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.