Pierre Joly

Etude des effets de mélanges d'herbicides employés sur le maïs, sur les communautés microbiennes édaphiques : approche en microcosmes: Ces travaux s’inscrivent dans une problématique actuelle de compréhension des effets, encore peu connus, des mélanges de pesticides dans le sol. Les formulations d’herbicides S-métolachlore (Dual Gold Safeneur®), mésotrione (Callisto®) et nicosulfuron (Milagro®) ont été appliquées en mélanges et à la dose agronomique recommandée. Les molécules formulées sont plus toxiques que les molécules actives seules (Microtox®) et aucun effet synergique ou antagoniste des mélanges n’a été observé. Cependant, ces mélanges impactent transitoirement et à court terme les communautés microbiennes du sol de Limagne, sans modifier les paramètres généraux de structure et d’abondance. Toutefois, des effets ont été mis en évidence sur des communautés sensibles telles que les phototrophes ou celles impliquées dans les processus de nitrification et de dénitrification. De plus, une comparaison des effets des mélanges sur les communautés phototrophes de deux sols souligne l’importante toxicité du Dual Gold Safeneur® et remet en cause son utilisation d’un point de vue écotoxicologique.

Agriculture is in need of alternative products to conventional phytopharmaceutical treatments from chemical industry. One solution is the use of natural microorganisms with beneficial properties to ensure crop yields and plant health. In the present study, we focused our analyses on a bacterium referred as strain B25 and belonging to the species Bacillus velezensis (synonym B. amyloliquefaciens subsp. plantarum or B. methylotrophicus), a promising plant growth promoting rhizobacterium (PGPR) and an inhibitor of pathogenic fungi inducing crops diseases. B25 strain activities were investigated. Its genes are well preserved, with their majority being common with other Bacillus spp. strains and responsible for the biosynthesis of secondary metabolites known to be involved in biocontrol and plant growth-promoting activities. No antibiotic resistance genes were found in the B25 strain plasmid. In vitro and in planta tests were conducted to confirm these PGPR and biocontrol properties, showing its efficiency against 13 different pathogenic fungi through antibiosis mechanism. B25 strain also showed good capacities to quickly colonize its environment, to solubilize phosphorus and to produce siderophores and little amounts of auxin-type phytohormones (around 13,051 µg/mL after 32 h). All these findings combined to the fact that B25 demonstrated good properties for industrialization of the production and an environmental-friendly profile, led to its commercialization under market authorization since 2018 in several biostimulant preparations and opened its potential use as a biocontrol agent.

Modern agriculture needs proper solutions to face the current trend of pesticides and fertilizers reduction. One of the available leverages to support this transition is the use of bioproducts that are more environmentally friendly and less hazardous for human health. Among them, blue biotechnology and more precisely seaweed and microalgae gain interest every year in the scientific community. In agriculture, seaweeds (Macroalgae) have been used in the production of plant biostimulants while microalgae still remain unexploited. Microalgae are widely described as renewable sources of biofuels, bioingredients and biologically active compounds, such as polyunsaturated fatty acids (PUFAs), carotenoids, phycobiliproteins, sterols, vitamins and polysaccharides, which attract considerable interest in both scientific and industrial communities. They affect agricultural crops for enhancement of plant growth, seedling growth. They can also improve nutrient incorporation, fruit setting, resistance properties against pests and diseases, improving stress management (drought, salinity and temperature). The present review aimed at the interest of blue biotechnology in agronomy, with a specific focus on microalgae, their biological activities and their possible application in agriculture as a potentially sustainable alternative for enhanced crop performance, nutrient uptake and resilience to environmental stress. This review does not only present a comprehensive study of microalgae as plant biostimulants but also as biofertilizers, with a particular emphasis on future challenges these solutions will have to deal with, microalgae being able to synthesize secondary metabolites with potential biopesticidal action.

Biological control is a great hope for reducing the overutilization of pesticides in agricultural soils. It often involves microorganisms or molecules produced by microorganisms that will be able to interact with either a plant or pathogens of this plant to reduce the growth of the pathogen and limit its negative impact on the host plant. When new biocontrol products are developed, strains were mostly selected based on their ability to inhibit a pathogen of interest under in vitro conditions via antagonistic effects. Strains with no in vitro effect are often discarded and not tested in planta. But is the in vitro selection of bacterial agents according to their antagonism activities towards a plant pathogen the best way to get effective biocontrol products? To answer this question, we used wheat and the fungal pathogen Fusarium graminearum as a study pathosystem model. A library of 205 soil bacteria was screened in 2 types of in vitro growth inhibition tests against F. graminearum, and in an in planta experiment. We find strains which do not have inhibition phenotypes in vitro but good efficacy in planta. Interestingly, some strains belong to species (Microbacterium, Arthrobacter, Variovorax) that are not known in the literature for their ability to protect plants against fungal pathogens. Thus, developing a biocontrol product against F. graminearum must be preferentially based on the direct screening of strains for their protective activity on wheat plants against fungal diseases, rather than on their in vitro antagonistic effects on fungal growth.

