Abstract
Leveillula taurica is a major pathogen of tomato and several other crops that can cause substantial yield losses in favorable conditions for the fungus. Quinone outside inhibitor fungicides (QoIs) are routinely used for the control of the pathogen in tomato fields across California, but their recurrent use could lead to the emergence of resistance against these compounds. Here, we partially cloned the cytochrome b gene from L. taurica (Lt cytb) and searched within populations of the fungus collected from tomato fields across California for mutations that confer resistance to QoIs. A total of 21 single nucleotide polymorphisms (SNPs) were identified within a 704 bp fragment of the Lt cytb gene analyzed, of which five were non-synonymous substitutions. Among the most frequent SNPs encountered within field populations of the pathogen was the G143A substitution that confers high levels of resistance against QoIs in several fungi. The other four amino acid substitutions were novel mutations, whose effect on QoI resistance is currently unknown. Sequencing of the Lt cytb gene from individual single-cell conidia of the fungus further revealed that most SNPs, including the one leading to the G143A substitution, were present in a heteroplasmic state, indicating the co-existence of multiple mitotypes in single cells. Analysis of the field samples showed that the G143A substitution is predominantly heteroplasmic also within field populations of L. taurica in California, suggesting that QoI resistance in this fungus is likely to be quantitative rather than qualitative.
Highlights
Powdery mildews are obligate biotrophic fungi that cause extensive damage in a wide range of crops (Ridout, 2009)
When translated with the yeast mitochondrial genetic code, the 95 amino acid long product spanned the C-terminal region of transmembrane helix C and the surface cd1 loop region of cytb that contain three of the residues associated with resistance to Quinone outside inhibitor fungicides (QoIs) in fungal plant pathogens, i.e., F129, G137, and G143 (Gisi et al, 2002; Fisher and Meunier, 2008)
The sequencing analysis identified several polymorphisms within the L. taurica cytb Gene (Lt cytb) gene fragment analyzed, indicating a high mutation rate. This is perhaps not surprising as mtDNA is believed to exhibit higher mutation rates compared to nuclear DNA, mainly as a result of less efficient DNA repair mechanisms and a more mutagenic intracellular environment that is created by the production of free radicals during mitochondrial respiration, such as reactive oxygen intermediates (ROIs) (Palmer et al, 2000; Gillooly et al, 2005; Lynch et al, 2006; Haag-Liautard et al, 2008)
Summary
Powdery mildews are obligate biotrophic fungi that cause extensive damage in a wide range of crops (Ridout, 2009). The fungus mainly infects the leaves, where it enters through the stomata and grows predominately endophytically, penetrating deeply into the mesophyll of the leaf tissue (Thomson and Jones, 1981; Palti, 1988). This is a fundamental difference between L. taurica and the other powdery mildew species infecting tomato that are primarily epiphytic (Jones et al, 2001; Kiss et al, 2001). Eventually turn yellow to necrotic and become shriveled, whereas defoliation can occur in severe infections (Correll et al, 1987)
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