Arbuscular mycorrhizal inoculum sources influence bacterial, archaeal, and fungal communities' structures of historically dioxin/furan-contaminated soil but not the pollutant dissipation rate.

Little is known about the influence of arbuscular mycorrhizal fungi (AMF) inoculum sources on phytoremediation efficiency. Therefore, the aim of this study was to compare the effects of two mycorrhizal inocula (indigenous and commercial inocula) in association with alfalfa and tall fescue on the plant growth, the bacterial, fungal, and archaeal communities, and on the removal of dioxin/furan (PCDD/F) from a historically polluted soil after 24 weeks of culture in microcosms. Our results showed that both mycorrhizal indigenous and commercial inocula were able to colonize plant roots, and the growth response depends on the AMF inoculum. Nevertheless, the improvement of root dry weight in inoculated alfalfa with indigenous inoculum and in inoculated tall fescue with commercial inoculum was clearly correlated with the highest mycorrhizal colonization of the roots in both plant species. The highest shoot dry weight was obtained in inoculated alfalfa and tall fescue with the commercial inoculum. AMF inoculation differently affected the number of bacterial and archaeal OTUs and bacterial diversity, with elevated bacterial and archaeal OTUs and bacterial diversity observed with indigenous inoculum. Mycorrhizal inoculation increases the abundance of bacterial OTUs (in particular with indigenous inoculum) and microbial richness but it does not improve PCDD/F dissipation. Vegetation had no effect on the abundance of microbial OTUs nor on richness but stimulated specific communities (Planctomycetia and Gammaproteobacteria) likely to be involved in the dissipation of PCDD/F. The reduction of toxic equivalency PCDD/F concentration also could be explained by the stimulation of soil microbial activities estimated with dehydrogenase and fluorescein diacetate hydrolase.

Phosphorus supply, arbuscular mycorrhizal fungal species, and plant genotype impact on the protective efficacy of mycorrhizal inoculation against wheat powdery mildew.

A potential alternative strategy to chemical control of plant diseases could be the stimulation of plant defense by arbuscular mycorrhizal fungi (AMF). In the present study, the influence of three parameters (phosphorus supply, mycorrhizal inoculation, and wheat cultivar) on AMF protective efficiency against Blumeria graminis f. sp. tritici, responsible for powdery mildew, was investigated under controlled conditions. A 5-fold reduction (P/5) in the level of phosphorus supply commonly recommended for wheat in France improved Funneliformis mosseae colonization and promoted protection against B. graminis f. sp. tritici in a more susceptible wheat cultivar. However, a further decrease in P affected plant growth, even under mycorrhizal conditions. Two commercially available AMF inocula (F. mosseae, Solrize®) and one laboratory inoculum (Rhizophagus irregularis) were tested for mycorrhizal development and protection against B. graminis f. sp. tritici of two moderately susceptible and resistant wheat cultivars at P/5. Mycorrhizal levels were the highest with F. mosseae (38 %), followed by R. irregularis (19 %) and Solrize® (SZE, 8 %). On the other hand, the highest protection level against B. graminis f. sp. tritici was obtained with F. mosseae (74 %), followed by SZE (58 %) and R. irregularis (34 %), suggesting that inoculum type rather than mycorrhizal levels determines the protection level of wheat against B. graminis f. sp. tritici. The mycorrhizal protective effect was associated with a reduction in the number of conidia with haustorium and with an accumulation of polyphenolic compounds at B. graminis f. sp. tritici infection sites. Both the moderately susceptible and the most resistant wheat cultivar were protected against B. graminis f. sp. tritici infection by F. mosseae inoculation at P/5, although the underlying mechanisms appear rather different between the two cultivars. This study emphasizes the importance of taking into account the considered parameters when considering the use of AMF as biocontrol agents.

Side effects of the sterol biosynthesis inhibitor fungicide, propiconazole, on a beneficial arbuscular mycorrhizal fungus.