The rhizosphere hosts a considerable microbial community. Among that community, bacteria called plant growth-promoting rhizobacteria (PGPR) can promote plant growth and defense against diseases using diverse distinct plant-beneficial functions. Crop inoculation with PGPR could allow to reduce the use of pesticides and fertilizers in agrosystems. However, microbial crop protection and growth stimulation would be more efficient if cooperation between rhizosphere bacterial populations was taken into account when developing biocontrol agents and biostimulants. Rhizospheric bacteria live in multi-species biofilms formed all along the root surface or sometimes inside the plants (i.e., endophyte). PGPR cooperate with their host plants and also with other microbial populations inside biofilms. These interactions are mediated by a large diversity of microbial metabolites and physical signals that trigger cell–cell communication and appropriate responses. A better understanding of bacterial behavior and microbial cooperation in the rhizosphere could allow for a more successful use of bacteria in sustainable agriculture. This review presents an ecological view of microbial cooperation in agrosystems and lays the emphasis on the main microbial metabolites involved in microbial cooperation, plant health protection, and plant growth stimulation. Eco-friendly inoculant consortia that will foster microbe–microbe and microbe–plant cooperation can be developed to promote crop growth and restore biodiversity and functions lost in agrosystems.

Mesotrione (2‐[4‐(methylsulfonyl)‐2‐nitrobenzoyl]‐cylohexane‐1,3‐dione) is an herbicide brought to the market 15 years ago and commonly used on maize cultures. This review summarizes the results obtained from worldwide scientific studies on its efficiency, effects, and fate in environment, in order to bring first overall conclusions about its safety of use. The comprehensive review of the available scientific literature reveals that mesotrione applied as recommended ensures the desired effects on sensitive crops, mainly in post‐emergence application, while exhibiting favorable toxicological, and environmental profile. According to the studies realized so far, mesotrione presents no significant risks to humans and other non‐target organisms or to the environment. Its degradation by soil microorganisms is fast, ensuring a low possibility to retrieve mesotrione in ground water. Its terminal residues in soil, maize, grain, and maize straw are below the limit of quantification. All these results suggest that mesotrione applied as directed ensures the respect of current environmental standards and food safety. Future prospects should focus on its associated pesticides when applied in mixtures on field, and on the detection, quantification, and risk assessment of its transformation products in order to exclude any toxicological synergistic effects.

Soil phototrophic microorganisms, contributors to soil health and food webs, share their particular metabolism with plants. Current agricultural practices employ mixtures of pesticides to ensure the crops yields and can potentially impair these non-target organisms. However despite this environmental reality, studies dealing the susceptibility of phototrophic microorganisms to pesticide mixtures are scarce. We designed a 3 months microcosm study to assess the ecotoxicity of realistic herbicide mixtures of formulated S-metolachlor (Dual Gold Safeneur^®), mesotrione (Callisto^®) and nicosulfuron (Milagro^®) on phototrophic communities of two soils (Limagne vertisol and Versailles luvisol). The soils presented different colonizing communities, with diatoms and chlorophyceae dominating communities in Limagne soil and cyanobacteria and bryophyta communities in Versailles soil. The results highlighted the strong impairment of Dual Gold Safeneur^® treated microcosms on the biomass and the composition of both soil phototrophic communities, with no resilience after a delay of 3 months. This study also excluded any significant mixture effect on these organisms for Callisto^® and Milagro^® herbicides. We strongly recommend carrying on extensive soil studies on S-metolachlor and its commercial formulations, in order to reconsider its use from an ecotoxicological point of view.

In order to reduce the amounts of pesticides used, and thereby their associated risks, new generations of less environmentally dangerous molecules with lower weight are currently being used in the mixtures sprayed on crops. Few studies have been made, however, to analyse their impact on the soil, and more particularly on the microorganisms living in the soil which maintain the essential functions of this ecosystem. By taking a microcosmic approach, we were able to assess the impact of the maize herbicides “cocktail” Mesotrione and S-metolachlor on global soil microbial activity, biomass, and structures, by using the formulated compounds, respectively, Callisto and Dual Gold (both registered brands of Syngenta). Our results highlighted a synergetic effect in “cocktail” microcosms resulting in an increase in the Mesotrione herbicide dissipation time and in an impact on the microbial community at onefold field rate equally to more than a single herbicide used at tenfold field rate.