Strobilurin A

The ecotoxicological effects of pyraoxystrobin, a novel strobilurin fungicide, were studied using outdoor freshwater microcosms and the species sensitivity distribution approach. The microcosms were treated with pyraoxystrobin at concentrations of 0, 1.0, 3.0, 10, 30 and 100µg/L. Species sensitivity distribution (SSD) curves were constructed by means of acute toxicity data using the BurrliOZ model for fourteen representatives of sensitive invertebrates, algae and fish and eleven taxa of invertebrates and algae, respectively. The responses of zooplankton, phytoplankton and physical and chemical endpoints in microcosms were studied. Zooplankton, especially Sinodiaptomus sarsi was the most sensitive to pyraoxystrobin exposure in the microcosms. Short-term toxic effects (281 days) for S. sarsi and the zooplankton community. Based on the results obtained from the organisms in the microcosm system, 1µg/L was recommended as the NOEAEC (no observed ecologically adverse effect concentration). Also, 0.33µg/L was derived as the Regulatory Acceptable Concentration based on the ecological recovery option (ERO-RAC) of pyraoxystrobin. For all fourteen tested species, the median HC5 (hazardous concentration affecting 5% of species) was 0.86µg/L, and the lower limit HC5 (LL-HC5) was 0.39µg/L. For the eleven taxa of invertebrates and algae tested, the median HC5 was 1.1µg/L, and the LL-HC5 was 0.26µg/L. The present study positively contributes to the suggestion of adequately using acute L(E)C50-based HC5/ LL-HC5 for deriving protective concentrations for strobilurin fungicides, and it should be valuable for full comprehension of the potential toxicity of pyraoxystrobin in aquatic ecosystems.

Strobilurins are potent antifungal polyketides produced by basidiomycete fungi. Two genes encoding O-methyltransferases (O-MeT) are present in the biosynthetic gene cluster of strobilurin A 1. In previous studies, the two O-MeT enzymes Str2 and Str3 were found to catalyse the final steps of the biosynthesis of 1. Here, we show by in vivo expression experiments, that O-methylation during strobilurin biosynthesis is regiospecific. O-MeT Str2 acts first and selectively catalyses the methylation of the carboxyl group of strobilurin and bolineol precursors. Str3 catalyses the subsequent methyl transfer to the enol group to form strobilurin A 1, but cannot methylate bolineol 4. Toxicity tests showed increasing antifungal activity of intermediates through the pathway and that bolineol 4 shows antifungal activity against A. oryzae NSAR1 with an MIC of 0.1 mg ml-1.

Strobilurin fungicides (SFs), a class of new fungicides, use strobilurin A as a lead compound. However, with excessive production and usage, the SF residues in soil and aquatic ecosystems may lead to environmental pollution. The mechanism of action (MOA) of SFs is respiratory inhibition of fungal mitochondria. Specifically, azoxystrobin (AZO), pyraclostrobin (PYR), trifloxystrobin (TRI), fluoxastrobin (FLUO), picoxystrobin (PICO), and kresoxim-methyl (KRE) are considered the most widely used SFs. The toxicities of those six fungicides in the environment are still unclear. The present review summarized the toxicities of the six SFs to terrestrial and aquatic biota, including mice, amphibians, aquatic organisms (fish, daphnia, algae, etc.), apoidea, soil animals (earthworms and Folsomia fimetaria), and soil microorganisms. We also review the residue, fate, and transportation of SFs. The results indicate that SFs are highly toxic to aquatic and soil organisms and pose potential risks to ecosystems. Current toxicology studies are more focused on acute or chronic toxicity, but the underlying mechanisms are still unclear and require further analysis. In addition, a simple and scientific analysis method is needed to compare the toxicity differences of different SFs to the same test organisms or differences in the same SFs to different test organisms.