Thomas Pfleeger

A composite transcriptional signature differentiates responses towards closely related herbicides in Arabidopsis thaliana and Brassica napus

In this study, genome-wide expression profiling based on Affymetrix ATH1 arrays was used to identify discriminating responses of Arabidopsis thaliana to five herbicides, which contain active ingredients targeting two different branches of amino acid biosynthesis. One herbicide contained glyphosate, which targets 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), while the other four herbicides contain different acetolactate synthase (ALS) inhibiting compounds. In contrast to the herbicide containing glyphosate, which affected only a few transcripts, many effects of the ALS inhibiting herbicides were revealed based on transcriptional changes related to ribosome biogenesis and translation, secondary metabolism, cell wall modification and growth. The expression pattern of a set of 101 genes provided a specific, composite signature that was distinct from other major stress responses and differentiated among herbicides targeting the same enzyme (ALS) or containing the same chemical class of active ingredient (sulfonylurea). A set of homologous genes could be identified in Brassica napus that exhibited a similar expression pattern and correctly distinguished exposure to the five herbicides. Our results show the ability of a limited number of genes to classify and differentiate responses to closely related herbicides in A. thaliana and B. napus and the transferability of a complex transcriptional signature across species.

Influence of root exposure concentration on the fate of nitrobenzene in soybean

Abstract The influence of exposure concentration on the physical and chemical fate of nitrobenzene was studied by providing 0.02, 0.2, 2.0 and 100 μg/ml to roots of intact soybean plants. The results of this study illustrate how plants may govern the physical and chemical fate of environmental pollutants, and emphasize the importance of plants as a vector for foodchain contamination even when biomagnification does not occur.

Wheat Leaf Rust Severity as Affected by Plant Density and Species Proportion in Simple Communities of Wheat and Wild Oats

ABSTRACT While it is generally accepted that dense stands of plants exacerbate epidemics caused by foliar pathogens, there is little experimental evidence to support this view. We grew model plant communities consisting of wheat and wild oats at different densities and proportions and exposed these communities to Puccinia recondita to induce wheat leaf rust. Wild oats was included because it is a common competitor of wheat and may act as a barrier to the dispersal of P. recondita spores among wheat plants. Disease severity was estimated as percentage of wheat flag leaves covered by rust lesions. Seeding density rarely had a significant influence on rust severity, probably because of compensation due to increased tillering at low seeding densities. In contrast, increasing the proportion of wheat in mixtures with wild oats consistently increased wheat leaf rust severity. Regression parameters describing wheat leaf rust severity as a function of wheat seeding density did not differ significantly between pure wheat stands and wheat-wild oat mixtures and, thus, failed to support an effect of wild oats on wheat leaf rust other than through its competitive impact on wheat tiller density.

Selecting and Evaluating Native Plants for Region-Specific Phytotoxicity Testing

ABSTRACT In this study, we evaluated methodology to determine risks to terrestrial native plant species from potential herbicide drift, focusing on 1) selection of native species for testing, 2) growth of these species, and 3) variability in herbicide response among native species and compared with crop plants. Native plant species were selected for initial testing on the basis of spatial analysis, which indicated that species from Illinois, USA, were at potential risk for off-target effects of herbicide drift. On the basis of preliminary seed germination tests, 5 native plant species (Andropogon gerardi, Polygonum lapathifolium, Solidago canadensis, Symphyotrichum lateriflorum, and Tridens flavus) were selected for comparison with crops grown in Illinois, normally used in the US Environmental Protection Agencys (USEPAs) Vegetative Vigor Test (Avena sativa, Daucus carota, Glycine max, Solanum lycopersicon, and Zea mays), or both. When treated with low concentrations of a test herbicide, sulfometuron methyl, 2 native species, P. lapathifolium and S. canadensis, were as sensitive as the 5 crop species. The effective herbicide concentrations producing a 25% reduction in shoot dry weight (EC25) for these species, ranged from 0.00015 to 0.0014 times a field application concentration of 52 g/ha active ingredient of sulfometuron methyl. S. lateriflorum and T. flavus were less sensitive than the other native species, whereas A. gerardi was tolerant to sulfometuron methyl with no growth reduction at any herbicide concentration tested. This study indicated that native species can be successfully selected and grown, used in the suite of species used in the USEPAs phytotoxicity test to assess risks of chemical herbicides to nontarget plants. It also showed (with a limited number of species) that native species varied more in sensitivity to simulated herbicide drift than crop species often used in phytotoxicity testing and that a Weibull function was useful to calculate EC25 values when low concentrations of herbicides was used.

