Faculté des sciences

Above- and belowground antifungal resistance in maize : aspects of organ-specific defense

Balmer, Dirk ; Mauch-Mani, Brigitte (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2013.

The hemibiotrophic fungus Colletotrichum graminicola causes devastating anthracnose on maize (Zea mays) and is responsible for annual losses of up to 1 billion dollars in the U.S. A key factor for its success is the capability to infect different plant organs. The predominant symptoms are leaf blight and stalk rot, but C. graminicola also infects roots. The vast majority of... Plus

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    Summary
    The hemibiotrophic fungus Colletotrichum graminicola causes devastating anthracnose on maize (Zea mays) and is responsible for annual losses of up to 1 billion dollars in the U.S. A key factor for its success is the capability to infect different plant organs. The predominant symptoms are leaf blight and stalk rot, but C. graminicola also infects roots. The vast majority of phytopathological studies were conducted on aerial disease stages, and only little is known about belowground defense responses. Moreover, most studies on antifungal resistance focus on either above- or belowground immune systems. Thus, this thesis investigated the local and systemic organ-specific interactions of maize and C. graminicola.
    Firstly, a soil-free plant growth system was developed, allowing non-destructive in vivo observations of C. graminicola infection strategies on maize roots. This system consists of pouches containing nutrient-soaked filter paperwhich supplies the plants with nutrients adapted to the host.
    Secondly, local and systemic molecular and chemical changes upon C. graminicola attack on maize leaves and roots were investigated. Distinct gene expression patterns in leaves and roots were found, in agreement with different dynamics of phytohormone induction. In roots defense-related genes were induced faster than in leaves, and roots also exhibited higher hormone levels upon infection. Local leaf and root infections triggered leaf-leaf and root-leaf systemic transcriptional and hormonal adaptations, including the induction of defense-related genes and hormones. Interestingly, local leaf and root infection also resulted in a systemic resistance against C. graminicola in distal leaves. Performing metabolomic fingerprinting, several local and systemic organ-specific compounds were identified, which could serve as chemical arsenal during antifungal immunity in maize.
    Thirdly, the organ-specific microRNA (miRNA) transcriptome of maize during C. graminicola infection was examined. Several miRNAs were identified which are specifically induced or downregulated in fungal infected shoots or roots, but not upon challenge with the herbivore Spodoptera frugiperda. Some of those miRNAs target defense-related genes, thus miRNAs might play an important role in organ-specific antifungal defense.
    In conclusion, exploiting organ-specific plant defense might be a prominent target for future crop enhancing programs.