Evolution of Target-Site Resistance to Glyphosate in an Amaranthus palmeri Population from Argentina and Its Expression at Different Plant Growth Temperatures

Investigating the origins and evolution of a glyphosate-resistant weed invasion in South America

The global invasion, and subsequent spread and evolution of weeds provides unique opportunities to address fundamental questions in evolutionary and invasion ecology. Amaranthus palmeri is a widespread glyphosate-resistant (GR) weed in the USA. Since 2015, GR populations of A. palmeri have been confirmed in South America, raising questions about introduction pathways and the importance of pre- vs. post-invasion evolution of GR traits. We used RAD-sequencing genotyping to characterize genetic structure of populations from Brazil, Argentina, Uruguay and the USA. We also quantified gene copy number of the glyphosate target, 5-enolpyruvyl-3-shikimate phosphate synthase (EPSPS), and the presence of an extrachromosomal circular DNA (eccDNA) replicon known to confer glyphosate resistance in USA populations. Populations in Brazil, Argentina and Uruguay were only weakly differentiated (pairwise FST  ≤0.043) in comparison to USA populations (mean pairwise FST  =0.161, range =0.068-0.258), suggesting a single major invasion event. However, elevated EPSPS copy number and the EPSPS replicon were identified in all populations from Brazil and Uruguay, but only in a single Argentinean population. These observations are consistent with independent in situ evolution of glyphosate resistance in Argentina, followed by some limited recent migration of the eccDNA-based mechanism from Brazil to Argentina. Taken together, our results are consistent with an initial introduction of A. palmeri into South America sometime before the 1980s, and local evolution of GR in Argentina, followed by a secondary invasion of GR A. palmeri with the unique eccDNA-based mechanism from the USA into Brazil and Uruguay during the 2010s.

Fitness Cost Associated With Enhanced EPSPS Gene Copy Number and Glyphosate Resistance in an Amaranthus tuberculatus Population

The evolution of resistance to pesticides in agricultural systems provides an opportunity to study the fitness costs and benefits of novel adaptive traits. Here, we studied a population of Amaranthus tuberculatus (common waterhemp), which has evolved resistance to glyphosate. The growth and fitness of seed families with contrasting levels of glyphosate resistance was assessed in the absence of glyphosate to determine their ability to compete for resources under intra- and interspecific competition. We identified a positive correlation between the level of glyphosate resistance and gene copy number for the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) glyphosate target, thus identifying gene amplification as the mechanism of resistance within the population. Resistant A. tuberculatus plants were found to have a lower competitive response when compared to the susceptible phenotypes with 2.76 glyphosate resistant plants being required to have an equal competitive effect as a single susceptible plant. A growth trade-off was associated with the gene amplification mechanism under intra-phenotypic competition where 20 extra gene copies were associated with a 26.5 % reduction in dry biomass. Interestingly, this growth trade-off was mitigated when assessed under interspecific competition from maize.

Syngenta's contribution to herbicide resistance research and management

The evolution of weed resistance to herbicides is an ever-increasing problem that affects crop yield and food production. In Syngenta, we believe that this difficult and complex issue can be most efficiently addressed through a deep understanding of the evolutionary dynamics and mechanism of resistance. A profound knowledge of resistance is key to developing the next generation of resistance-breaking compounds with existing or novel herbicide sites of action. We use a multidisciplinary laboratory-based, glasshouse and field biology approach to study herbicide resistance and provide strong science-based solutions to delay the onset and manage resistance. We have developed and implemented simple early-season resistance detection methods to allow farmers make an informed decision for effective weed control. We have built mechanistic, individual-based computer models to design profitable, long-term sustainable weed management programs. Our zero tolerance approaches employ herbicides with different sites of action, applied in mixtures and sequences, to minimise the risk of resistance evolution. Weeds are targeted at the right growth stage with optimal herbicide formulation and spray technology for maximising weed control and depleting the seed bank. We are promoting the use of competitive crop varieties and other nonchemical methods for an integrated weed management strategy. We have a global web of external collaborations for studying and managing herbicide resistance. We are committed to farmers' education and training on herbicide resistance, and regularly share our methods and findings via conferences and peer-reviewed scientific publications for the benefit of the wider weed science community and field practitioners. © 2020 Society of Chemical Industry.