Pea (Pisum sativum) seed production as an assay for reproductive effects due to herbicides

Even though herbicide drift can affect plant reproduction, current plant testing protocols emphasize effects on vegetative growth. In this study, we determined whether a short-growing season plant can indicate potential effects of herbicides on seed production. Pea (Pisum sativum cv. Dakota) plants were grown in mineral soil in pots under greenhouse conditions. Plants were treated with a variety of herbicides (dicamba, clopyralid, glufosinate, glyphosate, 2-methyl-4-chlorophenoxyacetic acid, primisulfuron, or sulfometuron) at below standard field application rates applied at a vegetative stage of growth (approximately 14 d after emergence) or at flowering (approximately 20 d after emergence). Pea seed production was greatly reduced by sulfometuron at the minimum concentration used (0.001 x field application rate), with an effective concentration producing a 25% reduction in seed dry weight of 0.00007 x field application rate. Primisulfuron and glyphosate had a 25% reduction in seed dry weight for seed dry weight of 0.0035 and 0.0096 x field application rate, respectively. Clopyralid and dicamba reduced pea seed dry weight at a 25% reduction in seed dry weight of approximately 0.07 x field application rate. Glufosinate only reduced pea seed weight in one experiment, with a 25% reduction in seed dry weight of 0.07 and 0.008 x field application rate at vegetative growth and flowering stages, respectively. Pea seed dry weight was not affected by 2-methyl-4-chlorophenoxyacetic acid. Plant developmental stage had no consistent effect on herbicide responses. Reduced seed production occurred with some herbicides (especially acetolactate synthase inhibitors), which caused little or no reduction in plant height or shoot biomass and little visible injury. Thus, pea may be a model species to indicate seed reproductive responses to herbicides, with seed production obtained by extending plant growth for usually only 7 d longer than the period usually used in the vegetative vigor test.

Comparing effects of low levels of herbicides on greenhouse- and field-grown potatoes (Solanum tuberosum L.), soybeans (Glycine max L.), and peas (Pisum sativum L.)

Although laboratory toxicology tests are generally easy to perform, cost effective, and readily interpreted, they have been questioned for their environmental relevance. In contrast, field tests are considered realistic while producing results that are difficult to interpret and expensive to obtain. Toxicology tests were conducted on potatoes, peas, and soybeans grown in a native soil in pots in the greenhouse and were compared to plants grown outside under natural environmental conditions to determine toxicological differences between environments, whether different plant developmental stages were more sensitive to herbicides, and whether these species were good candidates for plant reproductive tests. The reproductive and vegetative endpoints of the greenhouse plants and field-grown plants were also compared. The herbicides bromoxynil, glyphosate, MCPA ([4-chloro-2-methylphenoxy] acetic acid), and sulfometuron-methyl were applied at below field application rates to potato plants at two developmental stages. Peas and soybeans were exposed to sulfometuron-methyl at similar rates at three developmental stages. The effective herbicide concentrations producing a 25% reduction in a given measure differed between experimental conditions but were generally within a single order of magnitude within a species, even though there were differences in plant morphology. This study demonstrated that potatoes, peas, and soybeans grown in pots in a greenhouse produce phytotoxicity results similar to those grown outside in pots; that reproductive endpoints in many cases were more sensitive than vegetative ones; and that potato and pea plants are reasonable candidates for asexual and sexual reproductive phytotoxicity tests, respectively. Plants grown in pots in a greenhouse and outside varied little in toxicity. However, extrapolating those toxicity results to native plant communities in the field is basically unknown and in need of research.

Using a geographic information system to identify areas with potential for off-target pesticide exposure

In many countries, numerous tests are required as part of the risk assessment process before chemical registration to protect human health and the environment from unintended effects of chemical releases. Most of these tests are not based on ecological or environmental relevance but, rather, on consistent performance in the laboratory. A conceptual approach based on Geographic Information System (GIS) technology has been developed to identify areas that are vulnerable to nontarget chemical exposure. This GIS-based approach uses wind speed, frequency of those winds, pesticide application rates, and spatial location of agricultural crops to identify areas with the highest potential for pesticide exposure. A test scenario based on an incident in Idaho (USA) was used to identify the relative magnitude of risk from off-target movement of herbicides to plants in the conterminous United States. This analysis indicated that the western portion of the Corn Belt, the central California valley, southeastern Washington, the Willamette Valley of Oregon, and agricultural areas bordering the Great Lakes are among those areas in the United States that appear to have the greatest potential for off-target movement of herbicides via drift. Agricultural areas, such as the Mississippi River Valley and the southeastern United States, appears to have less potential, possibly due to lower average wind speeds. Ecological risk assessments developed for pesticide registration would be improved by using response data from species common to high-risk areas instead of extrapolating test data from species unrelated to those areas with the highest potential for exposure.

