The arbuscular mycorrhizal Rhizophagus irregularis activates storage lipid biosynthesis to cope with the benzo[a]pyrene oxidative stress

The phytoremediation assisted by arbuscular mycorrhizal fungi (AMF) could constitute an ecological and economic method to restore polycyclic aromatic hydrocarbon (PAH) polluted soils. Unfortunately, little is known about the PAH impact on the beneficial symbiotic AMF. Using radiolabelling experiments, our work aims to understand how benzo[a]pyrene (B[a]P), a representative of high molecular weight PAH, acts on the AMF lipid metabolism. Our results showed decreases in the sterol precursors as well as in total phospholipid quantities, in link with the [1-(14)C]acetate incorporation decreases in these lipids. Interestingly, a concomitant increase of [1-(14)C]acetate incorporation by 29.5% into phosphatidylcholine with its content decrease in Rhizophagus irregularis extraradical mycelium was observed, suggesting a membrane regeneration. A second concomitant increase (estimated to 69%) of [1-(14)C]acetate incorporation into triacylglycerols (TAG) with the content decrease was also observed. This suggests a fungal TAG biosynthesis activation probably to offset the decrease in storage lipid content when the fungus was grown under B[a]P pollution. In addition, our findings showed that lipase activity was induced by more than 3 fold in the presence of B[a]P in comparison to the control indicating that the drop in TAG content could be a consequence of their active degradation. Taken together, our data suggest the involvement of the fungal TAG metabolism to cope B[a]P toxicity through two means: (i) by providing carbon skeletons and energy necessary for membrane regeneration and/or for B[a]P translocation and degradation as well as (ii) by activating the phosphatidic acid and hexose metabolisms which may be involved in cellular stress defence.

Role of arbuscular mycorrhizal symbiosis in root mineral uptake under CaCO3 stress

This study investigated the effects of increasing CaCO(3) concentrations (0, 5, 10, 20 mM) on arbuscular mycorrhizal (AM) symbiosis establishment as well as on chicory root growth and mineral nutrient uptake in a monoxenic system. Although CaCO(3) treatments significantly decreased root growth and altered the symbiosis-related development steps of the AM fungus Rhizophagus irregularis (germination, germination hypha elongation, root colonization rate, extraradical hyphal development, sporulation), the fungus was able to completely fulfill its life cycle. Even when root growth decreased more drastically in mycorrhizal roots than in non-mycorrhizal ones in the presence of high CaCO(3) levels, the AM symbiosis was found to be beneficial for root mineral uptake. Significant increases in P, N, Fe, Zn and Cu concentrations were recorded in the mycorrhizal roots. Whereas acid and alkaline phosphatase enzymatic activities remained constant in mycorrhizal roots, they were affected in non-mycorrhizal roots grown in the presence of CaCO(3) when compared with the control.

Propiconazole inhibits the sterol 14α-demethylase in Glomus irregulare like in phytopathogenic fungi

The increasing concentrations impact (0.02, 0.2 and 2 mg L(-1)) of a Sterol Biosynthesis Inhibitor (SBI) fungicide, propiconazole, was evaluated on development and sterol metabolism of two non-target organisms: mycorrhizal or non-mycorrhizal transformed chicory roots and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare using monoxenic cultures. In this work, we provide the first evidence of a direct impact of propiconazole on the AMF by disturbing its sterol metabolism. A significant decrease in end-products sterols contents (24-methylcholesterol and in 24-ethylcholesterol) was observed concomitantly to a 24-methylenedihydrolanosterol accumulation indicating the inhibition of a key enzyme in sterol biosynthesis pathway, the sterol 14α-demethylase like in phytopathogenic fungi. A decrease in end-product sterol contents in propiconazole-treated roots was also observed suggesting a slowing down of the sterol metabolism in plant. Taken together, our findings suggest that the inhibition of the both AM symbiotic partners development by propiconazole results from their sterol metabolism alterations.

Benzo[a]pyrene induced lipid changes in the monoxenic arbuscular mycorrhizal chicory roots

Arbuscular mycorrhizal (AM) colonization may be one of the means that protects plants and allows them to thrive on polycyclic aromatic hydrocarbon-polluted soils including the carcinogenic benzo(a)pyrene (B[a]P). To understand the mechanisms involved in the AM symbiosis tolerance to B[a]P toxicity, the purpose of this study was to compare the lipid compositions as well as the contents between mycorrhizal and non-mycorrhizal chicory root cultures grown in vitro under B[a]P pollution. Firstly, B[a]P induced significant decreases of the Glomalean lipid markers: C16:1ω5 and 24-methyl/methylene sterol amounts in AM roots indicating a reduced AM fungal development inside the roots. Secondly, whereas increases in fatty acid amounts after B[a]P application were measured in non-mycorrhizal roots, no changes were shown in mycorrhizal roots. On the other hand, while, after treatment with B[a]P, the total phospholipid contents were unmodified in non-mycorrhizal roots in comparison with the control, drastic reductions were observed in mycorrhizal roots, mainly owing to decreases in phosphatidylethanolamine and phosphatidylcholine. Moreover, B[a]P affected AM root sterols by reducing stigmasterol. In conclusion, the findings presented in this paper have highlighted, for the first time, significant changes in the AM root lipid metabolism under B[a]P pollution and have culminated on their role in the defense/protection mechanisms.

