A Conserved Mechanism for Hormesis in Molecular Systems.

Hormesis refers to dose-response phenomena where low dose treatments elicit a response that is opposite the response observed at higher doses. Hormetic dose-response relationships have been observed throughout all of biology, but the underlying determinants of many reported hormetic dose-responses have not been identified. In this report, we describe a conserved mechanism for hormesis on the molecular level where low dose treatments enhance a response that becomes reduced at higher doses.

Low glyphosate doses change reproduction and produce tolerant offspring in dense populations of Hordeum vulgare.

Background: Low toxin doses that do not affect mean responses in plant populations can still change the growth of subpopulations. Studies covering vegetative stages ascribed fast-growing plants higher thresholds for growth stimulation and inhibition, compared with the rest of the population. We hypothesized that such selective effects also play a role after reproduction; that is, the offspring of glyphosate-treated tolerant, fast-growing phenotypes is more tolerant than the offspring of untreated plants.

Low herbicide doses can change the responses of weeds to subsequent treatments in the next generation: metamitron exposed PSII-target-site resistant Chenopodium album as a case study.

Background: It is well known that exposure to mild stress can precondition organisms to better tolerate subsequent stress exposure in the same or future generations. Since herbicide hormesis also represents a moderate stress to exposed plants, a transgenerational priming is likely but not proven. Especially in herbicide-resistant weeds showing enhanced reproductive fitness after regular herbicide treatments, the ability to induce resilient offspring phenotypes via hormesis may hasten the evolution of herbicide resistance in weeds.

Low toxin doses change plant size distribution in dense populations - Glyphosate exposed Hordeum vulgare as a greenhouse case study.

Numerous intentionally released toxins persist in agricultural or natural environments at low concentrations. Such low toxin doses are regularly associated with hormesis, i.e.

Realistic low-doses of two emerging contaminants change size distribution of an annual flowering plant population.

HHCB [1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta(g)-2-benzopyran] and 4-tert-octylphenol [4-(1,1,3,3-tetramethylbutyl)phenol] are widely used emerging contaminants that have the potential to cause adverse effects in the environment. The purpose of this study was to observe if and how environmentally realistic concentrations of these contaminants alter growth in plant populations. It was hypothesized that within an exposed Gypsophila elegans Bieb (annual baby's breath) population especially fast-growing seedlings are impaired even when the population mean is unaffected, and small doses can cause hormesis and, thus, an increase in shoot or root length.

A quantitative assessment of hormetic responses of plants to ozone.

Evaluations of ozone effects on vegetation across the globe over the last seven decades have mostly incorporated exposure levels that were multi-fold the preindustrial concentrations. As such, global risk assessments and derivation of critical levels for protecting plants and food supplies were based on extrapolation from high to low exposure levels. These were developed in an era when it was thought that stress biology is framed around a linear dose-response.

Predicting the effect of ozone on vegetation via linear non-threshold (LNT), threshold and hormetic dose-response models.

Unlabelled: The nature of the dose-response relationship in the low dose zone and how this concept may be used by regulatory agencies for science-based policy guidance and risk assessment practices are addressed here by using the effects of surface ozone (O) on plants as a key example for dynamic ecosystems sustainability. This paper evaluates the current use of the linear non-threshold (LNT) dose-response model for O. The LNT model has been typically applied in limited field studies which measured damage from high exposures, and used to estimate responses to lower concentrations.

Enhancing and Extending Biological Performance and Resilience.

Human performance, endurance, and resilience have biological limits that are genetically and epigenetically predetermined but perhaps not yet optimized. There are few systematic, rigorous studies on how to raise these limits and reach the true maxima. Achieving this goal might accelerate translation of the theoretical concepts of conditioning, hormesis, and stress adaptation into technological advancements.

Herbicide hormesis can act as a driver of resistance evolution in weeds - PSII-target site resistance in Chenopodium album L. as a case study.

