Current outlook and future research needs for harvest weed seed control in North American cropping systems.

Harvest weed seed control (HWSC) comprises a set of tools and tactics that prevents the addition of weed seed to the soil seed bank, attenuating weed infestations and providing a method to combat the development and spread of herbicide-resistant weed populations. Initial HWSC research efforts in North America are summarized and, combined with the vast area of crops suitable for HWSC, clearly indicate strong potential for this technology. However, potential limitations exist that are not present in Australian cropping systems where HWSC was developed. These include rotations with crops that are not currently amenable to HWSC (e.g. corn), high moisture content at harvest, untimely harvest, and others. Concerns about weeds becoming resistant to HWSC (i.e. adapting) exist, as do shifts in weed species composition, particularly with the diversity of weeds in North America. Currently the potential of HWSC vastly outweighs any drawbacks, necessitating further research. Such expanded efforts should foremost include chaff lining and impact mill commercial scale evaluation, as this will address potential limitations as well as economics. Growers must be integrated into large-scale, on-farm research and development activities aimed at alleviating the problems of using HWSC systems in North America and drive greater adoption subsequently. © 2020 Society of Chemical Industry.

Survey of the genomic landscape surrounding the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene in glyphosate-resistant Amaranthus palmeri from geographically distant populations in the USA.

BACKGROUND: Glyphosate resistance in Amaranthus palmeri, one of the most prevalent herbicide-resistant weeds in the USA, is attributable to amplification and increased expression of the gene encoding the target site of glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). The EPSPS gene and the surrounding 287 kilobases (kb) of amplified sequence are unique to glyphosate-resistant plants and termed the EPSPS cassette. It has only been sequenced in one A. palmeri population from Mississippi. This research compares EPSPS cassettes in seven resistant and five sensitive populations from geographically distant locations within the USA, including Mississippi, Arizona, Kansas, Maryland, Delaware and Georgia. RESULTS: Polymerase chain reaction (PCR) products from 40 primer pairs specific to the cassette were similar in size and sequence in resistant populations. Several primer pairs failed to generate PCR products in sensitive populations. Regions of the cassette sequenced in the resistant populations were found to be nearly identical to those from Mississippi. Gene expression analysis showed that both EPSPS and another gene in the cassette, a reverse transcriptase, were elevated in all resistant populations tested relative to the sensitive populations. CONCLUSION: EPSPS cassettes from distant resistant populations were nearly homologous. Considering the complexity of the cassette, and the degree of similarity among some cassette sequences, the results are consistent with the hypothesis that glyphosate resistance probably evolved once and then rapidly spread across the USA. © 2017 Society of Chemical Industry.

Transfer of glyphosate resistance: evidence of hybridization in Conyza (Asteraceae).

Transfer of herbicide resistance genes between crops and weeds is relatively well documented; however, far less information exists for weed-to-weed interactions. The hybridization between the weedy diploids Conyza canadensis (2n = 18) and C. ramosissima (2n = 18) was investigated by monitoring transmission of the allele conferring resistance to N-phosphonomethyl glycine (glyphosate). In a multivariate quantitative trait analysis, we described the phylogenic relationship of the plants, whereas we tested seed viability to assess potential postzygotic reproductive barriers (PZRB) thus affecting the potential establishment of hybrid populations in the wild. When inflorescences were allowed to interact freely, approximately 3% of C. ramosissima or C. canadensis ova were fertilized by pollen of the opposing species and produced viable seeds; >95% of the ova were fertilized under no-pollen competition conditions (emasculation). The interspecific Conyza hybrid ( ) demonstrated an intermediate phenotype between the parents but superior resistance to glyphosate compared to the resistant C. canadensis parent. Inheritance of glyphosate resistance in the selfed ( ) followed the partially dominant nuclear, single-gene model; backcrosses confirmed successful introgression of the resistance allele to either parent. Negligible PZRB were observed in the hybrid progenies, confirming fertility of the C. canadensis × C. ramosissima nothotaxa. The implications of introgressive hybridization for herbicide resistance management and taxonomy of Conyza are discussed.