Herbicides as anthropogenic drivers of eco-evo feedbacks in plant communities at the agro-ecological interface

Plasticity in plant mating systems

Many plants are extremely plastic in their vegetative and life-history traits, allowing them to deal with a variety of environmental conditions during their lifetime. However, in our understanding of plant reproduction, plasticity in mating system is not broadly considered. Even though mating system shifts are well studied on an evolutionary timescale, we show that many traits affecting plant mating system also show plasticity within an ecological timeframe. This plasticity in reproduction can be found in prepollination, in interactions with pollinators, and in various postpollination processes. We bring together molecular and ecological work on plant reproduction and guide future research on mating systems to embrace trait plasticity and context dependency of mating strategies.

Meeting Report on the Symposium "Evolutionary Applications" at the 3rd Joint Congress on Evolutionary Biology

The symposium "Evolutionary Applications" took place on June 28, 2024 in the virtual part of the 3rd Joint Congress on Evolutionary Biology. It was contributed to the conference by the European Society for Evolutionary Biology (ESEB). The symposium highlighted research on evolutionary biology applied to address questions and contemporary problems in medicine and public health, conservation biology, and food production and agriculture. Each of the six talks covered a different application and a different organism: domestication of cheese-making fungi, restoration of long-lived bird populations, evolution of herbicide resistance, coral reef conservation, gene drive systems targeting Malaria vectors, and antibiotic resistance evolution in bacteria. By including speakers who are active in a consortium or work in an NGO, the symposium also showed how to make the step from scientific findings to practical application. The symposium furthermore featured a range of scientific methods, ranging from genomic analyses and mathematical modeling to laboratory evolution and field experiments. Speakers from across 15 time zones highlighted the potential of virtual symposia to foster global collaboration in evolutionary biology.

Off-target drift of the herbicide dicamba disrupts plant-pollinator interactions via novel pathways

While herbicide use in agriculture is expected to have many effects on surrounding weed communities, it is largely unknown how plant exposure to sublethal doses of herbicide may subsequently impact plant-pollinator interactions. We tested the hypothesis that sublethal herbicide exposure indirectly alters plant-pollinator interactions through changes in plant traits, and specifically through alterations in floral display. Using a common garden experiment, we exposed 11 weed species to the herbicide dicamba and examined the potential for changes in pollinator abundance and patterns of pollinator visitation as well as alterations to plant traits. We found variation among plant species in the extent of damage from dicamba drift, and variation in size, flowering time, and flower displays, with some plant species showing negative impacts and others showing little effect. Pollinator frequencies were reduced in dicamba-exposed plots, and pollinator visits were reduced for some weed species yet not for others. Structural equation modeling revealed that the relationship between flower display and pollinator visits was disrupted in the presence of dicamba. Our study provides the most comprehensive picture to date of the impacts of herbicide drift on plant-pollinator interactions, with findings that highlight an underappreciated role of services supplied by weedy communities.

Herbicidal interference: glyphosate drives both the ecology and evolution of plant-herbivore interactions

The coevolution of plants and their insect herbivores reflects eco-evolutionary dynamics at work - ecological interactions influence adaptive traits, which feed back to shape the broader ecological community. However, novel anthropogenic stressors like herbicide, which are strong selective agents, can disrupt these dynamics. Little is known about how the evolution of herbicide resistance may impact plant-herbivore interactions. We performed a common garden field experiment using Ipomoea purpurea (common morning glory) and the herbicide glyphosate (Roundup) to investigate the ecological effects of herbicide exposure on insect herbivory patterns and assess the potential evolutionary consequences. We find that plants treated with glyphosate experienced higher levels of herbivory and altered chewing herbivory damage patterns. Additionally, we found that glyphosate resistance is positively associated with herbivory resistance, and uncovered positive selection for increased glyphosate resistance, suggesting that selection for increased glyphosate resistance has the potential to lead to increased herbivory resistance. Positive selection for glyphosate resistance, coupled with the detection of genetic variation for this trait, suggests there is potential for glyphosate resistance - and herbivory resistance via hitchhiking - to further evolve. Our results show that herbicides cannot just influence, but potentially drive the eco-evolutionary dynamics of plant-herbivore interactions.