Background: Herbicide hormesis may play a role in the evolution of weed resistance by increasing resistance selection. A standard herbicide rate may be subtoxic to resistant plants and make them more fit than untreated plants. If this increase in fitness is ultimately expressed in reproductive traits, resistance genes can accumulate more rapidly and exacerbate resistance evolution by magnifying the selection differential between resistant and sensitive plants.

Low doses of six toxicants change plant size distribution in dense populations of Lactuca sativa.

Toxicants are known to have negligible or stimulatory, i.e. hormetic, effects at low doses below those that decrease the mean response of a plant population.

Does selective hormesis impact herbicide resistance evolution in weeds? ACCase-resistant populations of Alopecurus myosuroides Huds. as a case study.

Background: A field-evolved herbicide-resistant weed population can represent a heterogeneous composite of subpopulations that differ in their susceptibility and responsiveness to herbicide hormesis. Variable hormesis responsiveness can result in selection for and against certain subpopulations under low herbicide doses, and this has the potential to contribute to the evolution of resistance. The relevance of this hypothesis at practical field rates was studied for two field-collected acetyl-coenzyme A carboxylase (ACCase) target-site resistant (TSR) biotypes of Alopecurus myosuroides Huds.

Predicting biphasic responses in binary mixtures: Pelargonic acid versus glyphosate.

Predicting hormesis in mixtures is challenging, but essential considering that chemical exposures often occur in mixtures and at low doses. This study investigated mixture effects with two herbicides prone to induce hormesis and to interact, namely pelargonic acid versus glyphosate. Five independent mixture experiments were conducted in vitro to assess effects on root growth of lettuce.

Selective toxin effects on faster and slower growing individuals in the formation of hormesis at the population level - A case study with Lactuca sativa and PCIB.

Natural plant populations have large phenotypic plasticity that enhances acclimation to local stress factors such as toxin exposures. While consequences of high toxin exposures are well addressed, effects of low-dose toxin exposures on plant populations are seldom investigated. In particular, the importance of 'selective low-dose toxicity' and hormesis, i.

Investigating a Potential Auxin-Related Mode of Hormetic/Inhibitory Action of the Phytotoxin Parthenin.

Parthenin is a metabolite of Parthenium hysterophorus and is believed to contribute to the weed's invasiveness via allelopathy. Despite the potential of parthenin to suppress competitors, low doses stimulate plant growth. This biphasic action was hypothesized to be auxin-like and, therefore, an auxin-related mode of parthenin action was investigated using two approaches: joint action experiments with Lactuca sativa, and dose-response experiments with auxin/antiauxin-resistant Arabidopsis thaliana genotypes.

Plants Release Precursors of Histone Deacetylase Inhibitors to Suppress Growth of Competitors.

To secure their access to water, light, and nutrients, many plant species have developed allelopathic strategies to suppress competitors. To this end, they release into the rhizosphere phytotoxic substances that inhibit the germination and growth of neighbors. Despite the importance of allelopathy in shaping natural plant communities and for agricultural production, the underlying molecular mechanisms are largely unknown.

Statistical modeling of the hormetic dose zone and the toxic potency completes the quantitative description of hormetic dose responses.

Quantifying the characteristics of hormesis provides valuable insights into this low-dose phenomenon and helps to display and capture its variability. A prerequisite to do so is a statistical procedure allowing quantification of general hormetic features, namely the maximum stimulatory response, the dose range of hormesis, and the distance from the maximum stimulation to the dose where hormesis disappears. Applying extensions of a hormetic dose-response model that is well-established in plant biology provides a direct estimation of several quantities, except the hormetic dose range.

Herbicides and plant hormesis.

Herbicide hormesis is commonly observed at subtoxic doses of herbicides and other phytotoxins. The occurrence and magnitude of this phenomenon are influenced by plant growth stage and physiological status, environmental factors, the endpoint measured and the timing between treatment and endpoint measurement. The mechanism in some cases of herbicide hormesis appears to be related to the target site of the herbicide, whereas in other examples hormesis may be by overcompensation to moderate stress induced by the herbicides or a response to disturbed homeostasis.

