Ambrosia grayi as a new alien causal species in Israel: plant biology and chemical management.

BACKGROUND: Ambrosia grayi is a perennial weed native to northern Mexico, which can also be found in the Great Plains of the US. Outside the Americas, A. grayi has only been documented in Israel, where it is currently categorized as a casual species at advanced eradication stages. Here, we studied the plant biology and chemical weed management options of A. grayi. RESULTS: Only large achenes of A. grayi (~5% of all achenes) contain seeds; moreover, the viability of seeds extracted from large achenes was ~25%. Examination of plant anatomy revealed that underground vegetative segments show an anatomical structure of stems (rhizomes) with anomalous secondary growth. The optimal (night/day) temperature for the emergence of A. grayi rhizomes was 20/30 °C, and the emergence rate increased under elevated temperatures. Emergence may occur at different soil moisture content (25-60%); rhizomes were able to emerge even after 1 month of drought conditions (20%, 25% and 30%). Herbicide combinations, such as fluroxypyr + glufosinate, fluroxypyr + glyphosate, and glyphosate + saflufenacil + surfactant, were tested under quarantine conditions and showed high efficacy for the control of A. grayi. However, the efficiency of these treatments was highly correlated with plant growth stage. CONCLUSION: In Israel, the spread of A. grayi occurs mainly via rhizomes that can emerge under a wide range of temperatures and soil moisture conditions. Data regarding herbicide efficacy will aid in improving the eradication efforts taken by Israel's Plant Protection and Inspection Services. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Variation in seed properties and germination capabilities among populations of the invasive weed Parthenium hysterophorus L. (Asteraceae).

Introduction: Parthenium hysterophorus (Asteraceae) is an invasive weed species that has invaded over 50 countries worldwide. It was first detected in 1980 at Tirat-Zvi, in eastern-northern Israel. In recent years, there has been an increasing concern over the spread of this weed in agricultural and non-agricultural habitats across the country. However, very little is known about the biology of P. hysterophorus and its variation among populations. Methods: Seeds collected from five locations across Israel were germinated and plants were grown in pollen-proof cages under uniform conditions to produce the progeny populations. Spatial parameters, weight and germination under different environmental conditions were recorded for field and progeny populations. Results: Seeds originating from field populations were significantly smaller and lighter than seeds of the progeny populations. Germination occurred in the range of 10°C to 30°C (To ranges from 19°C to 22.3°C, Tb ranged from 9°C to 15°C, Tc ranged from 24 ℃ to 30.5°C), depending on generation and population. A water potential-based model was developed to estimate germination under different soil water content using specific parameters (b - slope, d - upper limit, e - infliction point). The model suggests a correlation between germination and water potential. Indeed, reduced germination was recorded for the lower water potentials especially for the progeny populations. Spatial parameters, weight and germination under different environmental conditions were recorded for field and progeny populations. Discussion: We identified differences in seed size and weight, germination under different temperatures, and osmotic potential among P. hysterophorus Israeli populations. Differences across generations may arise due to the transgenerational effects. Our results, may shed light on the germination abilities of P. hysterophorus populations and provide vital insight into understanding the invasive capabilities of this highly noxious weed.

Solanum elaeagnifolium and S. rostratum as potential hosts of the tomato brown rugose fruit virus.

Invasive weeds cause significant crop yield and economic losses in agriculture. The highest indirect impact may be attributed to the role of invasive weeds as virus reservoirs within commercial growing areas. The new tobamovirus tomato brown rugose fruit virus (ToBRFV), first identified in the Middle East, overcame the Tm-22 resistance allele of cultivated tomato varieties and caused severe damage to crops. In this study, we determined the role of invasive weed species as potential hosts of ToBRFV and a mild strain of pepino mosaic virus (PepMV-IL). Of newly tested weed species, only the invasive species Solanum elaeagnifolium and S. rostratum, sap inoculated with ToBRFV, were susceptible to ToBRFV infection. S. rostratum was also susceptible to PepMV-IL infection. No phenotype was observed on ToBRFV-infected S. elaeagnifolium grown in the wild or following ToBRFV sap inoculation. S. rostratum plants inoculated with ToBRFV contained a high ToBRFV titer compared to ToBRFV-infected S. elaeagnifolium plants. Mixed infection with ToBRFV and PepMV-IL of S. rostratum plants, as well as S. nigrum plants (a known host of ToBRFV and PepMV), displayed synergism between the two viruses, manifested by increasing PepMV-IL levels. Additionally, when inoculated with either ToBRFV or PepMV-IL, disease symptoms were apparent in S. rostratum plants and the symptoms were exacerbated upon mixed infections with both viruses. In a bioassay, ToBRFV-inoculated S. elaeagnifolium, S. rostratum and S. nigrum plants infected tomato plants harboring the Tm-22 resistant allele with ToBRFV. The distribution and abundance of these Solanaceae species increase the risks of virus transmission between species.

