Fungicides With Contrasting Mode of Action Differentially Affect Hyphal Healing Mechanism in Gigaspora sp. and Rhizophagus irregularis

Development of a high-throughput spore germination test to assess the toxicity of pesticides on arbuscular mycorrhizal fungi

Pesticides are essential agricultural inputs that help securing crop yields. However, they can affect non-target soil microorganisms, including arbuscular mycorrhizal (AM) fungi, that are potential indicators of the toxicity of pesticides on the soil microbiota. Here, we developed a fast-track high-throughput spore germination test, for AM fungi produced in vitro. This test allows the determination of EC50 values and the nature of the effects of pesticides on AM fungal spores (fungicidal or fungistatic). First, 19 active ingredients were tested on Rhizophagus intraradices MUCL 49410. Secondly, five of these compounds, varying in their toxicity to R. intraradices, were tested on three additional AM fungi (Rhizophagus irregularis MUCL 41833, Rhizophagus clarus MUCL 46238 and Rhizophagus aggregatus MUCL 49408). Our results showed that the toxicity of pesticides varied according to their chemical nature, concentration and AM fungal species tested. With the exception of 3,5,6-trichloro-2-pyridinol (TCP, a transformation product of chlorpyrifos), insecticides and herbicides had no detrimental effect on spore germination at the concentration expected in soil upon application of the recommended dose, unlike most fungicides, which had an impact on one or more AM fungi. Fludioxonil and pyraclostrobin were by far the most problematic fungicide and R. aggregatus the most sensitive strain to pesticides. This AM fungus could thus be a good indicator to be used in standard ecotoxicity testing. In conclusion, we present a fast-track, high-throughput testing system for assessing the toxicity of pesticides on AM fungi, using spore germination as a relevant endpoint, that could be used as a first-tier screening tool in pesticide risk assessment.

The microbiome orchestrates contaminant low-dose phytostimulation

Our understanding of the physiological mechanisms of the plant hormetic response to countless environmental contaminants is rapidly advancing. However, the microbiome is a critical determinant of plant responses to stressors, thus possibly influencing hormetic responses. Here, we review the otherwise neglected role of microbes in shaping plant stimulation by subtoxic concentrations of contaminants and vice versa. Numerous contaminants at subtoxic levels enhance microorganisms and proliferate symbionts, such as mycorrhizae and other plant beneficial microbes, leading to both direct and indirect improvements in plant physiological performance. Microbial symbiosis facilitates nutrient uptake by plants, indicating an important contribution of symbionts to phytostimulation under subtoxic contamination. We also discuss the mechanisms and implications of the stimulation of plant-microbe systems by subtoxic contaminants.

Evaluation of Pencycuron Residue Dynamics in Eggplant Using LC-MS/MS and Establishment of Pre-Harvest Residue Limits

Pencycuron is a fungicide whose maximum residue limit (MRL) in eggplant is either set at very low levels (0.02 mg/kg in European Union) or remains unestablished in many countries, necessitating stringent pesticide management. To enable timely interventions by farmers and regulators, pre-harvest residue limits (PHRLs) propose maximum allowable pesticide concentrations for each day during the pre-harvest period. An analytical method was developed to conduct residue determination trials, demonstrating that graphitized carbon black (GCB) effectively removes eggplant matrices during sample preparation. The LC-MS/MS method was established with a limit of quantification (LOQ) of 0.005 mg/kg, recovery rates ranging from 102.6% to 106.1% with relative standard deviation (RSD; 2.3-6.4%), and a matrix effect (%ME) of +8.1%. Residue analysis revealed a concentration of 0.045 mg/kg at 0 days after treatment (DAT), decreasing to 0.006 mg/kg at 14 DAT. The residue dynamics followed a first-order kinetic model, as confirmed by the F-test, with a rate constant of 0.1405. Therefore, the half-life was determined to be 4.9 d. Based on the MRL value of 0.02 mg/kg at 0 days before harvest (DBH), the PHRL was determined using both k and kmin, resulting in values of 0.04 mg/kg and 0.02 mg/kg at 5 days and 0.08 mg/kg and 0.03 mg/kg at 10 DBH, respectively. Using kmin yields more conservative results, which ensures food safety under conditions of slower degradation rates.

