Engineered glyphosate resistance is the most widely adopted genetically modified trait in agriculture, gaining widespread acceptance by providing a simple robust weed control system. However, extensive and sustained use of glyphosate as a sole weed control mechanism has led to field selection for glyphosate-resistant weeds and has induced significant population shifts to weeds with inherent tolerance to glyphosate. Additional weed control mechanisms that can complement glyphosate-resistant crops are, therefore, urgently needed. 2,4-dichlorophenoxyacetic acid (2,4-D) is an effective lowcost, broad-spectrum herbicide that controls many of the weeds developing resistance to glyphosate. We investigated the substrate preferences of bacterial aryloxyalkanoate dioxygenase enzymes (AADs) that can effectively degrade 2,4-D and have found that some members of this class can act on other widely used herbicides in addition to their activity on 2,4-D. AAD-1 cleaves the aryloxyphenoxypropionate family of grass-active herbicides, and AAD-12 acts on pyridyloxyacetate auxin herbicides such as triclopyr and fluroxypyr. Maize plants transformed with an AAD-1 gene showed robust crop resistance to aryloxyphenoxypropionate herbicides over four generations and were also not injured by 2,4-D applications at any growth stage. Arabidopsis plants expressing AAD-12 were resistant to 2,4-D as well as triclopyr and fluroxypyr, and transgenic soybean plants expressing AAD-12 maintained field resistance to 2,4-D over five generations. These results show that single AAD transgenes can provide simultaneous resistance to a broad repertoire of agronomically important classes of herbicides, including 2,4-D, with utility in both monocot and dicot crops. These transgenes can help preserve the productivity and environmental benefits of herbicide-resistant crops.herbicide resistance | weed management | genetically modified crops |

Since its discovery and initial commercialization in the 1940s, 2,4-D has been an important tool for weed control in a wide variety of crop and noncrop uses. Work studying its chemistry, physiology, mode of action, toxicology, environmental behavior, and efficacy has not only helped elucidate the characteristics of 2,4-D but also provided basic methods that have been used to investigate the properties of hundreds of herbicides that followed it. Much of the information published by researchers over 60 yr ago is still pertinent to understanding the performance of 2,4-D today. Further, new studies continue to be published, especially regarding the mechanisms of 2,4-D action at the molecular level. New uses for 2,4-D, sometimes enabled by biotechnology, continue to be developed. This review strives to provide an overall understanding of 2,4-D activity in plants, plant sensitivity to 2,4-D, toxicological impacts, and current and future uses.

Herbicide-resistant weeds have been observed since the early years of synthetic herbicide development in the 1950s and 1960s. Since that time there has been a consistent increase in the number of cases of herbicide resistance and the impact of herbicide-resistant weeds. Although the nature of crop production varies widely around the world, herbicides have become a primary tool for weed control in most areas. Dependence on herbicides continues to increase as global populations migrate away from rural areas to cities and the agricultural labor force declines. This increased use of herbicides and the concurrent selection pressure have resulted in a rise in cases of multiple resistance, leaving some farmers with few or no herbicide options for certain weed infestations. Global population and economic forces drive many farmer choices regarding crop production and weed control. The challenge is how to insert best management practices into the decision-making process while addressing various economic and regulatory needs. This review endeavors to provide a current overview of herbicide resistance challenges in the major crop production areas of the world and discusses some research initiatives designed to address portions of the problem. © 2017 Society of Chemical Industry.

Although the roots of alfalfa (Medicago sativa L.) usually form pink nodules capable of fixing atmospheric N2 when inoculated with Rhizobium meliloti Dang., one clone incapable of forming nodules (non‐nod) and five clones which produced ineffective nodules were recently discovered. These unusual nodulation traits are useful for research on root infection, nodule formation, and N2 fixation. The objectives of this study were to determine the inheritance of the non‐nodulating trait and of the ineffective nodulation in each of the five clones, to compare the genetic controls for ineffective nodulation in the five clones, and to determine the genetic relationship between ineffective nodules and non‐nodulation.The non‐nod clone and the five ineffective clones were self pollinated, crossed in a diallel mating design, and crossed to two normal effective nodulating clones. F2 and backcross seed was produced on non‐nod ✕ normal and ineffective ✕ normal crosses. F2 seed was also produced on non‐nod ✕ ineffective crosses and ineffective ✕ ineffective crosses. Progenies were evaluated under nil‐nitrate greenhouse sand culture. Six weeks after planting, non‐nod plants were small, chlorotic, and had no nodules. Plants with ineffective nodules were small, chlorotic, and had white nodules. Plants with effective nodules were vigorous, dark green, and had pink nodules. Four of the five clones had a different basis for inheritance of ineffectiveness. Three were conditioned by a single tetrasomically inherited recessive gene (in1, in2, or in3), the nulliplex genotype in each instance producing ineffective nodules. The fourth was conditioned by two recessive genes (in4 and in5), the nulliplex condition at both loci being required for production of ineffective nodules. The non‐nod trait was conditioned by two tetrasomically inherited recessive genes (nn1, and nn2). The nulliplex condition at both loci resulted in non‐nodulation. The non‐nod trait and the four different types of ineffective nodules appeared to segregate independently of one another.

The response of corn (Zea maysL. ‘Sokota TS 46’), flax (Linum usitatissimumL. ‘Culbert 79’), grain sorghum [Sorghum bicolor(L.) Merr. ‘Sokota 466’), soybeans [Glycine max(L.) Merr. ‘Corsoy 79’], and sunflowers (Helianthus annuusL. ‘Sokata 4000’) to soil residues 12 and 24 months after application of 17, 34, and 68 g ai/ha chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] benzenesulfonamide} was determined at two locations, Redfield and Watertown, in eastern South Dakota. All crops at Redfield were injured significantly at 17 g/ha, 12 months after application as determined by plant dry weight and visual evaluations. Corn and sorghum were the most susceptible while flax was the least susceptible. Injury at Watertown was significantly less than at Redfield. Differences in carryover were related to a lower soil pH at Watertown.