Simulated herbicide drift alters native plant flowering phenology

Data for herbicide effects on plant flowering are needed to determine potential impacts on plant reproduction. Thus, flowering phenology was determined for up to 12 weeks after herbicide treatment for native Willamette Valley plants growing in small plots on two Oregon State University experimental farms. Six perennial species were evaluated: Camassia leichtlinii (CALE), Elymus glaucus (ELGL), Eriophyllum lanatum (ERLA), Festuca idahoensis subsp. roemeri (FEID), Iris tenax (IRTE), and Prunella vulgaris var. lanceolata (PRVU). Effects of glyphosate and dicamba, alone and in combination, were determined using simulated drift rates of 0.1 or 0.2 x field application rates (FAR) of 1119 g ha-1 active ingredient (a.i.) (830 g ha-1 acid glyphosate) for glyphosate and 560 g ha-1 a.i. for dicamba. Flowering phenology was evaluated as stage of development on a scale from no buds (converted to 0), buds (1), pre-flowering (2), flowering (3), post-flowering (4), to mature seeds (5) before herbicide treatment and for 12 weeks after treatment. Flowering response to herbicides varied by species and farm; but, in general, dicamba and glyphosate resulted in earlier flowering stages (delayed or not full flowering) for the dicot ERLA, and to a lesser extent, PRVU; and glyphosate resulted in earlier flowering stages for the monocot IRTE. Based on these data, the concentration of herbicide affecting flowering stage was 0.1 x FAR. Once flowering stage was inhibited by dicamba and glyphosate, plants generally did not recover to full flowering. This study provided evidence that common herbicides can affect flowering phenology of native plants with implications for seed production.

Rhizobial variation, more than plant variation, mediates plant symbiotic and fitness responses to herbicide stress

Symbiotic mutualisms provide critical ecosystem services throughout the world. Anthropogenic stressors, however, may disrupt mutualistic interactions and impact ecosystem health. The plant-rhizobia symbiosis promotes plant growth and contributes to the nitrogen (N) cycle. While off-target herbicide exposure is recognized as a significant stressor impacting wild plants, we lack knowledge about how it affects the symbiotic relationship between plants and rhizobia. Moreover, we do not know whether the impact of herbicide exposure on symbiotic traits or plant fitness might be ameliorated by plant or rhizobial genetic variation. To address these gaps, we conducted a greenhouse study where we grew 17 full-sibling genetic families of red clover (Trifolium pratense) either alone (uninoculated) or in symbiosis with one of two genetic strains of rhizobia (Rhizobium leguminosarum) and exposed them to a concentration of the herbicide dicamba that simulated "drift" (i.e., off-target atmospheric movement) or a control solution. We recorded responses in immediate vegetative injury, key features of the plant-rhizobia mutualism (nodule number, nodule size, and N fixation), mutualism outcomes, and plant fitness (biomass). In general, we found that rhizobial variation more than plant variation determined outcomes of mutualism and plant fitness in response to herbicide exposure. Herbicide damage response depended on plant family, but also whether plants were inoculated with rhizobia and if so, with which strain. Rhizobial strain variation determined nodule number and size, but this was herbicide treatment-dependent. In contrast, strain and herbicide treatment independently impacted symbiotic N fixation. And while herbicide exposure significantly reduced plant fitness, this effect depended on inoculation state. Furthermore, the differential fitness benefits that the two rhizobial strains provided plants seemed to diminish under herbicidal conditions. Altogether, these findings suggest that exposure to low levels of herbicide impact key components of the plant-rhizobia mutualism as well as plant fitness, but genetic variation in the partners determines the magnitude and/or direction of these effects. In particular, our results highlight a strong role of rhizobial strain identity in driving both symbiotic and plant growth responses to herbicide stress.

Elucidating Factors Contributing to Dicamba Volatilization by Characterizing Chemical Speciation in Dried Dicamba-Amine Residues

Dicamba is a semivolatile herbicide that has caused widespread unintentional damage to vegetation due to its volatilization from genetically engineered dicamba-tolerant crops. Strategies to reduce dicamba volatilization rely on the use of formulations containing amines, which deprotonate dicamba to generate a nonvolatile anion in aqueous solution. Dicamba volatilization in the field is also expected to occur after aqueous spray droplets dry to produce a residue; however, dicamba speciation in this phase is poorly understood. We applied Fourier transform infrared (FTIR) spectroscopy to evaluate dicamba protonation state in dried dicamba-amine residues. We first demonstrated that commercially relevant amines such as diglycolamine (DGA) and n,n-bis(3-aminopropyl)methylamine (BAPMA) fully deprotonated dicamba when applied at an equimolar molar ratio, while dimethylamine (DMA) allowed neutral dicamba to remain detectable, which corresponded to greater dicamba volatilization. Expanding the amines tested, we determined that dicamba speciation in the residues was unrelated to solution-phase amine pKa, but instead was affected by other amine characteristics (i.e., number of hydrogen bonding sites) that also correlated with greater dicamba volatilization. Finally, we characterized dicamba-amine residues containing an additional component (i.e., the herbicide S-metolachlor registered for use alongside dicamba) to investigate dicamba speciation in a more complex chemical environment encountered in field applications.