Distinctive foliar features and volatile profiles in three Ambrosia species (Asteraceae).

MAIN CONCLUSION: Ambrosia species differ both in their trichome types and in metabolic profiles of leaf volatiles. The current study provides tools for easier taxonomic identification of ragweed species. The genus Ambrosia (Asteraceae) includes some of the most noxious allergenic invasive weeds in the world. Due to high polymorphism in this genus, identification of species is often difficult. This study focuses on microscopic investigation of foliar features and GC-MS identification of the main leaf volatile components of three Ambrosia species currently found in Israel-invasive species Ambrosia confertiflora and A. tenuifolia, and transient A. grayi. A. confertiflora and A. tenuifolia have three trichome types: non-glandular trichomes, capitate glandular trichomes and linear glandular trichomes. Their non-glandular trichomes and capitate trichomes have distinct structures and can serve as taxonomic characters. A. grayi (the least successful invader) has only very dense covering trichomes. All three Ambrosia species have secretory structures in their leaf midrib. A. confertiflora, the most problematic invasive plant in Israel, had a ten times higher volatiles content than the other two species. In A. confertiflora, the most abundant volatiles were chrysanthenone (25.5%), borneol (18%), germacrene D and (E)-caryophyllene (both around 12%). In A. tenuifolia, the most abundant volatiles were ß-myrcene (32.9%), (2E)-hexenal (13%) and 1,8-cineole (11.7%). In A. grayi, the most abundant volatiles were ß-myrcene (17.9%), germacrene D (17.8%) and limonene (14%). The three examined species have distinct trichome types and metabolic profiles. Non-glandular trichomes show structural diversification between species and are a good descriptive character. Considering the anthropocentric significance of this highly problematic genus, the current study provides tools for easier identification of ragweed species.

Wild sunflower goes viral: Citizen science and comparative genomics allow tracking the origin and establishment of invasive sunflower in the Levant.

Globalization and intensified volume of trade and transport around the world are accelerating the rate of biological invasions. It is therefore increasingly important to understand the processes through which invasive species colonize new habitats, often to the detriment of native flora. The initial steps of an invasion are particularly critical, as the introduced species relies on limited genetic diversity to adapt to a new environment. However, our understanding of this critical stage of the invasion is currently limited. We used a citizen science approach and social media to survey the distribution of invasive sunflower in Israel. We then sampled and sequenced a representative collection and compared it with available genomic data sets of North American wild sunflower, landraces and cultivars. We show that invasive wild sunflower is rapidly establishing throughout Israel, probably from a single, recent introduction from Texas, while maintaining high genetic diversity through ongoing gene flow. Since its introduction, invasive sunflower has spread quickly to most regions, and differentiation was detected despite extensive gene flow between clusters. Our findings suggest that rapid spread followed by continuous gene flow between diverging populations can serve as an efficient mechanism for maintaining sufficient genetic diversity at the early stages of invasion, promoting rapid adaptation and establishment in the new territory.

Sex dimorphism in dioecious Palmer amaranth (Amaranthus palmeri) in response to water stress.

