The plant-pathogenic fungus Mycosphaerella graminicola (asexual stage: Septoria tritici) causes septoria tritici blotch, a disease that greatly reduces the yield and quality of wheat. This disease is economically important in most wheat-growing areas worldwide and threatens global food production. Control of the disease has been hampered by a limited understanding of the genetic and biochemical bases of pathogenicity, including mechanisms of infection and of resistance in the host. Unlike most other plant pathogens, M. graminicola has a long latent period during which it evades host defenses. Although this type of stealth pathogenicity occurs commonly in Mycosphaerella and other Dothideomycetes, the largest class of plant-pathogenic fungi, its genetic basis is not known. To address this problem, the genome of M. graminicola was sequenced completely. The finished genome contains 21 chromosomes, eight of which could be lost with no visible effect on the fungus and thus are dispensable. This eight-chromosome dispensome is dynamic in field and progeny isolates, is different from the core genome in gene and repeat content, and appears to have originated by ancient horizontal transfer from an unknown donor. Synteny plots of the M. graminicola chromosomes versus those of the only other sequenced Dothideomycete, Stagonospora nodorum, revealed conservation of gene content but not order or orientation, suggesting a high rate of intra-chromosomal rearrangement in one or both species. This observed “mesosynteny” is very different from synteny seen between other organisms. A surprising feature of the M. graminicola genome compared to other sequenced plant pathogens was that it contained very few genes for enzymes that break down plant cell walls, which was more similar to endophytes than to pathogens. The stealth pathogenesis of M. graminicola probably involves degradation of proteins rather than carbohydrates to evade host defenses during the biotrophic stage of infection and may have evolved from endophytic ancestors.

Drug transporters are membrane proteins that provide protection for organisms against natural toxic products and fungicides. In plant pathogens, drug transporters function in baseline sensitivity to fungicides, multidrug resistance (MDR) and virulence on host plants. This paper describes drug transporters of the filamentous fungi Aspergillus nidulans (Eidam) Winter, Botrytis cinerea Pers and Mycosphaerella graminicola (Fückel) Schroter that function in fungicide sensitivity and resistance. The fungi possess ATP-binding cassette (ABC) drug transporters that mediate MDR to fungicides in laboratory mutants. Similar mutants are not pronounced in field resistance to most classes of fungicide but may play a role in resistance to azoles. MDR may also explain historical cases of resistance to aromatic hydrocarbon fungicides and dodine. In clinical situations, MDR development in Candida albicans (Robin) Berkhout mediated by ABC transporters in patients suffering from candidiasis is common after prolonged treatment with azoles. Factors that can explain this striking difference between agricultural and clinical situations are discussed. Attention is also paid to the risk of MDR development in plant pathogens in the future. Finally, the paper describes the impact of fungal drug transporters on drug discovery.

No abstract

The dimorphic ascomycete pathogen Mycosphaerella graminicola switches from a yeastlike form to an infectious filamentous form that penetrates the host foliage through stomata. We examined the biological function of the mitogen-activated protein kinase-encoding gene MgHog1 in M. graminicola. Interestingly, MgHog1 mutants were unable to switch to filamentous growth on water agar that mimics the nutritionally poor conditions on the foliar surface and, hence, exclusively developed by a yeastlike budding process. Consequently, due to impaired initiation of infectious germ tubes, as revealed by detailed in planta cytological analyses, the MgHog1 mutants failed to infect wheat leaves. We, therefore, conclude that MgHog1 is a new pathogenicity factor involved in the regulation of dimorphism in M. graminicola. Furthermore, MgHog1 mutants are osmosensitive, resistant to phenylpyrrole and dicarboximide fungicides, and do not melanize.

Agrobacterium tumefaciens-mediated transformation has been successfully applied to the wheat pathogen Mycosphaerella graminicola. Both protoplasts and intact cells have been transformed to hygromycin B resistance. Furthermore, A. tumefaciens-mediated transformation using homologous DNA originating from the M. graminicola ABC transporter gene MgAtr2 resulted in the efficient generation of disruption mutants. In 44% of the transformants, disruption of MgAtr2 was achieved and transformants resulted from the integration of a single copy of the transforming DNA. These results indicate that A. tumefaciens-mediated transformation is a useful tool to generate targeted gene disruption in the phytopathogen M. graminicola, where gene targeting by conventional methods is hardly possible.