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The ABC Transporter MgAtr4 Is a Virulence Factor of Mycosphaerella graminicola that Affects Colonization of Substomatal Cavities in Wheat Leaves

August 2003 , Volume 16 , Number  8
Pages  689 - 698

Ioannis Stergiopoulos , Lute-Harm Zwiers , and Maarten A. De Waard

Laboratory of Phytopathology, Department of Plant Sciences, Wageningen University, Binnenhaven 5, 6709 PD, P.O. Box 8025, 6700 EE Wageningen, The Netherlands

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Accepted 11 April 2003.

The role in virulence of the ATP-binding cassette (ABC) transporters MgAtr1, MgAtr2, MgAtr3, MgAtr4, and MgAtr5 from Mycosphaerella graminicola was analyzed by gene disruption or replacement on seedlings of the susceptible wheat cultivar Obelisk. Disruption strains of MgAtr1 and MgAtr2 and replacement strains of MgAtr3 and MgAtr5 displayed the same phenotype as control strains, while virulence of the MgAtr4 disruption strains was significantly reduced. This reduction in virulence was independent of the wheat cultivar used. Histopathological analysis of the infection process revealed that MgAtr4 disruption strains colonize substomatal cavities less efficiently and display reduced intercellular growth in the apoplast of wheat leaves. In vitro growth experiments in different media showed no fitness penalty associated with the disruption of MgAtr4. Expression analysis demonstrated that transcripts of the constitutively expressed gene CYP51 encoding the fungal-specific cytochrome P450 sterol 14α-demethylase from M. graminicola were not detectable in interaction RNA from wheat infected with MgAtr4 disruption strains, thus confirming the reduced intercellular growth of these strains. The results indicate that MgAtr4 is a virulence factor of M. graminicola during pathogenesis on wheat and may function in protection against fungitoxic compounds present around the substomatal cavities of wheat leaves. MgAtr4 is the first virulence factor cloned from this important plant pathogen.

Additional keywords: compatible interaction, incompatible interaction, pathogenicity, plant defense compounds, Septoria tritici, toxins.

© 2003 The American Phytopathological Society