MAIN CONCLUSION: Phenological isolation can potentially reduce seed output and may be exploited as a novel tool for ecological management of dioecious weeds. Dioecious plants may benefit from a maximized outcrossing and optimal sex-specific resource allocation; however, this breeding system may also be exploited for weed management. Seed production in dioecious species is contingent upon the co-occurrence and co-flowering of the two genders and can be further disturbed by flowering asynchrony. We explored dimorphism in secondary sex characters in Amaranthus palmeri, and tested if reproductive synchrony can be affected by water stress. We have used seeds of A. palmeri from California, Kansas and Texas, and studied secondary sex characters under natural conditions and in response to water stress. Seeds of A. palmeri from California (CA) and Kansas (KS) were cordially provided by Dr. Anil Shrestha (California State University, Fresno, California) and Dr. Dallas E. Peterson (Kansas State University, Manhattan, Kansas), respectively. Seeds of a third population were collected from mature plants (about 30 plants) from a set-aside field in College Station, Texas. A. palmeri showed no sexual dimorphism with regard to the timing of emergence, plant height, and relative growth rate. While the initiation of flowering occurred earlier in males than females, females preceded males in timing of anthesis. Water stress delayed anthesis in males to a greater extent than females increasing the anthesis mismatch between the two sexes by seven days. Our data provide the first evidence of environment-controlled flowering asynchrony in A. palmeri. From a practical point of view, phenological isolation can potentially reduce seed output and may be exploited as a novel tool for ecological management of dioecious weeds.

Using biotechnological approaches to develop crop resistance to root parasitic weeds.

MAIN CONCLUSION: New transgenic and biotechnological approaches may serve as a key component in achieving crop resistance to root parasitic weeds. Root parasitic weeds inflict severe damage to numerous crops, reducing yield quantity and quality. A lack of new sources of resistance limits our ability to manage newly developing, more virulent races. Having no effective means to control the parasites in most crops, innovative biotechnological solutions are needed. Several novel biotechnological strategies using regulatory RNA molecules, the CRISPR/Cas9 system, and T-DNA insertions have been acknowledged for engineering resistance against parasitic weeds. Significant breakthroughs have been made over the years in deciphering the plant genome and its functions, including the genomes of parasitic weeds. However, the basis of biotechnological strategies to generate host resistance to root parasitic weeds needs to be further developed. Gene-silencing and editing tools should be used to target key processes of host-parasite interactions, such as strigolactone biosynthesis and signaling, haustorium development, and degradation and penetration of the host cell wall. In this review, we summarize and discuss the main areas of research leading to the discovery and functional analysis of genes involved in host-induced gene silencing that target key parasite genes, transgenic host modification, and host gene editing to generate sustainable resistance to root parasitic weeds.

Effect of Herbicides on the Management of the Invasive Weed Solanum rostratum Dunal (Solanaceae).

Solanum rostratum Dunal is an invasive weed species that invaded Israel in the 1950s. The weed appears in several germination flashes, from early spring until late summer. Recently, an increase in its distribution range was observed, alongside the identification of new populations in the northern part of Israel. This study aimed to investigate the efficacy of herbicide application for the control of S. rostratum using two field populations originated from the Golan Heights and the Jezreel Valley. While minor differences in herbicide efficacy were recorded between populations, plant growth stage had a significant effect on herbicide response. Carfentrazone-ethyl was found to be highly effective in controlling plants at both early and late growth stages. Metribuzin, oxadiazon, oxyfluorfen and tembutrione showed reduced efficacy when applied at later growth stage (8-9 cm height), as compared to the application at an early growth stage (4-5 cm height). Tank mixes of oxadiazon and oxyfluorfen with different concentrations of surfactant improved later growth stage plant control. Taken together, our study highlights several herbicides that can improve weed control and may be used as chemical solutions alongside diversified crop rotation options. Thus, they may aid in preventing the spread and further buildup of S. rostratum field populations.

Subcritical water extraction as a circular economy approach to recover energy and agrochemicals from sewage sludge.

Global population growth is creating severe pressure on wastewater treatment plants, and specifically on sludge management. Meanwhile, the global challenge of achieving food-security requires the development of green pest control practices that maximize crop productivity. The hydrothermal technology, using subcritical water as the conversion media has been intensively studied, mostly for energy recovery purposes. Here, we focused on the aqueous phase by-product of this process and studied the subcritical water extraction of sewage sludge to recover valuable agrochemicals, with high potential of pre-emergent herbicidal activity. Full characterization of hydrothermal extracts from different reaction temperatures (200-300 °C) and times (30-120 min) highlighted the formation of pyrazine derivatives. Seed germination bioassays with three different species suggested a positive correlation between reaction temperature and extract herbicidal activity. Moreover, differences in seed viability and final root elongation between the tested crop (Trriticum aestivum) and weeds (Lapidium sativum and Amaranthus palmeri) may indicate the competitive abilities that can play a key role in weed management. Our results suggest that subcritical water can be applied as a green solvent for extracting a valuable agrochemical from sewage sludge and improving the circular economy for wastewater treatment.

