Sarah M. Ward

Reducing the Risks of Herbicide Resistance: Best Management Practices and Recommendations

Herbicides are the foundation of weed control in commercial crop-production systems. However, herbicide-resistant (HR) weed populations are evolving rapidly as a natural response to selection pressure imposed by modern agricultural management activities. Mitigating the evolution of herbicide resistance depends on reducing selection through diversification of weed control techniques, minimizing the spread of resistance genes and genotypes via pollen or propagule dispersal, and eliminating additions of weed seed to the soil seedbank. Effective deployment of such a multifaceted approach will require shifting from the current concept of basing weed management on single-year economic thresholds.

Palmer Amaranth ( Amaranthus palmeri ): A Review

Abstract In little over 20 yr, Palmer amaranth has risen from relative obscurity to its current status as one of the most widespread, troublesome, and economically damaging agronomic weeds in the southeastern U.S. Numerous factors have enabled Palmer amaranth to become such a dominant and difficult-to-control weed, including its rapid growth rate, high fecundity, genetic diversity, ability to tolerate adverse conditions, and its facility for evolving herbicide resistance. It is both a serious threat to several U.S. cropping systems and a fascinating model weed. In this paper, we review the growing body of literature on Palmer amaranth to summarize the current state of knowledge on the biology, agricultural impacts, and management of this weed, and we suggest future directions for research. Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA. Resumen En poco más de 20 años, Amaranthus palmeri ha salido de una relativa oscuridad a su estado actual como una de las malezas agrícolas más ampliamente distribuida, más problemática y económicamente dañina en el sureste de los Estados Unidos. Numerosos factores le han permitido a A. palmeri convertirse en una maleza tan dominante y difícil de controlar, incluyendo su rápida tasa de crecimiento, alta fecundidad, diversidad genética, habilidad para tolerar condiciones adversas, y su facilidad para evolucionar resistencia a herbicidas. Es una amenaza para varios sistemas de cultivos en los Estados Unidos, pero también es una maleza modelo fascinante. En este artículo, revisamos la cantidad creciente de literatura sobre A. palmeri para resumir el estado actual de conocimiento sobre la biología, impactos agrícolas, y manejo de esta maleza, y sugerimos futuras direcciones para su investigación.

Mechanism of Resistance of Evolved Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

Evolved glyphosate resistance in weedy species represents a challenge for the continued success and utility of glyphosate-resistant crops. Glyphosate functions by inhibiting the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). The resistance mechanism was determined in a population of glyphosate-resistant Palmer amaranth from Georgia (U.S.). Within this population, glyphosate resistance correlates with increases in (a) genomic copy number of EPSPS, (b) expression of the EPSPS transcript, (c) EPSPS protein level, and (d) EPSPS enzymatic activity. Dose response results from the resistant and an F(2) population suggest that between 30 and 50 EPSPS genomic copies are necessary to survive glyphosate rates between 0.5 and 1.0 kg ha(-1). These results further confirm the role of EPSPS gene amplification in conferring glyphosate resistance in this population of Palmer amaranth. Questions remain related to how the EPSPS amplification initially occurred and the occurrence of this mechanism in other Palmer amaranth populations and other glyphosate-resistant species.

Inheritance of Resistance to The Auxinic Herbicide Dicamba in Kochia (Kochia scoparia)

Abstract The inheritance of resistance to the auxinic herbicide dicamba was examined in a kochia population from Nebraska. An inbred, resistant line was developed by selection and selfing over seven generations to ensure any resistance alleles would be homozygous in the parents. An inbred, susceptible line was similarly developed, but without selection. Dose–response experiments with dicamba determined a glyphosate-resistant concentration required to inhibit dry weight accumulation by 50% (GR50) of 45 and 1,331 g ae ha−1 for the susceptible and resistant populations, respectively. F1 crosses were made between resistant and susceptible inbred individuals by hand-pollination, and the F1 plants were selfed to produce F2 plants. The F2 population was screened with 280 g ha−1 dicamba, a rate that could discriminate between susceptible and resistant plants. A total of eight F2 families were screened twice. In the first screen, seven F2 families segregated in a 3:1 ratio, consistent with a single dominant allele controlling resistance, and in the second screen six F2 families segregated in a 3:1 ratio. F2 individuals were selfed, the F3 progeny were tested with 280 g ha−1 dicamba, and the genotype of each F2 parent was determined based on F3 progeny segregation. F3 family segregation was consistent with the F2 parents having a 1:2:1 homozygous-susceptible:heterozygote:homozygous-resistant pattern, confirming that resistance to dicamba in kochia is likely conferred by a single allele with a high degree of dominance. Nomenclature: Dicamba, kochia, Kochia scoparia (L.) Schrad. KCHSC