Organic pesticide modification of species interactions in annual plant communities.

A method is proposed and tested for assessing multispecies responses to three pesticides (atrazine, 2,4-D and malathion). Pesticides were applied at two concentrations, on model plant communities grown in raised beds using soil containing a natural seed bank. Cover by species was monitored over time in nested 10 and 20 cm diameter neighbourhoods around Poa annua and Calandrinia ciliata target plants. All tested compounds modified relative species abundance, altered dominance and simplified the treated communities. Community biomass decreased with atrazine and 2,4-D treatments, but not with malathion. Each chemical altered species interactions for all treated communities, including the identities of interacting species and the timing of interactions. Each target species had its own suite of interacting species that individually changed with chemical treatment. When cover was used as a predictor of neighbour influence, analysis of species interactions using 10 cm neighbourhoods indicated more interactions than using 20 cm neighbourhoods. When biomass was used as the predictor, use of the 20 cm neighbourhoods indicated more interactions. This method of using model plant communities for field toxicity testing is simple, economical and effective. It uses naturally occurring plants while reducing the environmental heterogeneity common in most field studies.

Effects of low concentrations of herbicides on full-season, field-grown potatoes.

Current phytotoxicity plant test protocols for US pesticide registration require testing for effects on seedling emergence and early growth without regard to other important factors, such as plant reproduction. Yield and quality reduction can have significant economic and ecological effects. Therefore, field trials were conducted to determine if potato (Solanum tubersum L.) vegetative growth and tuber yield and quality were affected by herbicides at below recommended field rates. Potatoes were grown in fields at the Oregon State University Horticulture Farm with herbicides applied at below recommended field application rates 14 d after emergence (DAE) or at 28 DAE. Plant height was measured before and 14 d after application. Visual foliar injury was rated 14 d after application, and tuber yield and quality parameters were measured at harvest (120 DAE). Some tubers were grown in the greenhouse the following year to determine if there were carry-over effects. Potato vegetation and tuber yield quality were generally more affected by herbicides applied at 14 DAE than at 28 DAE. Tuber yield and quality parameters were more affected by lower herbicide rates than were plant height or injury. There were significant yield losses caused by low rates of sulfometuron methyl and imazapyr and, to a lesser extent, with glyphosate and cloransulam-methyl. Bromoxynil and MCPA ((4-chloro-2-methylphenoxy)acetic) acid had little effect on the plants. Vegetative responses did not accurately predict yield and quality responses of tubers; therefore, reproductive responses should be considered in phytotoxicity test protocols for pesticide registration in the USA.

Phytotoxicity assay for seed production using Brassica rapa L.

Although pesticide drift can affect crop yield adversely, current plant testing protocols emphasize only the potential impacts on vegetative plant growth. The present study was conducted to determine whether a plant species with a short life cycle, such as Brassica rapa L. Wisconsin Fast Plants®, can be used to indicate potential effects on seed production of herbicides applied at relatively low levels (e.g., low field application rates [FAR]). The effects of ≤0.1 × FAR of aminopyralid, cloransulam, glyphosate, primisulfuron, or sulfometuron applied 14 d after emergence (DAE), were evaluated for B. rapa grown in mineral soil in pots under greenhouse conditions. Effects were expressed as the effective concentration of the herbicide producing a 25% reduction in a response (EC25) based on nonlinear regression. Brassica rapa seed dry weight was reduced by sulfometuron at an EC25 of 0.00014 × a field application rate (FAR) of 53 g active ingredient (a.i.) ha(-1), primisulfuron at 0.008 (experiment 1) or 0.0050 (experiment 2) × FAR of 40 g a.i. ha(-1), cloransulam at 0.022 × FAR of 18 g a.i. ha(-1), glyphosate at 0.0399 × FAR of 834 g a.i. ha(-1), and by aminopyralid at 0.005 × FAR of 123 g a.i. ha(-1), but only for 1 of 2 experiments. Reduced seed production occurred at less than the FAR that reduced shoot dry weight with sulfometuron and primisulfuron, whereas neither aminopyralid, cloransulam, nor glyphosate affected shoot dry weight. A short life cycle form of B. rapa could be used to indicate reduced seed production with plants grown only 1 week longer (∼35 DAE) than as the current vegetative vigor test for nontarget herbicide effects on plants.