Independently evolved pollution resistance in four killifish populations is largely explained by few variants of large effect

The genetic architecture of phenotypic traits can affect the mode and tempo of trait evolution. Human-altered environments can impose strong natural selection, where successful evolutionary adaptation requires swift and large phenotypic shifts. In these scenarios, theory predicts that adaptation is due to a few adaptive variants of large effect, but empirical studies that have revealed the genetic architecture of rapidly evolved phenotypes are rare, especially for populations inhabiting polluted environments. Fundulus killifish have repeatedly evolved adaptive resistance to extreme pollution in urban estuaries. Prior studies, including genome scans for signatures of natural selection, have revealed some of the genes and pathways important for evolved pollution resistance, and provide context for the genotype-phenotype association studies reported here. We created multiple quantitative trait locus (QTL) mapping families using progenitors from four different resistant populations, and using RAD-seq genetically mapped variation in sensitivity (developmental perturbations) following embryonic exposure to a model toxicant PCB-126. We found that one to two large-effect QTL loci accounted for resistance to PCB-mediated developmental toxicity. QTLs harbored candidate genes that govern the regulation of aryl hydrocarbon receptor (AHR) signaling. One QTL locus was shared across all populations and another was shared across three populations. One QTL locus showed strong signatures of recent natural selection in the corresponding wild population but another QTL locus did not. Some candidate genes for PCB resistance inferred from genome scans in wild populations were identified as QTL, but some key candidate genes were not. We conclude that rapidly evolved resistance to the developmental defects normally caused by PCB-126 is governed by few genes of large effect. However, other aspects of resistance beyond developmental phenotypes may be governed by additional loci, such that comprehensive resistance to PCB-126, and to the mixtures of chemicals that distinguish urban estuaries more broadly, may be more genetically complex.

Herbicide resistance status impacts the profile of non-anthocyanin polyphenolics and some phytomedical properties of edible cornflower (Centaurea cyanus L.) flowers

To ensure sufficient food supply worldwide, plants are treated with pesticides to provide protection against pathogens and pests. Herbicides are the most commonly utilised pesticides, used to reduce the growth of weeds. However, their long-term use has resulted in the emergence of herbicide-resistant biotypes in many weed species. Cornflower (Centaurea cyanus L., Asteraceae) is one of these plants, whose biotypes resistant to herbicides from the group of acetolactate synthase (ALS) inhibitors have begun to emerge in recent years. Some plants, although undesirable in crops and considered as weeds, are of great importance in phytomedicine and food production, and characterised by a high content of health-promoting substances, including antioxidants. Our study aimed to investigate how the acquisition of herbicide resistance affects the health-promoting properties of plants on the example of cornflower, as well as how they are affected by herbicide treatment. To this end, we analysed non-anthocyanin polyphenols and antioxidant capacity in flowers of C. cyanus from herbicide-resistant and susceptible biotypes. Our results indicated significant compositional changes associated with an increase in the content of substances and activities that have health-promoting properties. High antioxidant activity and higher total phenolic and flavonoid compounds as well as reducing power were observed in resistant biotypes. The latter one increased additionally after herbicide treatment which might also suggest their role in the resistance acquisition mechanism. Overall, these results show that the herbicide resistance development, although unfavourable to crop production, may paradoxically have very positive effects for medicinal plants such as cornflower.

Ecosystem consequences of herbicides: the role of microbiome

Non-target organisms are globally exposed to herbicides. While many herbicides - for example, glyphosate - were initially considered safe, increasing evidence demonstrates that they have profound effects on ecosystem functions via altered microbial communities. We provide a comprehensive framework on how herbicide residues may modulate ecosystem-level outcomes via alteration of microbiomes. The changes in soil microbiome are likely to influence key nutrient cycling and plant-soil processes. Herbicide-altered microbiome affects plant and animal performance and can influence trophic interactions such as herbivory and pollination. These changes are expected to lead to ecosystem and even evolutionary consequences for both microbes and hosts. Tackling the threats caused by agrochemicals to ecosystem functions and services requires tools and solutions based on a comprehensive understanding of microbe-mediated risks.