Review: evolutionary drivers of agricultural adaptation in Lolium spp.

The genus Lolium comprises many species, of which L. perenne ssp. multiflorum, L. perenne ssp. perenne, and L. rigidum are of worldwide agricultural importance as both pasture crops and as weeds. These three species are inter-fertile, obligate out-crossers with a self-incompatible reproduction system. This combination contributes to high genetic diversity that supplies new variants during expansion to new natural areas and agricultural environments. Human dispersal, de-domestication and crop-weed hybridization events between Lolium spp., or with others such as Festuca spp., are likely associated with their distinct weediness abilities. Furthermore, new introductions followed by introgression may hasten adaptation to new environments. Most Lolium-related weed science studies have focused on adaptation leading to herbicide resistance, but other forms of adaptation may also occur. In this review, we explore how the wide genetic variation among Lolium species and hybridization with other species may contribute to range expansion, and adaptation to both new agricultural practices and future predicted climate change scenarios. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Male Linked Genomic Region Determines Sex in Dioecious Amaranthus palmeri.

Dioecy, the separation of reproductive organs on different individuals, has evolved repeatedly in different plant families. Several evolutionary paths to dioecy have been suggested, but the mechanisms behind sex determination is not well understood. The diploid dioecious Amaranthus palmeri represents a well-suited model system to study sex determination in plants. Despite the agricultural importance of the species, the genetic control and evolutionary state of dioecy in A. palmeri is currently unknown. Early cytogenetic experiments did not identify heteromorphic chromosomes. Here, we used whole-genome sequencing of male and female pools from 2 independent populations to elucidate the genetic control of dioecy in A. palmeri. Read alignment to a close monoecious relative and allele frequency comparisons between male and female pools did not reveal significant sex-linked genes. Consequently, we employed an alignment-free k-mer comparison which enabled us to identify a large number of male-specific k-mers. We assembled male-specific contigs comprising a total of almost 2 Mb sequence, proposing a XY sex-determination system in the species. We were able to identify the potential Y chromosome in the A. palmeri draft genome sequence as 90% of our male-specific sequence aligned to a single scaffold. Based on our findings, we suggest an intermediate evolutionary state of dioecy with a young Y chromosome in A. palmeri. Our findings give insight into the evolution of sex chromosomes in plants and may help to develop sustainable strategies for weed management.

Increased temperatures and elevated CO2 levels reduce the sensitivity of Conyza canadensis and Chenopodium album to glyphosate.

Herbicides are the most commonly used means of controlling weeds. Recently, there has been growing concern over the potential impacts of global climate change, specifically, increasing temperatures and elevated carbon dioxide (CO2) concentrations, on the sensitivity of weeds to herbicides. Here, glyphosate response of both Conyza canadensis and Chenopodium album was evaluated under different environmental conditions. Reduced glyphosate sensitivity was observed in both species in response to increased temperature, elevated CO2 level, and the combination of both factors. Increased temperature had greater effect on plant survival than elevated CO2 level. In combination, high temperature and elevated CO2 level resulted in loss of apical dominance and rapid necrosis in glyphosate-treated plants. To investigate the mechanistic basis of reduced glyphosate sensitivity, translocation was examined using 14C-glyphosate. In plants that were subjected to high temperatures and elevated CO2 level, glyphosate was more rapidly translocated out of the treated leaf to shoot meristems and roots than in plants grown under control conditions. These results suggest that altered glyphosate translocation and tissue-specific sequestration may be the basis of reduced plant sensitivity. Therefore, overreliance on glyphosate for weed control under changing climatic conditions may result in more weed control failures.

Climate change exacerbates pest damage through reduced pesticide efficacy.

