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Influence of the sterol biosynthesis inhibitor fungicides on the arbuscular mycorrhizal symbiosis

Final Activity Report Summary - FUNGIMYC (Influence of the sterol biosynthesis inhibitor fungicides on the arbuscular mycorrhizal symbiosis)

FUNGIMYC aimed to better understand the effects of Sterol biosynthesis inhibitor (SBI) fungicides on the non-target AM fungi. Two SBI fungicides were targeted (fenpropimorph and fenhexamid) and one AM fungi considered (Glomus intraradices). All the experiments were conducted under strict in vitro conditions using the Root organ culture (ROC) system.

The important phytotoxicity of fenpropimorph was clearly demonstrated. This molecule induced a reduction of root growth (Zocco et al., 2008; Campagnac et al., 2008), an induction of an oxidative stress (Campagnac et al., 2009), a decrease of root colonisation and modifications in the sterol profile of roots (Campagnac et al., 2008). Normal sterols which are crucial components of the membrane cells were replaced by unusual compounds (Campagnac et al., 2008). Fenpropimorph also strongly impacted the AM fungal development. Spore production and fungal biomass was decreased, and extraradical mycelium architecture was perturbed (Zocco et al., 2008; Campagnac et al., 2009). The capacity of the AM fungi to take up and translocate 33P to its host plant was also drastically decreased in presence of fenpropimorph (Zocco et al., in preparation). This seemed partially due to a decrease in the mRNA levels of the mycorrhiza-specific phosphate transporter MtPT4 (Zocco et al., in preparation). However, the impact of fenpropimorph was rather fungistatic than fungitoxic (Zocco et al., 2008). Spores incubated on the two SBI were able to germinate after their transplantation on a SBI-free medium (Zocco et al., 2008). The second SBI fungicide studied, fenhexamid, induced a more limited impact on the AM fungi, both in terms of development and function (Zocco et al., 2008; Campagnac et al., 2009).

Molecular studies allowed the identification of a C-4 Sterol methyl oxidase (SMO), the first gene involved in the sterol biosynthetic pathway, described so far from an AM fungi. The gene, called GintSMO, was characterised by means of sequence analyses, expression profiles and yeast complementation assays. In addition, two Sterol methyl transferases (SMT) and one Lanosterol synthase (LAS) encoding genes were identified. Expression studies carried out on the identified genes revealed that the SBI fungicide fenpropimorph had an impact on the sterol biosynthetic pathway of G. intraradices while the SBI fungicide fenhexamid did not exert any effect. The stronger impact of fenpropimorph compared to fenhexamid revealed by gene expression patterns was in line with what has been observed at a morphological level (a stronger negative effects on sporulation and hyphal elongation; Zocco et al., 2008), and at a biochemical level (considerable changes in the sterol profiles; Campagnac E. PhD Thesis). The investigations carried out on fenpropimorph revealed that antifungal effects might not be only due to the interference with sterol synthesis and suggested that reactive oxygen species ROS production and scavenging are involved in the response to SBI.

In conclusion, the results obtained in FUNGIMYC have clearly demonstrated that the SBI fungicide fenpropimorph has an impact on the sterol biosynthetic pathway of G. intraradices while the impact of fenhexamid is less pronounced. Our results also suggested that a normal sterol composition is essential to an optimal development of the symbiosis. A protective effect of mycorrhisation was noticed on the lipid peroxidation of roots after application with both fungicides. Finally, the ROC system appeared a suitable system to compare the impacts of different molecules on AM fungi, and for the initial screening of new candidate molecules before registration (Zocco et al., 2008). All these results not only contribute to improve our knowledge on the toxicity of agricultural chemicals on the AM symbiosis but also demonstrate that the biologic system used is a useful approach for ecotoxicology studies and risk assessment.