Amine Volatilization from Herbicide Salts: Implications for Herbicide Formulations and Atmospheric Chemistry

Amines are frequently included in formulations of the herbicides glyphosate, 2,4-D, and dicamba to increase herbicide solubility and reduce herbicide volatilization by producing herbicide-amine salts. Amines, which typically have higher vapor pressures than the corresponding herbicides, could potentially volatilize from these salts and enter the atmosphere, where they may impact atmospheric chemistry, human health, and climate. Amine volatilization from herbicide-amine salts may additionally contribute to volatilization of dicamba and 2,4-D. In this study, we established that amines applied in herbicide-amine salt formulations undergo extensive volatilization. Both dimethylamine and isopropylamine volatilized when aqueous salt solutions were dried to a residue at ∼20 °C, while lower-vapor pressure amines like diglycolamine and n,n-bis-(3-aminopropyl)methylamine did not. However, all four amines volatilized from salt residues at 40-80 °C. Because amine loss typically exceeded herbicide loss, we proposed that neutral amines dominated volatilization and that higher temperatures altered their protonation state and vapor pressure. Due to an estimated 4.0 Gg N/yr applied as amines to major U.S. crops, amine emissions from herbicide-amine salts may be important on regional scales. Further characterization of worldwide herbicide-amine use would enable this contribution to be compared to the 285 Gg N/yr of methylamines emitted globally.

Species prevalence and plant traits discriminate between herbicide resistant and susceptible weeds

BACKGROUND

Herbicide resistant weeds pose one of the most significant global challenges to sustainable food and fiber production. Plant traits are assumed to play a significant role in determining whether a weed is likely to evolve herbicide resistance but there have been few quantitative assessments to date. There is therefore an urgent need to investigate both the demographic and evolutionary characteristics of weeds to predict which weed species are likely to evolve herbicide resistance. Here, the discriminatory power of multiple plant traits was examined by comparing herbicide resistant and herbicide susceptible weeds in the United States.

RESULTS

Despite the taxonomic and agronomic similarity of herbicide resistant and susceptible weeds in the United States, differences between these groups were captured by a relatively small set of explanatory variables. Herbicide resistant weeds were found across more states than susceptible species and this suggests widespread weeds also happen to be more problematic in crops and therefore specifically targeted for weed control. In terms of traits, herbicide resistant species were more likely to be outcrossing, have unisexual flowers and be wind pollinated as well as have larger chromosome numbers and seed size than herbicide susceptible weeds.

CONCLUSIONS

A trait-based approach to understanding herbicide resistance confirms many assumptions as to the genetic attributes that make a weed more likely to evolve herbicide resistance. Scope therefore exists to build better risk assessment tools to identify future herbicide resistance hazards by incorporating plant traits, environmental tolerances, and evidence of herbicide resistance elsewhere in the world. © 2021 Society of Chemical Industry.

Evolutionary Rescue as a Mechanism Allowing a Clonal Grass to Adapt to Novel Climates

Filing gaps in our understanding of species' abilities to adapt to novel climates is a key challenge for predicting future range shifts and biodiversity loss. Key knowledge gaps are related to the potential for evolutionary rescue in response to climate, especially in long-lived clonally reproducing species. We illustrate a novel approach to assess the potential for evolutionary rescue using a combination of reciprocal transplant experiment in the field to assess performance under a changing climate and independent growth chamber assays to assess growth- and physiology-related plant trait maxima and plasticities of the same clones. We use a clonal grass, Festuca rubra, as a model species. We propagated individual clones and used them in a transplant experiment across broad-scale temperature and precipitation gradients, simulating the projected direction of climate change in the region. Independent information on trait maxima and plasticities of the same clones was obtained by cultivating them in four growth chambers representing climate extremes. Plant survival was affected by interaction between plant traits and climate change, with both trait plasticities and maxima being important for adaptation to novel climates. Key traits include plasticity in extravaginal ramets, aboveground biomass, and osmotic potential. The direction of selection in response to a given climatic change detected in this study mostly contradicted the natural trait clines indicating that short-term selection pressure as identified here does not match long-term selection outcomes. Long-lived clonal species exposed to different climatic changes are subjected to consistent selection pressures on key traits, a necessary condition for adaptation to novel conditions. This points to evolutionary rescue as an important mechanism for dealing with climate change in these species. Our experimental approach may be applied also in other model systems broadening our understanding of evolutionary rescue. Such knowledge cannot be easily deduced from observing the existing field clines.