A holistic approach in herbicide resistance research and management: from resistance detection to sustainable weed control

Agricultural weeds can adapt rapidly to human activities as exemplified by the evolution of resistance to herbicides. Despite its multi-faceted nature, herbicide resistance has rarely been researched in a holistic manner. A novel approach combining timely resistance confirmation, investigation of resistance mechanisms, alternative control solutions and population modelling was adopted for the sustainable management of the Amaranthus palmeri weed in soybean production systems in Argentina. Here, we show that resistance to glyphosate in the studied population from Cordoba province was mainly due to a P106S target-site mutation in the 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) gene, with minor contributions from EPSPS gene duplication/overexpression. Alternative herbicides, such as fomesafen, effectively controlled the glyphosate-resistant plants. Model simulations revealed the tendency of a solo herbicidal input to primarily select for a single resistance mechanism and suggested that residual herbicides, alongside chemical diversity, were important for the sustainable use of these herbicides. We also discuss the value of an interdisciplinary approach for improved understanding of evolving weeds.

Predominance of Metabolic Resistance in a Six-Way-Resistant Palmer Amaranth (Amaranthus palmeri) Population

Evolution of multiple herbicide resistance in Palmer amaranth across the United States is a serious challenge for its management. Recently, a Palmer amaranth population (KCTR; Kansas Conservation Tillage Resistant) from a long-term conservation tillage research project in Kansas, United States, was found uncontrolled by several commonly used herbicides. Importantly, this field did not have a history of repeated use of some of the herbicides for which the KCTR Palmer amaranth population showed lack of control. The objectives of this study were to confirm the evolution of multiple resistances and determine possible mechanism(s) of resistance in KCTR Palmer amaranth plants. In response to post-emergence application, 28-100% of KCTR Palmer amaranth survived field recommended rates of 2,4-D, ALS-, PS II-, EPSPS-, PPO-, HPPD-inhibitor herbicides, or tank- or pre-mixture of PS II- and HPPD-inhibitor herbicides, confirming evolution of six-way resistance in this Palmer amaranth population. However, this population was found susceptible to the PS I- and glutamine synthetase inhibitor herbicides. Chlorsulfuron-, imazethapyr-, and atrazine-resistant plants did not show any previously reported mutation in ALS and psbA genes, the target sites of these herbicides, respectively. However, the survivors of glyphosate treatment showed amplification of EPSPS gene (up to 88 copies). The KCTR plants pretreated with cytochrome P450 or GST inhibitors along with atrazine, 2,4-D, lactofen, or mesotrione had significantly less biomass accumulation than those treated with herbicides alone. Plants treated with P450 inhibitor followed by imazethapyr showed moderate reduction of biomass in KCTR which was statistically similar to a susceptible Palmer amaranth population treated with imazethapyr. These results suggest predominance of metabolic resistance possibly mediated by cytochrome P450 and GST enzyme activity that may have predisposed the KCTR Palmer amaranth population to evolve resistance to multiple herbicides. This is the first report of evolution of six-way resistance in a single Palmer amaranth population. Appropriate management strategies, including integration of cultural, and mechanical, and herbicide mixtures, are warranted to control such Palmer amaranth populations.

Treatment of Sweet Pepper with Stress Tolerance-Inducing Compounds Alleviates Salinity Stress Oxidative Damage by Mediating the Physio-Biochemical Activities and Antioxidant Systems

Salinity stress occurs due to the accumulation of high levels of salts in soil, which ultimately leads to the impairment of plant growth and crop loss. Stress tolerance-inducing compounds have a remarkable ability to improve growth and minimize the effects of salinity stress without negatively affecting the environment by controlling the physiological and molecular activities in plants. Two pot experiments were carried out in 2017 and 2018 to study the influence of salicylic acid (1 mM), yeast extract (6 g L−1), and proline (10 mM) on the physiological and biochemical parameters of sweet pepper plants under saline conditions (2000 and 4000 ppm). The results showed that salt stress led to decreasing the chlorophyll content, relative water content, and fruit yields, whereas electrolyte leakage, malondialdehyde (MDA), proline concentration, reactive oxygen species (ROS), and the activities of antioxidant enzymes increased in salt-stressed plants. The application of salicylic acid (1 mM), yeast extract (6 g L−1), and proline (10 mM) markedly improved the physiological characteristics and fruit yields of salt-stressed plants compared with untreated stressed plants. A significant reduction in electrolyte leakage, MDA, and ROS was also recorded for all treatments. In conclusion, our results reveal the important role of proline, SA, and yeast extracts in enhancing sweet pepper growth and tolerance to salinity stress via modulation of the physiological parameters and antioxidants machinery. Interestingly, proline proved to be the best treatment.