Quinone outside Inhibitors (QoIs) are considered as single-site of action fungicides which inhibit the electron transfer in mitochondria by binding to the cytochrome bc1 enzyme complex. In many phytopathogenic fungi a single point mutation in codon 143 of the CYTB gene, encoding cytochrome b, confers complete resistance to QoI fungicides. However, in some fungi neither QoI resistance nor this mutation (G143A), have been detected so far. This was explained by the presence of an intron in the CYTB gene after codon 143: it was predicted that a G143A mutation would prevent the splicing of this intron and, thus, the production of functional cytochrome b proteins.
Guignardia bidwellii, the causal agent of grape black rot, is considered as a CYTB intron-containing species with low risk for the development of QoI resistance. We amplified and cloned CYTB fragments in several G. bidwellii strains, and also in some other Guignardia spp., including authentic strains of G. citricarpa, G. gaultheriae, G. mangiferae and G. aesculi, obtained from CBS, to sequence the intron located after codon 143. Surprisingly, no intron was detected in the predicted position in several G. bidwellii strains isolated from different grape varieties in Hungarian vineyards. Also, the intron was not found in either an authentic G. bidwellii strain obtained from LGC ATCC, or the G. aesculi and a G. gaultheriae strains included this study, while the intron was identified, and sequenced, in all other Guignardia spp. strains examined by us. In vitro fungicide resistance tests did not show a clear correlation between the presence/absence of the intron in Guignardia spp. strains and their sensibility to QoIs. This suggests that other mechanisms may also be involved in their QoI resistance; and some G. bidwellii strains may develop QoI resistance in the field through the G143A mutation.
This work was supported by GINOP-2.3.2-15-2016-00061 project.