C-reactive protein and erythrocyte sedimentation rate discordance: frequency and causes in adults.

C-reactive protein (CRP) levels and the erythrocyte sedimentation rate (ESR) are widely used tests of inflammation that sometimes show opposite results. We performed a retrospective cohort study to clarify the frequency and causes of CRP/ESR discordance in adults. Between January and December of 2011, the laboratories of Texas Health Presbyterian Hospital performed 2150 paired CRP/ESR measurements in 1753 patients, 1731 of whom were nonpregnant adults aged ≥ 18 years.

Modeling effective dosages in hormetic dose-response studies.

Background: Two hormetic modifications of a monotonically decreasing log-logistic dose-response function are most often used to model stimulatory effects of low dosages of a toxicant in plant biology. As just one of these empirical models is yet properly parameterized to allow inference about quantities of interest, this study contributes the parameterized functions for the second hormetic model and compares the estimates of effective dosages between both models based on 23 hormetic data sets. Based on this, the impact on effective dosage estimations was evaluated, especially in case of a substantially inferior fit by one of the two models.

Soil degradation of parthenin-does it contradict the role of allelopathy in the invasive weed Parthenium hysterophorus L.?

The invasive success of Parthenium hysterophorus L. is thought to be partially attributable to allelopathy mediated by the plant metabolite parthenin. To assess the ecological significance of parthenin release from plant material, its persistence and phytotoxicity in soil was studied.

Stimulation versus inhibition--bioactivity of parthenin, a phytochemical from Parthenium hysterophorus L.

Parthenium hysterophorus L. is an invasive weed that biosynthesizes several phytochemicals. The sesquiterpene lactone parthenin receives most attention regarding allelopathy of the plant or potential herbicidal properties.

Hormesis in mixtures -- can it be predicted?

Binary mixture studies are well established for mixtures of pollutants, pesticides, or allelochemicals and sound statistical methods are available to evaluate the results in relation to reference models. The majority of mixture studies are conducted to investigate the effect of one compound on the inhibitory action of another. However, since stimulatory responses to low concentrations of chemicals are gaining increased attention and improved statistical models are available to describe this phenomenon of hormesis, scientists are challenged by the question of what will happen in the low concentration range when all or some of the chemicals in a mixture induce hormesis? Can the mixture effects still be predicted and can the size and concentration range of hormesis be predicted? The present study focused on binary mixtures with one or both compounds inducing hormesis and evaluated six data sets of root length of Lactuca sativa L.

Allelopathy in crop/weed interactions--an update.

Since varietal differences in allelopathy of crops against weeds were discovered in the 1970s, much research has documented the potential that allelopathic crops offer for integrated weed management with substantially reduced herbicide rates. Research groups worldwide have identified several crop species possessing potent allelopathic interference mediated by root exudation of allelochemicals. Rice, wheat, barley and sorghum have attracted most attention.

Dose-response-a challenge for allelopathy?

The response of an organism to a chemical depends, among other things, on the dose. Nonlinear dose-response relationships occur across a broad range of research fields, and are a well established tool to describe the basic mechanisms of phytotoxicity. The responses of plants to allelochemicals as biosynthesized phytotoxins, relate as well to nonlinearity and, thus, allelopathic effects can be adequately quantified by nonlinear mathematical modeling.

Differential exudation of two benzoxazinoids--one of the determining factors for seedling allelopathy of Triticeae species.

Benzoxazinoids (Bx) are natural phytotoxins that function as chemical defense compounds in several species. The release of Bx by intact plant roots associated these compounds with root allelopathy in Triticeae species; however, the significance of exudate concentrations of Bx for plant-plant interactions is still a controversial question. A biological screening of 146 cultivars of four Triticeae species (Triticum aestivum L.