Impact of Genetic Background in Fitness Cost Studies: An Example from Glyphosate-Resistant Palmer Amaranth

Abstract Since its discovery in 2005, glyphosate-resistant Palmer amaranth has become a major problem for many farmers in the southern United States. One mechanism of resistance found in a Georgia population of glyphosate-resistant Palmer amaranth is amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene throughout the genome, with some resistant plants containing and expressing more than 100 EPSPS genes. Such high numbers of EPSPS genes and protein production could result in a fitness cost to resistant plants due to (1) metabolic cost of overproduction of this enzyme and (2) disruption of other genes after insertion of the EPSPS gene. A greenhouse experiment was set up to investigate differences in growth and reproduction between glyphosate-susceptible and -resistant Palmer amaranth plants. Measurements included growth rate, plant height/volume ratio, final biomass, photosynthetic rate, inflorescence length, pollen viability, and seed set. This study found no significant fitness costs for plants with the resistance trait. This study also provided a clear example of how controlling for genetic background is important in fitness cost studies and how potentially misleading results can be obtained if only a few fitness traits are measured. These results indicate that glyphosate-resistant Palmer amaranth plants with high EPSPS gene copy numbers are likely to persist in field populations, even in the absence of glyphosate, potentially leading to long-term loss of glyphosate as a control option for Palmer amaranth. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats AMAPA.

Interspecific hybridization transfers a previously unknown glyphosate resistance mechanism in Amaranthus species

A previously unknown glyphosate resistance mechanism, amplification of the 5‐enolpyruvyl shikimate‐3‐phosphate synthase gene, was recently reported in Amaranthus palmeri. This evolved mechanism could introgress to other weedy Amaranthus species through interspecific hybridization, representing an avenue for acquisition of a novel adaptive trait. The objective of this study was to evaluate the potential for this glyphosate resistance trait to transfer via pollen from A. palmeri to five other weedy Amaranthus species (Amaranthus hybridus, Amaranthus powellii, Amaranthus retroflexus, Amaranthus spinosus, and Amaranthus tuberculatus). Field and greenhouse crosses were conducted using glyphosate‐resistant male A. palmeri as pollen donors and the other Amaranthus species as pollen recipients. Hybridization between A. palmeri and A. spinosus occurred with frequencies in the field studies ranging from <0.01% to 0.4%, and 1.4% in greenhouse crosses. A majority of the A. spinosus × A. palmeri hybrids grown to flowering were monoecious and produced viable seed. Hybridization occurred in the field study between A. palmeri and A. tuberculatus (<0.2%), and between A. palmeri and A. hybridus (<0.01%). This is the first documentation of hybridization between A. palmeri and both A. spinosus and A. hybridus.

Genetic Analysis of Invasive Plant Populations at Different Spatial Scales

Measuring genetic diversity requires selection of a spatial scale of analysis. Different levels of genetic structuring are revealed at different spatial scales, however, and the relative importance of factors driving genetic structuring varies along the spatial scale continuum. Unequal gene flow is a major factor determining genetic structure in plant populations at the local level, while the effect of selection imposed by environmental heterogeneity increases with the spatial scale of analysis. At a continental and global scale genetic structure of invasive plant populations is significantly affected by founder effect and propagule transport via human vectors. Although genetic analysis at one spatial scale provides only partial information about the invasion process, little published research reports such data for the same species at multiple scales. A multi-faceted approach to investigating the genetic structure of invasive plant species that incorporates sampling at different spatial and temporal scales would provide a more complete picture of the role of genetic forces in invasion.

Genetic diversity of jointed goatgrass (Aegilops cylindrica) determined with RAPD and AFLP markers.