The Sterol Biosynthesis Inhibitor (SBI) fungicide, propiconazole, is extensively used in modern agriculture to control fungal diseases. Unfortunately, little is known about its potential side effects on non-target plant-beneficial soil organisms such as arbuscular mycorrhizal fungi (AMF). The direct impact of increasing propiconazole concentrations (0.02; 0.2 and 2 mg x L(-1)) on the lipid metabolism of the AMF Glomus irregulare in relation with its development, was studied by using axenic cultures. The propiconazole impact on G. irregulare was investigated, firstly, through sterol (the target-metabolism of SBI fungicides), phospholipids (PL) and their associated fatty acids (PLFA) analysis (the main membrane components) and secondly by measuring malondialdehyde (MDA) (a biomarker of lipid peroxidation) formation. Finally, the storage lipid quantity, triacylglycerol (TAG), was quantified. Our results demonstrated that the drastic reduction of G. irregulare development (germination, germ tube elongation, colonization, extraradical hyphae growth and sporulation) could be explained not only by the decreases of the total sterol end-products (24-methylcholesterol and 24-ethylcholesterol) and by 24-methylene dihydrolanosterol (a sterol precursor) accumulation, suggesting an inhibition of a key enzyme in sterol biosynthesis pathway (14alpha-demethylase), but also by the increases in phosphatidylcholine (PC) and PLFA (C16:0; C18:0 and C18:3) quantities as well as by MDA accumulation. Moreover, TAG quantity was found to be reduced in the presence of propiconazole, suggesting their use by G. irregulare in a response to propiconazole toxicity. In conclusion, taken together, the findings of the current study highlighted a relationship between the SBI fungicide toxicity against the beneficial AMF G. irregulare and (1) the disturbance in the sterol metabolism, (2) the membrane alteration (PC decrease, lipid peroxidation) as well as (3) the reduction in storage lipids, TAG. More generally, this work could contribute to investigate the toxicity of agricultural chemicals on AMF and underlined the emergency of using sustainable alternative method to control plant diseases. Furthermore, these data can provide a useful approach in soil ecotoxicology studies and risk assessment.

Fenpropimorph slows down the sterol pathway and the development of the arbuscular mycorrhizal fungus Glomus intraradices.

The direct impact of fenpropimorph on the sterol biosynthesis pathway of Glomus intraradices when extraradical mycelia alone are in contact with the fungicide was investigated using monoxenic cultures. Bi-compartmental Petri plates allowed culture of mycorrhizal chicory roots in a compartment without fenpropimorph and exposure of extraradical hyphae to the presence of increasing concentrations of fenpropimorph (0, 0.02, 0.2, 2, 20 mg l(-1)). In the fungal compartment, sporulation, hyphal growth, and fungal biomass were already reduced at the lowest fungicide concentration. A decrease in total sterols, in addition to an increase in the amount of squalene and no accumulation of abnormal sterols, suggests that the sterol pathway is severely slowed down or that squalene epoxidase was inhibited by fenpropimorph in G. intraradices. In the root compartment, neither extraradical and intraradical development of the arbuscular mycorrhizal (AM) fungus nor root growth was affected when they were not in direct contact with the fungicide; only hyphal length was significantly affected at 2 mg l(-1) of fenpropimorph. Our results clearly demonstrate a direct impact of fenpropimorph on the AM fungus by a perturbation of its sterol metabolism.

Effects of anthracene on development of an arbuscular mycorrhizal fungus and contribution of the symbiotic association to pollutant dissipation.

The influence of anthracene, a low molecular weight polycyclic aromatic hydrocarbon (PAH), on chicory root colonization by Glomus intraradices and the effect of the root colonization on PAH degradation were investigated in vitro. The fungus presented a reduced development of extraradical mycelium and a decrease in sporulation, root colonization, and spore germination when exposed to anthracene. Mycorrhization improved the growth of the roots in the medium supplemented containing 140 mg l(-1) anthracene, suggesting a positive contribution of G. intraradices to the PAH tolerance of roots. Anthracene disappearance from the culture medium was quantified; results suggested that nonmycorrhizal chicory roots growing in vitro were able to contribute to anthracene dissipation, and in addition, that mycorrhization significantly enhanced anthracene dissipation. These monoxenic experiments demonstrated a positive contribution of the symbiotic association to anthracene dissipation in the absence of other microorganisms. In addition to anthracene dissipation, intracellular accumulation of anthracene was detected in lipid bodies of plant cells and fungal hyphae, indicating intracellular storage capacity of the pollutant by the roots and the mycorrhizal fungus.

Trehalose induces resistance to powdery mildew in wheat.

• Reduction in the degree of powdery mildew infection of wheat leaves is observed after treatments with trehalose, a nonreducing disaccharide commonly found in a wide variety of organisms, including fungi. • Wheat (Triticum aestivum) cv. Sideral plants grown in phytotrons were inoculated with Blumeria graminis f.sp. tritici. In addition to degree of infection, the effect of trehalose solution was further investigated using light and fluorescence microscopy and enzyme assays. • Infection in wheat leaves was reduced by 50 and 95% with trehalose solution (15 g l-1 ) following a single spraying and three sprayings, respectively; in a detached leaf assay, trehalose was effective at concentrations as low as 0.01 g l-1 . Trehalose did not inhibit conidial germination and differentiation of appressoria (in vitro or on the leaf epidermis), but enhanced papilla deposition in epidermal cells. Trehalose also enhanced phenylalanine ammonia-lyase (PAL) and peroxidase (PO) activities; both markers of plant defence responses. However, the level of three cinnamyl alcohol dehydrogenase (CAD) activities (conyferyl, p-coumaryl and sinapyl alcohol dehydrogenase) was unchanged. • Trehalose treatment of wheat confers resistance to B. graminis infection by activating plant defence responses (e.g. papilla deposition, PAL and PO activities).