Location: Crop Production Systems Research2011 Annual Report
1a. Objectives (from AD-416)
1: Establish the level of resistance and pattern of cross-resistance/multiple resistance of various weeds to herbicides, and herbicide interactions that promote control of these weeds. 2: Discover potential resistance mechanisms using structural, biochemical and genetic probes. 3: Evaluate the contribution of dispersal mechanisms to the spread and distribution of resistance. 4: Establish structural, biochemical, and genetic characteristics that limit herbicide efficacy against recalcitrant weeds, such as vines (redvine and kudzu) and nutsedges.
1b. Approach (from AD-416)
The approach is holistic, examining not only the mechanism or control of the resistant biotypes, but evaluating the potential for spread of resistance and developing alternative strategies for weed control measures to minimize further development of resistant weeds. Basic growth analyses, assays and bioassays using whole plant and plant tissues will determine major changes that occur in the resistant biotypes in response to herbicide exposure. Studies will involve microscopic approaches (light, fluorescence, standard and scanning and electron microscopy). Subsequent biochemical, genetic, proteomic, immunochemical and radiological techniques will identify more specific sites or differences in herbicide resistant and sensitive weed biotypes within species. This will result in a greater understanding of the biochemistry, physiology and genetics of the mechanisms and spread of resistant weeds, and provide insight on recommendations for better weed control.
3. Progress Report
As a fundamental part of this project, the search for herbicide resistant weeds continued. A Mississippi state-wide (82 counties) herbicide resistance screening study on Palmer amaranth populations was completed in the greenhouse. Resistance to glyphosate was widespread over the state. Thirteen counties among 65 non-Delta counties had glyphosate resistance, 27 counties among all 82 counties of Mississippi had pyrithiobac resistance, and thirteen counties exhibited both glyphosate and pyrithiobac resistance in at least 1 population. No county had populations resistant to trifluralin or flumioxazin. Greenhouse/laboratory resistance screening studies are underway to evaluate populations of barnyardgrass, junglerice, common lambsquarters, giant ragweed, goosegrass, Johnsongrass, slender amaranth, and spiny amaranth. Other efforts continue to discover resistance in other species, to increase seed stocks so that homozygous biotypes can be developed to carry out proposed biological and ecological studies, and to propagate sensitive and resistant weeds for biochemical and molecular biological characterization studies. Molecular studies on herbicide resistance continued. Laboratory studies and progress has been achieved on several fronts and projects are underway towards: characterizing the physiological mechanism of glyphosate resistance in waterhemp; creating a fosmid library of Palmer amaranth DNA [clones are being screened for the presence of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase gene), positive clones will sequenced to determine the nature of the amplification event conferring resistance]; cloning and sequencing the EPSP synthase gene from Palmer amaranth; examining the EPSP synthase gene sequence from purple nutsedge to determine the basis for higher tolerance to glyphosate in sedges; cloning and sequencing the acetolactate synthase (ALS) gene from resistant and sensitive accessions of Palmer amaranth; developing a single nucleotide polymorphism assay for the point mutation in glyphosate-resistant goosegrass to allow rapid screening for this trait; using antibodies to characterize glyphosate sequestration and transport processes in resistant versus susceptible weed biotypes; and using immunological techniques to identify, localize and quantify target-sites associated with or responsible for herbicide resistance in weeds. Other important progress associated with this project include: forwarding the sequence of the resistant and sensitive EPSP synthase genes of goosegrass to GenBank (May 2011), preparation/submission (May 2011) of a Competitive Grant to National Institute of Food and Agriculture, to support research toward a better understanding of the biology and ecology of herbicide-resistant Italian ryegrass that is threatening conservation tillage practices in Mississippi and the broader Mid South area, and the posting of a Post-Doctoral job announcement (July 2011) to support this project.
1. Discovery of herbicide resistant weed biotypes. A Mississippi state-wide (82 counties) herbicide resistance greenhouse screening study on Palmer amaranth was completed by ARS scientists at Stoneville, MS, to ascertain the distribution of resistance to glyphosate in this species. Thirteen counties among 65 non-Delta counties had glyphosate resistance. Twenty seven counties among all counties exhibited pyrithiobac resistance and thirteen counties had both glyphosate and pyrithiobac resistance. Thus glyphosate resistance will be a major challenge and significant resistance to pyrithiobac also exists. This survey provides benchmarks by which the spread of resistance to these herbicides can be measured.
2. Interactions of herbicides with biological agents. Positive interactions of herbicides with phytopathogen for biological weed control (bioherbicdes) can increase weed control efficacy. Scientists at USDA-ARS, Stoneville, MS, found that application of the bioherbicidal fungus, Myrothecium verrucaria (MV) with glyphosate can synergyze herbicidal injury on some invasive weeds. In greenhouse tests on kudzu, the auxin-type herbicide quinclorac applied with MV as mixtures (both at sub-lethal concentrations) caused additive and/or synergistic herbicidal effects. These findings are the first regarding interactions of this herbicide and bioherbicide and provide the basis for characterization of these interactions under field conditions for control of this recalcitrant weed.
3. Molecular biological studies on Palmer amaranth. A study of glyphosate-resistant Palmer amaranth from Mississippi by ARS scientists at Stoneville, MS, revealed that the resistance mechanism was amplification of the gene for EPSP synthase (glyphosate target site). Results agree with findings of other researchers studying glyphosate-resistant Palmer amaranth from Georgia. Results are important since they demonstrate commonality of the glyphosate resistance mechanism in this one weed species. However, the etiology related to the rapid spread of this genetic trait remains unknown.
4. Glyphosate resistance in goosegrass. Scientists at USDA-ARS, Stoneville, MS, isolated/sequenced the gene for EPSP synthase from glyphosate-resistant goosegrass. A point mutation was discovered which conferred moderate resistance to glyphosate. The sequences have been submitted to GenBank. A single nucleotide polymorphism assay was developed for the point mutation in glyphosate-resistant goosegrass which allows rapid screening for this trait. A rapid assay for detection of herbicide resistance will facilitate screening/detection of resistance in numerous biotypes over wide and diverse geographic areas.
Mellon, J.E., Vaughn, K.C. 2011. Immunohistochemical investigation of cotton carpel tissue exposed to xylanolytic hydrolases of Aspergillus flavus. Physiological and Molecular Plant Pathology. 76:34-38.