Pesticide efficacy is strongly associated with environmental conditions. Conditional resistance defined as a reduction in pesticide sensitivity under changed environmental conditions has been widely detected under climatic changes such as elevated temperatures and CO2 enrichment. Given the effects of environmental conditions on pesticide sensitivity, many of the putative resistance reports made by farmers may be due to pesticide application followed by non-optimal environmental conditions rather than the evolution of resistance. This type of conditional resistance may be the result of phenotypic plasticity or epigenetic changes in response to environmental changes. Elevated temperatures and CO2 enrichment can directly lead to reduced pesticide efficacy by altering pesticide metabolism and translocation, or indirectly increasing pesticide detoxification in host-plants thus reducing pesticide availability for the target pest. Stress-related signal transduction pathways, as well as physiological changes, can both be associated with accelerated pesticide detoxification under climatic changes. The possibility for parallel mechanisms controlling these responses in different pest species should be considered. It is proposed that the same mechanisms leading to non-target site resistance in pests may also play a role in conditional resistance, suggesting we can predict the pesticides to which pests are likely to be less responsive under changing climatic conditions. Using adjuvants to improve pesticide translocation or reduce pesticide metabolism, alongside with new technologies such as using nanoparticles may result in higher pesticide functionality under the projected climate change. Exploring the physiological, transcriptional and biochemical basis underlying conditional resistance is crucial in maintaining future pest management under changing environmental conditions. © 2018 Society of Chemical Industry.

Unraveling the Transcriptional Basis of Temperature-Dependent Pinoxaden Resistance in Brachypodium hybridum.

Climate change endangers food security and our ability to feed the ever-increasing human population. Weeds are the most important biotic stress, reducing crop-plant productivity worldwide. Chemical control, the main approach for weed management, can be strongly affected by temperature. Previously, we have shown that temperature-dependent non-target site (NTS) resistance of Brachypodium hybridum is due to enhanced detoxification of acetyl-CoA carboxylase inhibitors. Here, we explored the transcriptional basis of this phenomenon. Plants were characterized for the transcriptional response to herbicide application, high-temperature and their combination, in an attempt to uncover the genetic basis of temperature-dependent pinoxaden resistance. Even though most of the variance among treatments was due to pinoxaden application (61%), plants were able to survive pinoxaden application only when grown under high-temperatures. Biological pathways and expression patterns of members of specific gene families, previously shown to be involved in NTS metabolic resistance to different herbicides, were examined. Cytochrome P450, glucosyl transferase and glutathione-S-transferase genes were found to be up-regulated in response to pinoxaden application under both control and high-temperature conditions. However, biological pathways related to oxidation and glucose conjugation were found to be significantly enriched only under the combination of pinoxaden application and high-temperature. Analysis of reactive oxygen species (ROS) was conducted at several time points after treatment using a probe detecting H2O2/peroxides. Comparison of ROS accumulation among treatments revealed a significant reduction in ROS quantities 24 h after pinoxaden application only under high-temperature conditions. These results may indicate significant activity of enzymatic ROS scavengers that can be correlated with the activation of herbicide-resistance mechanisms. This study shows that up-regulation of genes related to metabolic resistance is not sufficient to explain temperature-dependent pinoxaden resistance. We suggest that elevated activity of enzymatic processes at high-temperature may induce rapid and efficient pinoxaden metabolism leading to temperature-dependent herbicide resistance.

Different Mutations Endowing Resistance to Acetyl-CoA Carboxylase Inhibitors Results in Changes in Ecological Fitness of Lolium rigidum Populations.

Various mutations altering the herbicide target site (TS), can lead to structural modifications that decrease binding efficiency and results in herbicide resistant weed. In most cases, such a mutation will be associated with ecological fitness penalty under herbicide free environmental conditions. Here we describe the effect of various mutations, endowing resistance to acetyl-CoA carboxylase (ACCase) inhibitors, on the ecological fitness penalty of Lolium rigidum populations. The TS resistant populations, MH (substitution of isoleucine 1781 to leucine) and NO (cysteine 2088 to arginine), were examined and compared to a sensitive population (AL). Grain weight (GW) characterization of individual plants from both MH and NO populations, showed that resistant individuals had significantly lower GW compared with sensitive ones. Under high temperatures, both TS resistant populations exhibited lower germination rate as compared with the sensitive (AL) population. Likewise, early vigor of plants from both TS resistant populations was significantly lower than the one measured in plants of the sensitive population. Under crop-weed intra-species competition, we found an opposite trend in the response of plants from different populations. Relatively to inter-population competition conditions, plants of MH population were less affected and presented higher reproduction abilities compared to plants from both AL and NO populations. On the basis of our results, a non-chemical approach can be taken to favor the sensitive individuals, eventually leading to a decline in resistant individuals in the population.

