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Resistance to fludioxonil in Botrytis cinerea isolates from blackberry and strawberry
Abstract:
Site-specific fungicides, including the phenylpyrrole fludioxonil, are frequently used for gray mold control but are at risk for the development of resistance. In this study, field isolates that were low-resistant (LR) and moderately resistant (MR) to fludioxonil from blackberry and strawberry fields of North Carolina, South Carolina, and Virginia were characterized. Genes involved in osmoregulation, including bcsak1, BcOS4, bos5, and BRRG-1, were cloned and sequenced to detect potential target gene alterations; however, none were found. A previously described mutation (R632I) in transcription factor Mrr1, which is known to increase the expression of ATP-binding cassette transporter AtrB, was found in MR but not in sensitive (S) or LR isolates. Expression of atrB in MR isolates was ≈200-fold increased compared with an S isolate; however, 30- to 100-fold overexpression was also detected in LR isolates. Both MR isolates exhibited increased sensitivity to salt stress in the form of mycelial growth inhibition at 4% NaCl, indicating a disruption of osmoregulatory processes in those strains. However, the glycerol content was indistinguishable between S, LR, and MR isolates with and without exposure to fludioxonil, suggesting that the glycerol synthesis pathway may not be a part of the resistance mechanism in LR or MR strains. An investigation into the origin of LR and MR isolates from blackberry revealed two insertions in the mrr1 gene consistent with those found in the Botrytis clade group S. The emergence of strains overexpressing atrB in European and now in North American strawberry fields underscores the importance of this resistance mechanism for development of resistance to fludioxonil in Botrytis cinerea.
Insights
Fungicide resistance in Botrytis cinerea is increasing. Studies show mutations in the Mrr1 transcription factor lead to increased atrB expression, contributing to fludioxonil resistance in strawberry and blackberry pathogens.
Area of Science:
- Plant Pathology
- Fungal Genetics
- Agricultural Science
Background:
- Fludioxonil is a key fungicide for controlling gray mold (Botrytis cinerea).
- Development of resistance to fludioxonil poses a significant threat to crop protection.
- Understanding resistance mechanisms is crucial for sustainable disease management.
Purpose of the Study:
- To characterize field isolates of Botrytis cinerea with varying resistance levels to fludioxonil.
- To investigate the genetic basis of fludioxonil resistance, focusing on osmoregulation and transporter genes.
- To identify mutations and gene expression changes associated with resistance in blackberry and strawberry pathogens.
Main Methods:
- Cloning and sequencing of osmoregulation genes (bcsak1, BcOS4, bos5, BRRG-1).
- Mutation analysis of the Mrr1 transcription factor.
- Quantitative analysis of atrB gene expression.
- Assessment of mycelial growth inhibition under salt stress.
- Measurement of intracellular glycerol content.
Main Results:
- No alterations were found in the targeted osmoregulation genes.
- A R632I mutation in Mrr1 was identified in moderately resistant (MR) isolates, correlating with increased atrB expression (≈200-fold).
- Low-resistant (LR) isolates also showed significant atrB overexpression (30-100-fold) without the R632I mutation.
- MR isolates displayed increased sensitivity to salt stress, indicating disrupted osmoregulation.
- Glycerol content remained unchanged across sensitive, LR, and MR isolates, suggesting it's not a key resistance factor.
Conclusions:
- The overexpression of the atrB gene, driven by mutations in Mrr1 or other mechanisms, is a significant factor in fludioxonil resistance in Botrytis cinerea.
- Disruption of osmoregulatory processes is linked to moderate resistance.
- The emergence of atrB-overexpressing strains in North America highlights the global importance of this resistance mechanism for fludioxonil.
- Further research into resistance evolution and management strategies is warranted.