Calcareous impact on arbuscular mycorrhizal fungus development and on lipid peroxidation in monoxenic roots

The present work underlined the negative effects of increasing CaCO(3) concentrations (5, 10 and 20 mM) both on the chicory root growth and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare development in monoxenic system. CaCO(3) was found to reduce drastically the main stages of G. irregulare life cycle (spore germination, germinative hyphae elongation, root colonization, extraradical hyphae development and sporulation) but not to inhibit it completely. The root colonization drop was confirmed by the decrease in the arbuscular mycorrhizal fungal marker C16:1ω5 amounts in the mycorrhizal chicory roots grown in the presence of CaCO(3). Oxidative damage evaluated by lipid peroxidation increase measured by (i) malondialdehyde (MDA) production and (ii) the antioxidant enzyme peroxidase (POD) activities, was highlighted in chicory roots grown in the presence of CaCO(3). However, MDA formation was significantly higher in non-mycorrhizal roots as compared to mycorrhizal ones. This study pointed out the ability of arbuscular mycorrhizal symbiosis to enhance plant tolerance to high levels of CaCO(3) by preventing lipid peroxidation and so less cell membrane damage.

Influence of fly ash aided phytostabilisation of Pb, Cd and Zn highly contaminated soils on Lolium perenne and Trifolium repens metal transfer and physiological stress

Due to anthropogenic activities, large extends of soils are highly contaminated by Metal Trace Element (MTE). Aided phytostabilisation aims to establish a vegetation cover in order to promote in situ immobilisation of trace elements by combining the use of metal-tolerant plants and inexpensive mineral or organic soil amendments. Eight years after Coal Fly Ash (CFA) soil amendment, MTE bioavailability and uptake by two plants, Lolium perenne and Trifolium repens, were evaluated, as some biological markers reflecting physiological stress. Results showed that the two plant species under study were suitable to reduce the mobility and the availability of these elements. Moreover, the plant growth was better on CFA amended MTE-contaminated soils, and the plant sensitivity to MTE-induced physiological stress, as studied through photosynthetic pigment contents and oxidative damage was lower or similar. In conclusion, these results supported the usefulness of aided phytostabilisation of MTE-highly contaminated soils.

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.

Arbuscular mycorrhiza partially protect chicory roots against oxidative stress induced by two fungicides, fenpropimorph and fenhexamid

The present work examined the oxidative stress induced by different concentrations (0.02 and 0.2 mg l-1) of two sterol biosynthesis inhibitor fungicides (fenpropimorph and fenhexamid) in non-target chicory root colonised or not by Glomus intraradices in a monoxenic system. The fungicides were found to cause oxidative damage by increasing lipid peroxidation measured by malondialdehyde production in non-colonised roots. Detoxification of the H(2)O(2) product was measured at 0.2 mg l-1 of fenpropimorph by an increase in peroxidase activities suggesting an antioxidant capacity in these roots. Moreover, this study pointed out the ability of arbuscular mycorrhiza to alleviate partially the oxidative stress in chicory roots, probably by lowering reactive oxygen species concentrations, resulting from increases in antioxidant defences. Our results suggest that the enhanced fungicide tolerance in the AM symbiosis could be related to less cell membrane damage.

Mycorrhization alleviates benzo[a]pyrene-induced oxidative stress in an in vitro chicory root model

Among chemicals that are widely spread both in terrestrial and aquatic ecosystems, benzo[a]pyrene is a major source of concern. However, little is known about its adverse effects on plants, as well as about the role of mycorrhization in protection of plant grown in benzo[a]pyrene-polluted conditions. Hence, to contribute to a better understanding of the adverse effects of polycyclic aromatic hydrocarbons on the partners of mycorrhizal symbiotic association, benzo[a]pyrene-induced oxidative stress was studied in transformed Cichorium intybus roots grown in vitro and colonized or not by Glomus intraradices. The arbuscular mycorrhizal fungus development (colonization, extraradical hyphae length, and spore formation) was significantly reduced in response to increasing concentrations of benzo[a]pyrene (35-280 microM). The higher length of arbuscular mycorrhizal roots, compared to non-arbuscular mycorrhizal roots following benzo[a]pyrene exposure, pointed out a lower toxicity of benzo[a]pyrene in arbuscular mycorrhizal roots, thereby suggesting protection of the roots by mycorrhization. Accordingly, in benzo[a]pyrene-exposed arbuscular mycorrhizal roots, statistically significant decreases were observed in malondialdehyde concentration and 8-hydroxy-2'-desoxyguanosine formation. The higher superoxide dismutase activity detected in mycorrhizal chicory roots could explain the benzo[a]pyrene tolerance of the colonized roots. Taken together, these results support an essential role of mycorrhizal fungi in protecting plants submitted to polycyclic aromatic hydrocarbon, notably by reducing polycyclic aromatic hydrocarbon-induced oxidative stress damage.