Hyperspectral Technologies for Assessing Seed Germination and Trifloxysulfuron-methyl Response in Amaranthus palmeri (Palmer Amaranth).

Weed infestations in agricultural systems constitute a serious challenge to agricultural sustainability and food security worldwide. Amaranthus palmeri S. Watson (Palmer amaranth) is one of the most noxious weeds causing significant yield reductions in various crops. The ability to estimate seed viability and herbicide susceptibility is a key factor in the development of a long-term management strategy, particularly since the misuse of herbicides is driving the evolution of herbicide response in various weed species. The limitations of most herbicide response studies are that they are conducted retrospectively and that they use in vitro destructive methods. Development of a non-destructive method for the prediction of herbicide response could vastly improve the efficacy of herbicide applications and potentially delay the evolution of herbicide resistance. Here, we propose a toolbox based on hyperspectral technologies and data analyses aimed to predict A. palmeri seed germination and response to the herbicide trifloxysulfuron-methyl. Complementary measurement of leaf physiological parameters, namely, photosynthetic rate, stomatal conductence and photosystem II efficiency, was performed to support the spectral analysis. Plant response to the herbicide was compared to image analysis estimates using mean gray value and area fraction variables. Hyperspectral reflectance profiles were used to determine seed germination and to classify herbicide response through examination of plant leaves. Using hyperspectral data, we have successfully distinguished between germinating and non-germinating seeds, hyperspectral classification of seeds showed accuracy of 81.9 and 76.4%, respectively. Sensitive and resistant plants were identified with high degrees of accuracy (88.5 and 90.9%, respectively) from leaf hyperspectral reflectance profiles acquired prior to herbicide application. A correlation between leaf physiological parameters and herbicide response (sensitivity/resistance) was also demonstrated. We demonstrated that hyperspectral reflectance analyses can provide reliable information about seed germination and levels of susceptibility in A. palmeri. The use of reflectance-based analyses can help to better understand the invasiveness of A. palmeri, and thus facilitate the development of targeted control methods. It also has enormous potential for impacting environmental management in that it can be used to prevent ineffective herbicide applications. It also has potential for use in mapping tempo-spatial population dynamics in agro-ecological landscapes.

Effects of Environmental Conditions on the Fitness Penalty in Herbicide Resistant Brachypodium hybridum.

Herbicide-resistance mutations may impose a fitness penalty in herbicide-free environments. Moreover, the fitness penalty associated with herbicide resistance is not a stable parameter and can be influenced by ecological factors. Here, we used two Brachypodium hybridum accessions collected from the same planted forest, sensitive (S) and target-site resistance (TSR) to photosystem II (PSII) inhibitors, to study the effect of agro-ecological parameters on fitness penalty. Both accessions were collected in the same habitat, thus, we can assume that the genetic variance between them is relatively low. This allow us to focus on the effect of PSII TSR on plant fitness. S plants grains were significantly larger than those of the TSR plants and this was associated with a higher rate of germination. Under low radiation, the TSR plants showed a significant fitness penalty relative to S plants. S plants exhibiting dominance when both types of plants were grown together in a low-light environment. In contrast to previous documented studies, under high-light environment our TSR accession didn't show any significant difference in fitness compared to the S accession. Nitrogen deficiency had significant effect on the R compared to the S accession and was demonstrated in significant yield reduction. TSR plants also expressed a high fitness penalty, relative to the S plants, when grown in competition with wheat plants. Two evolutionary scenarios can be suggested to explain the coexistence of both TSR and S plants in the same habitat. The application of PSII inhibitors may have created selective pressure toward TSR dominancy; termination of herbicide application gave an ecological advantage to S plants, creating changes in the composition of the seed bank. Alternatively, the high radiation intensities found in the Mediterranean-like climate may reduce the fitness penalty associated with TSR. Our results may suggest that by integrating non-herbicidal approaches into weed-management programs, we can reduce the agricultural costs associated with herbicide resistance.

Climate change increases the risk of herbicide-resistant weeds due to enhanced detoxification.

