 months burial). Seeds that form in summer germinate quickly at high temperatures, and the seedlings form tubers before winter. Thus, once herbicide‐resistant individuals emerge, the resistant gene or genes is/are easily distributed and fixed in the next generation. Autumn plowing and/or the spraying of herbicides is/are effective for controlling this weed. [ABSTRACT FROM AUTHOR]

The rapid range expansion of naturalized Italian ryegrass ( Lolium multiflorum Lam.) in farmland is a serious problem in Fukuroi city in Shizuoka Prefecture, Japan. Glyphosate has been used to control Italian ryegrass in the levees of rice paddy fields and wheat fields for ∼20 years, but this weed in Fukuroi city is poorly controlled by glyphosate. In order to elucidate the level of resistance to glyphosate in Italian ryegrass populations, seed bioassays and a foliar application experiment, using seeds collected from 16 wild populations in and around Fukuroi city and from three susceptible cultivars, were conducted. For the susceptible cultivars and one population from a site where glyphosate had not been applied for >10 years, the shoot length in the seed bioassays was greatly suppressed at a glyphosate concentration of 10 mg ai L−1 and no seedling survived after the foliar application of glyphosate at a rate of 2.3 kg ai ha−1. Nine wild populations from levees in the southern part of Fukuroi city showed vigorous shoot growth at a glyphosate concentration of 10 mg ai L−1 and had at least a 78% survival rate after the application of glyphosate at 2.3 kg ai ha−1. Four wild populations from levees in the northern part of Fukuroi city showed a slight suppression of the shoot growth as a result of the glyphosate treatment and their survival rates ranged from 20 to 64%. The results suggested that resistance to glyphosate has evolved in the wild populations of Italian ryegrass that are growing on the levees. This is the first report of a glyphosate-resistant weed in Japan. [ABSTRACT FROM AUTHOR]

As herbicide-resistant weeds have spread in the agricultural fields of grain-exporting countries, their seeds could be introduced into other countries as contaminants in imported grain. The spread of resistance genes through seed and pollen can cause significant economic loss. In order to assess the extent of the problem, we investigated the contamination by herbicide-resistant annual ryegrass ( Lolium rigidum) of wheat imported from Western Australia into Japan. Annual ryegrass seeds were recovered from wheat shipments and seed bioassays were conducted to identify resistance to the herbicides that are commonly used in Australia: diclofop-methyl, sethoxydim, chlorsulfuron, and glyphosate. Nearly 4500 ryegrass seeds were detected in 20 kg of wheat that was imported in both 2006 and 2007. About 35% and 15% of the seeds were resistant to diclofop-methyl, 5% and 6% were resistant to sethoxydim, and 56% and 60% were resistant to chlorsulfuron in 2006 and 2007, respectively. None was resistant to glyphosate in either year. As the contamination of crops by herbicide-resistant weeds is probably a common phenomenon, the monitoring of incoming grain shipments is necessary to stem the further spread of herbicide-resistant weeds into importing countries. [ABSTRACT FROM AUTHOR]