The systemic herbicide glyphosate affects the sporulation dynamics of Rhizophagus species more severely than mechanical defoliation or the contact herbicide diquat

Arbuscular mycorrhizal fungi (AMF) are totally dependent on a suitable host plant for their carbon resources. Here, we investigated under in vitro conditions, the impact of defoliation practices, i.e., mechanical defoliation or chemical defoliation with a contact herbicide (Reglone®, containing the active ingredient diquat) or systemic herbicide (RoundUp®, containing the active ingredient glyphosate), on the dynamics of spore production of Rhizophagus irregularis and Rhizophagus intraradices associated with Solanum tuberosum and/or Medicago truncatula. Glyphosate affected the spore production rate more rapidly and severely than diquat or mechanical defoliation. We hypothesize that this effect was related to disruption of the C metabolism in the whole plant combined with a possible direct effect of glyphosate on the fungus within the roots and/or perhaps in soil via the release of this active ingredient from decaying roots. No glyphosate could be detected in the roots due to technical constraints, while its release from the roots in the medium corresponded to 0.11% of the active ingredient applied to the leaves. The three defoliation practices strongly affected root colonization, compared to the non-defoliated plants. However, the amount of glyphosate released into the medium did not affect spore germination and germ tube growth. These results suggest that the effects of defoliation on the dynamics of spore production are mainly indirect via an impact on the plant, and that the effect is faster and more marked with the glyphosate-formulation, possibly via a direct effect on the fungus in the roots and more unlikely on spore germination.

Elucidation of furanone as ergosterol pathway inhibitor in Cryptococcus neoformans

In the era of antiretroviral therapy, the prevalence of Cryptococcal infection among HIV patients in developed countries has decreased considerably. However, C. neoformans ranks top among the critical priority pathogen that affects a wide range of immunocompromised individuals. The threat of C. neoformans is because of its incredibly multifaceted intracellular survival capabilities. Cell membrane sterols especially ergosterol and enzymes of its biosynthetic pathway are considered fascinating drug targets because of their structural stability. In this study, the ergosterol biosynthetic enzymes were modeled and docked with furanone derivatives. Among the tested ligands Compound 6 has shown a potential interaction with Lanosterol 14 α-demethylase. This best-docked protein-ligand complex was taken further to molecular dynamics simulation. In addition, Compound 6 was synthesized and an in vitro study was conducted to quantify the ergosterol in Compound 6 treated cells. Altogether the computational and in vitro study demonstrates that Compound 6 has anticryptococcal activity by targeting the biosynthetic pathway of ergosterol.Communicated by Ramaswamy H. Sarma.

A pipeline for rapidly evaluating activity and inferring mechanisms of action of prospective antifungal compounds

BACKGROUND

Fungal phytopathogens are a significant threat to crops and food security, and there is a constant need to develop safe and effective compounds that antagonize them. In-planta assays are complex and tedious and are thus not suitable for initial high-throughput screening of new candidate antifungal compounds. We propose an in vitro screening pipeline that integrates five rapid quantitative and qualitative methods to estimate the efficacy and mode of action of prospective antifungal compounds.

RESULTS

The pipeline was evaluated using five documented antifungal compounds (benomyl, catechol, cycloheximide, 2,4-diacetylphloroglucinol, and phenylacetic acid) that have different modes of action and efficacy, against the model soilborne fungal pathogen Fusarium oxysporum f. sp. radicis cucumerinum. We initially evaluated the five compounds' ability to inhibit fungal growth and metabolic activity using green fluorescent protein (GFP)-labeled F. oxysporum and PrestoBlue staining, respectively, in multiwell plate assays. We tested the compounds' inhibition of both conidial germination and hyphal elongation. We then employed FUN-1 and SYTO9/propidium iodide staining, coupled to confocal microscopy, to differentiate between fungal growth inhibition and death at the cellular level. Finally, using a reactive oxygen species (ROS)-detection assay, we were able to quantify ROS production in response to compound application.