MAIN CONCLUSION: Global warming will increase the incidence of metabolism-based reduced herbicide efficacy on weeds and, therefore, the risk for evolution of non-target site herbicide resistance. Climate changes affect food security both directly and indirectly. Weeds are the major biotic factor limiting crop production worldwide, and herbicides are the most cost-effective way for weed management. Processes associated with climatic changes, such as elevated temperatures, can strongly affect weed control efficiency. Responses of several grass weed populations to herbicides that inhibit acetyl-CoA carboxylase (ACCase) were examined under different temperature regimes. We characterized the mechanism of temperature-dependent sensitivity and the kinetics of pinoxaden detoxification. The products of pinoxaden detoxification were quantified. Decreased sensitivity to ACCase inhibitors was observed under elevated temperatures. Pre-treatment with the cytochrome-P450 inhibitor malathion supports a non-target site metabolism-based mechanism of herbicide resistance. The first 48 h after herbicide application were crucial for pinoxaden detoxification. The levels of the inactive glucose-conjugated pinoxaden product (M5) were found significantly higher under high- than low-temperature regime. Under high temperature, a rapid elevation in the level of the intermediate metabolite (M4) was found only in pinoxaden-resistant plants. Our results highlight the quantitative nature of non-target-site resistance. To the best of our knowledge, this is the first experimental evidence for temperature-dependent herbicide sensitivity based on metabolic detoxification. These findings suggest an increased risk for the evolution of herbicide-resistant weeds under predicted climatic conditions.

Conyza species: distribution and evolution of multiple target-site herbicide resistances.

MAIN CONCLUSION: Distribution of Conyza species is well correlated with human interference. Multiple herbicide resistance is caused by the attempt to overcome resistance to one mode of action by overuse of another. Conyza canadensis (CC) and Conyza bonariensis (CB) are troublesome weeds around the world. Extensive use of herbicides has led to the evolution of numerous Conyza spp. herbicide-resistant populations. Seeds of 91 CC and CB populations were collected across Israel. They were mostly found (86 %) in roadsides and urban habitats, two disturbed habitats that had been dramatically impacted by human activities, thus we classify these species as anthropogenic. Although pyrithiobac-sodium was only used in cotton fields, 90 % of Conyza spp. populations were identified as pyrithiobac-sodium resistant, suggesting possible natural resistance to pyrithiobac-sodium. CC21 and CC17 C. canadensis populations were highly resistant to all tested ALS inhibitors due to a substitution in the ALS gene from Trp574 to Leu. They were also atrazine resistant due to a substitution in the psbA gene from Ser264 to Gly. The high level of imazapyr and pyrithiobac-sodium resistance observed in the CC10 population was due to an Ala205 to Val substitution. However, high resistance to sulfometuron methyl and pyrithiobac-sodium in population CC6 was due to a point mutation at Pro197 to Ser. All resistant plants of CC21 population showed both psbA (Ser264 to Gly) and ALS (Trp574 to Leu) substitutions, leading us to the conclusion that the attempt to overcome resistance to one mode of action by overuse of another will most likely lead to multiple herbicide resistance. Furthermore, we concluded that only individuals that carry both mutations could survive the shift between the two modes of action and overcome the fitness cost associated with the PSII resistance.

Evolution of herbicide resistance mechanisms in grass weeds.

Herbicide resistant weeds are becoming increasingly common, threatening global food security. Here, we present BrIFAR: a new model system for the functional study of mechanisms of herbicide resistance in grass weeds. We have developed a large collection of Brachypodium accessions, the BrI collection, representing a wide range of habitats. Wide screening of the responses of the accessions to four major herbicide groups (PSII, ACCase, ALS/AHAS and EPSPS inhibitors) identified 28 herbicide-resistance candidate accessions. Target-site resistance to PSII inhibitors was found in accessions collected from habitats with a known history of herbicide applications. An amino acid substitution in the psbA gene (serine264 to glycine) conferred resistance and also significantly affected the flowering and shoot dry weight of the resistant accession, as compared to the sensitive accession. Non-target site resistance to ACCase inhibitors was found in accessions collected from habitats with a history of herbicide application and from a nature reserve. In-vitro enzyme activity tests and responses following pre-treatment with malathion (a cytochrome-P450 inhibitor) indicated sensitivity at the enzyme level, and give strong support to diclofop-methyl and pinoxaden enhanced detoxification as NTS resistance mechanism. BrIFAR can promote better understanding of the evolution of mechanisms of herbicide resistance and aid the implementation of integrative management approaches for sustainable agriculture.