Abstract Two DNA molecular marker techniques were used to evaluate genetic diversity in 58 accessions of jointed goatgrass and 6 accessions of the related wild species barb goatgrass. Random amplified polymorphic DNA (RAPD) assays were performed on 8 U.S. and 50 Eurasian jointed goatgrass accessions using 30 random decamer primers. The frequency of scorable polymorphic bands within jointed goatgrass was 6 out of 90 (6.7%). Cluster analysis of RAPD data showed small genetic distances (values of 0.005 or less) among jointed goatgrass accessions. To validate the effectiveness of RAPD techniques to detect genetic diversity in tetraploid Aegilops species, six accessions of barb goatgrass were assayed using a subset of 20 decamer primers (from the original 30). RAPD data for barb goatgrass were pooled with jointed goatgrass data from the same primers. A total of 63 scorable bands were generated, of which 27 (43%) were polymorphic between two or more accessions. RAPD analysis readily distinguished between the two species and detected much greater levels of genetic diversity within barb goatgrass than between the jointed goatgrass accessions. Amplified fragment length polymorphism (AFLP) assays were performed on a subset of the 58 jointed goatgrass accessions, 3 U.S. and 13 Eurasian. These accessions were selected to represent a range in geographic diversity within our collection. Ten primer combinations generated 560 scorable bands of which 28 (5%) were polymorphic. Cluster analysis of AFLP data showed a slightly smaller range in genetic distance (0.0002 to 0.0022) among accessions compared with RAPD results; however, AFLPs distinguished among all but 2 of the 16 accessions surveyed. Although AFLP produced more scorable bands than RAPD did, both methods revealed limited genetic diversity in jointed goatgrass. Nomenclature: Jointed goatgrass, Aegilops cylindrica L. AEGCY; barb goatgrass, Aegilops triuncialis L. AEGTR.

Genetic Variation in Invasive Populations of Yellow Toadflax (Linaria vulgaris) in the Western United States

Abstract Intraspecific genetic variation may contribute significantly to invasiveness and control problems, but has been characterized to date in relatively few invasive weed species. We examined 56 intersimple sequence repeat (ISSR) loci in 220 individuals from 11 invading populations of yellow toadflax sampled across five western states. All populations showed high levels of genetic diversity. Estimated values for Shannons diversity measure ranged from 0.217 to 0.388, and for expected heterozygosity from 0.178 to 0.260. Neis total gene diversity index (HT), on the basis of all individuals across all populations, was 0.267. Partitioning of genetic variance using analysis of molecular variance revealed 1.7% of genetic variation among regional population groups, 29.1% among populations within groups, and 69.2% within populations, consistent with expectations for an outcrossing species but suggesting little geographic differentiation. Pairs of adjacent individuals identical at all ISSR loci that appeared to be ramets of a single clone were detected in only one population. This indicates that patch expansion in yellow toadflax is driven more by sexual reproduction via seed than by rhizomatous clonal spread, at least at the spatial scale of sampling for this study. Eight populations had significant values for Mantels R at P  =  0.05, suggesting some fine-scale positive genetic structuring, possibly from restricted gene flow. Population clustering on the basis of Neis genetic distance between populations and unweighted pair group method with arithmetic mean did not reflect geographic location. It is likely that multiple introductions of this species have occurred across the Intermountain West, followed by extensive genetic recombination. High levels of genetic diversity within yellow toadflax populations pose management challenges, as already seen in reports of variable response to herbicide application and limited impacts of biocontrol agent releases.

Allotetraploid segregation for single-gene morphological characters in quinoa (Chenopodium quinoa} Willd.)

Cytological evidence suggests the Andean grain crop quinoa is an allotetraploid, but in the few genetic studies which have been published a functionally diploid (disomic-monogenic) model has been assumed for segregation at individual loci in this species. In this study, controlled crosses using male sterile plants as female parents produced F1 and F2 generations segregating for three different single-gene morphological traits. Allelic segregation analysis revealed a range of F1 and F2 ratios indicative of both disomic-digenic and tetrasomic inheritance in two of these traits, as well as distorted F2 ratios suggesting erratic multivalent formation at meiosis. These results are consistent with allotetraploidy in quinoa, with functional alleles having been retained at some duplicate loci and at least some association occurring between homoeologous chromosomes. Tetrasomic segregation ratios observed in a minority of families may be due to reciprocal fragment exchange between homoeologues. The occurrence of tetraploid segregations at some loci in quinoa complicates breeding and genetic studies in the crop.