CONCLUSIONS

Collectively, the proposed pipeline provides a wide array of quantitative and qualitative data on the tested compounds that can help pinpoint promising novel compounds; these can then be evaluated more vigorously using in planta screening assays. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Arbuscular Mycorrhizal inoculants and its regulatory landscape

One of the most prominent means for sustainable agriculture and ecosystem management are Arbuscular Mycorrhizal (AM) inoculants. These inoculants establish beneficial symbiotic relationships with land plant roots, offering a wide range of benefits, from enhanced nutrient absorption to improved resilience against environmental stressors. However, several currently available commercial AM inoculants face challenges such as inconsistency in field applications, ecological risks associated with non-native strains, and the absence of universal regulations. Currently, regulations for AM inoculants vary globally, with some regions leading efforts to standardize and ensure quality control. Proposed regulatory frameworks aim to establish parameters for composition, safety, and efficacy. Nevertheless, challenges persist in terms of scientific data, standardization, testing under real conditions, and the ecological impact of these inoculants. To address these challenges and unlock the full potential of AM inoculants, increased research funding, public-private partnerships, monitoring, awareness, and ecosystem impact studies are recommended. Future regulations have the potential to improve product quality, soil health, and crop productivity while reducing reliance on chemical inputs and benefiting the environment. However, addressing issues related to compliance, standardization, education, certification, monitoring, and cost is essential for realizing these benefits. Global harmonization and collaborative efforts are vital to maximize their impact on agriculture and ecosystem management, leading to healthier soils, increased crop yields, and a more sustainable agricultural industry.

Disentangling the effects of terroir, season, and vintage on the grapevine fungal pathobiome

The composition, diversity and dynamics of microbial communities associated with grapevines may be influenced by various environmental factors, including terroir, vintage, and season. Among these factors, terroir stands out as a unique possible determinant of the pathobiome, the community of plant-associated pathogens. This study employed high-throughput molecular techniques, including metabarcoding and network analysis, to investigate the compositional dynamics of grapevine fungal pathobiome across three microhabitats (soil, woody tissue, and bark) using the Furmint cultivar. Samples were collected during late winter and late summer in 2020 and 2021, across three distinct terroirs in Hungary's Tokaj wine region. Of the 123 plant pathogenic genera found, Diplodia, Phaeomoniella, and Fusarium displayed the highest richness in bark, wood, and soil, respectively. Both richness and abundance exhibited significant disparities across microhabitats, with plant pathogenic fungi known to cause grapevine trunk diseases (GTDs) demonstrating highest richness and abundance in wood and bark samples, and non-GTD pathogens prevailed soil. Abundance and richness, however, followed distinct patterns Terroir accounted for a substantial portion of the variance in fungal community composition, ranging from 14.46 to 24.67%. Season and vintage also contributed to the variation, explaining 1.84 to 2.98% and 3.67 to 6.39% of the variance, respectively. Notably, significant compositional differences in fungi between healthy and diseased grapevines were only identified in wood and bark samples. Cooccurrence networks analysis, using both unweighted and weighted metrics, revealed intricate relationships among pathogenic fungal genera. This involved mostly positive associations, potentially suggesting synergism, and a few negative relationships, potentially suggesting antagonistic interactions. In essence, the observed differences among terroirs may stem from environmental filtering due to varied edaphic and mesoclimatic conditions. Temporal weather and vine management practices could explain seasonal and vintage fungal dynamics. This study provides insights into the compositional dynamics of grapevine fungal pathobiome across different microhabitats, terroirs, seasons, and health statuses. The findings emphasize the importance of considering network-based approaches in studying microbial communities and have implications for developing improved viticultural plant health strategies.

