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Resistance in Monilinia fructicola to demethylation inhibitor (DMI) fungicides is beginning to emerge in North America, but its molecular basis is unknown. Two potential genetic determinants of DMI fungicide resistance including the 14alpha-demethylase gene (MfCYP51) and the ATP-binding cassette transporter gene MfABC1, were investigated in six resistant (DMI-R) and six sensitive (DMI-S) field isolates. No point mutations leading to an amino acid change were found in the MfCYP51 gene. The constitutive expression of the MfCYP51 gene in DMI-R isolates was significantly higher compared to DMI-S isolates. Gene expression was not induced in mycelium of DMI-R or DMI-S isolates treated with 0.3 mug of propiconazole/ml. A slightly higher average MfCYP51 copy number value was detected in DMI-R isolates (1.35) compared to DMI-S isolates (1.13); however, this difference could not be verified in Southern hybridization experiments or explain the up to 11-fold-increased MfCYP51 mRNA levels in DMI-R isolates. Analysis of the upstream nucleotide sequence of the MfCYP51 gene revealed a unique 65-bp repetitive element at base pair position -117 from the translational start site in DMI-R isolates but not in DMI-S isolates. This repetitive element contained a putative promoter and was named Mona. The link between Mona and the DMI resistance phenotype became even more apparent after studying the genetic diversity between the isolates. In contrast to DMI-S isolates, DMI-R isolates contained an MfCYP51 gene of identical nucleotide sequence associated with Mona. Still, DMI-R isolates were not genetically identical as revealed by Microsatellite-PCR analysis. Also, real-time PCR analysis of genomic DNA indicated that the relative copy number of Mona among DMI-S and DMI-R isolates varied, suggesting its potential for mobility. Interestingly, constitutive expression of the MfABC1 gene in DMI-R isolates was slightly lower than that of DMI-S isolates, but expression of the MfABC1 gene in DMI-R isolates was induced in mycelium after propiconazole treatment. Therefore, the MfABC1 gene may play a minor role in DMI fungicide resistance in M. fructicola. Our results strongly suggest that overexpression of the MfCYP51 gene is an important mechanism in conferring DMI fungicide resistance in M. fructicola field isolates from Georgia and that this overexpression is correlated with Mona located upstream of the MfCYP51 gene.
Luo et al. (Sat,) studied this question.