Abstract
Phenylpyrroles are chemical analogs of the natural antifungal compound pyrrolnitrin. Fenpiclonil, but mainly fludioxonil are registered against multiple fungal crop diseases since over 25 years for seed or foliar treatment. They have severe physiological impacts on the pathogen, including membrane hyperpolarization, changes in carbon metabolism and the accumulation of metabolites leading to hyphal swelling and burst. The selection and characterization of mutants resistant to phenylpyrroles have revealed that these fungicides activate the fungal osmotic signal transduction pathway through their perception by a typical fungal hybrid histidine kinase (HHK). The HHK is prone to point mutations that confer fungicide resistance and affect its sensor domain, composed of tandem repeats of HAMP motifs. Fludioxonil resistant mutants have been selected in many fungal species under laboratory conditions. Generally they present severe impacts on fitness parameters. Since only few cases of field resistance specific to phenylpyrroles have been reported one may suspect that the fitness penalty of phenylpyrrole resistance is the reason for the lack of field resistance.
Highlights
Specialty section: This article was submitted to Antimicrobials, Resistance and Chemotherapy, a section of the journal Frontiers in Microbiology
multidrug resistant (MDR) does not reach resistance levels sufficient to alter field efficacy of fungicides at their registered field rates, the MDR1h phenotype of B. cinerea group S strains leads to the highest resistance levels to fludioxonil reported for field isolates (Leroch et al, 2013) – besides the specific resistance reported from China (Ren et al, 2016) – and impacts fludioxonil efficacy at least in in vitro assays (Rupp et al, 2016)
All characterized resistance mutations have been mapped to class III hybrid histidine kinase (HHK) genes, the corresponding protein has never been demonstrated as phenylpyrrole target
Summary
Phenylpyrroles are chemical derivatives of pyrrolnitrin, a secondary metabolite produced by some bacteria from tryptophan (Floss et al, 1971) It was isolated for the first time from Pseudomonas pyrrocinia in the 1960s (Arima et al, 1965) and showed strong antifungal activity against various animal and plant pathogenic fungi even under greenhouse conditions. Fenpiclonil and fludioxonil are 3-cyano-4-phenylpyrrol analogs of pyrrolnitrin with largely increased photo-stability and similar antifungal activity (reviewed in Corran et al, 2008). These compounds differ by the substitutions at positions 2 and 3 of the phenyl ring (Figure 1). In the presence of fenpiclonil, [14C]-2-deoxyglucose accumulated intracellularly, while the intracellular concentration of [14C]-2-deoxyglucose-phosphate diminished indicating the inhibition of hexokinase activity during exposure of the mycelium to the phenylpyrrole. Either phenylpyrroles do not inhibit PK-III by itself, acting rather indirectly, or they may affect different cellular targets
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