Anastomosis within and between networks of Rhizophagus irregularis is differentially influenced by fungicides

Arbuscular mycorrhizal (AM) fungi play key roles in soil fertility of agroecosystems. They develop dense extraradical mycelial (ERM) networks via mechanisms such as hyphal anastomosis. These connections between hyphae can be affected by agricultural practices such as the use of fungicides, but how these compounds affect anastomosis formation within and more importantly between networks of the same AM fungal strain remains poorly unexplored. Here, the impact of azoxystrobin, pencycuron, flutolanil, and fenpropimorph at 0.02 and 2 mg L^-1 were tested in vitro on the anastomosis formation within and between networks of Rhizophagus irregularis MUCL 41833. Azoxystrobin and fenpropimorph had a particularly detrimental impact, at the highest concentration (2 mg L^-1), on the number of anastomoses within and between networks, and for fenpropimorph in particular at both concentrations (0.02 and 2 mg L^-1) on the number of anastomoses per length of hyphae. Curiously fenpropimorph at 0.02 mg L^-1 significantly stimulated spore production, while with azoxystrobin, the reverse was observed at 2 mg L^-1. The two other fungicides, pencycuron and flutolanil, had no detrimental effects on spore production or anastomosis formation within and between networks. These results suggest that fungicides with different modes of action and concentrations differentially affect anastomosis possibly by altering the hyphal tips of AM fungi and may thus affect the capacity of AM fungi to develop large hyphal networks exploring and exploiting the soil at the service of plants.

Fungi-on-a-Chip: microfluidic platforms for single-cell studies on fungi

This review highlights new advances in the emerging field of 'Fungi-on-a-Chip' microfluidics for single-cell studies on fungi and discusses several future frontiers, where we envisage microfluidic technology development to be instrumental in aiding our understanding of fungal biology. Fungi, with their enormous diversity, bear essential roles both in nature and our everyday lives. They inhabit a range of ecosystems, such as soil, where they are involved in organic matter degradation and bioremediation processes. More recently, fungi have been recognized as key components of the microbiome in other eukaryotes, such as humans, where they play a fundamental role not only in human pathogenesis, but also likely as commensals. In the food sector, fungi are used either directly or as fermenting agents and are often key players in the biotechnological industry, where they are responsible for the production of both bulk chemicals and antibiotics. Although the macroscopic fruiting bodies are immediately recognizable by most observers, the structure, function, and interactions of fungi with other microbes at the microscopic scale still remain largely hidden. Herein, we shed light on new advances in the emerging field of Fungi-on-a-Chip microfluidic technologies for single-cell studies on fungi. We discuss the development and application of microfluidic tools in the fields of medicine and biotechnology, as well as in-depth biological studies having significance for ecology and general natural processes. Finally, a future perspective is provided, highlighting new frontiers in which microfluidic technology can benefit this field.

Hormesis induced by silver iodide, hydrocarbons, microplastics, pesticides, and pharmaceuticals: Implications for agroforestry ecosystems health

Increasing amounts of silver iodide (AgI) in the environment are expected because of the recent massive expansion of weather modification programs. Concurrently, pharmaceuticals, microplastics, hydrocarbons, and pesticides in terrestrial ecosystems continue contaminating forests and agroforests. Our review supports that AgI induces hormesis, a biphasic dose response characterized by often beneficial low-dose responses and toxic high-dose effects, which adds to the evidence for pharmaceuticals, microplastics, hydrocarbons, and pesticides induced hormesis in numerous species. Doses smaller than the no-observed-adverse-effect-level (NOAEL) positively affect defense physiology, growth, biomass, yields, survival, lifespan, and reproduction. They also lead to negative or undesirable outcomes, including stimulation of pathogenic microbes, pest insects, and weeds with enhanced resistance to drugs and potential negative multi- or trans-generational effects. Such sub-NOAEL effects perplex terrestrial ecosystems managements and may compromise combating outbreaks of disease vectors that can threaten not only forest and agroforestry health but also sensitive human subpopulations living in remote forested areas.