A. thaliana root cortex
3.4 The V. dahliae oleate ∆12-fatty acid desaturase Ode1 promotes
3.4.4 V. dahliae ODE1 is dispensable for induction of disease symptoms in tomato
The ODE1 deletion strain was less impaired in growth in presence of linoleic acid ex planta. Also plants synthesize linoleic acid as an important precursor of defense-related compounds like plant oxylipins and cutin monomers and, therefore, might substitute the defect in fungal linoleic acid biosynthesis (Soliday & Kolattukudy, 1977; Brodhun &
Feussner, 2011). During tomato plant infection experiments the impact of the ODE1 deletion on virulence of V. dahliae was investigated.
Tomato plants inoculated with spores obtained from the ∆ODE1 strain displayed overall similar stunting and hypocotyl discolorations to wild type-infected plants with only slightly reduced numbers of plants with disease symptoms after 21 days. About 70% of the tomato plants inoculated with ΔODE1 spores displayed disease symptoms in comparison to 87% of the wild type treated plants (Figure 28).
In summary, the ODE1 deletion strain displayed generally impaired vegetative growth, correlated with decreased microsclerotia production and allows almost wild type-like induction of disease symptoms. Hardly any decrease in induced disease symptoms was observed in plants inoculated with spores obtained from the ODE1 deletion strain suggesting a minor or no role of the single gene ODE1 in virulence of V. dahliae. This virulent in planta phenotype of a strain with a severe growth defect might result from substitution of the fungal ODE1 product by linoleic acid provided in plant cells.
Figure 28: V. dahliae ODE1 is not required for induction of severe disease symptoms in S. lycopersicum. Ten-day-old tomato plants were inoculated by root dipping into water control (mock) or spores obtained from wild type JR2 (WT), ΔODE1 (VGB331, VGB332), or ODE1-GFP complementation (VGB359) strains. Representative plants and overview of 15 plants per genotype from one experiment are shown. Disease symptoms were assessed after 21 days and transferred into disease scores per plant visualized in a stack diagram.
Discoloration of hypocotyl dissections is shown. Data are shown from three independent experiments for wild type and ΔODE1 (VGB331), two independent experiments for a second ΔODE1 transformant (VGB332), and a single experiment for ODE1-GFP. Scale bar = 1 mm, n = total number of evaluated plants per genotype.
4 D ISCUSSION
The outcome of a host-fungus interaction can be fungal colonization due to plant susceptibility or fungal death combined with plant resistance. Between these two extremes several intermediate outcomes of disease severity can be observed. Several factors affect this relationship between a pathogen and a host, including environmental factors, nutritional conditions, age, and genome variations of both, plant and fungus.
Fungal signaling genes were in the focus of this study. The aim was a better understanding of the complex interplay of fungal signaling cascades favoring beneficial or detrimental outcomes in interactions with host plants.
It was shown, that pathogenic V. longisporum isolates possess genomic insertions which contribute to attenuation of disease symptoms induced in the interplay with rapeseed plants. These regions can be either species- or host-specific.
Virulence of V. dahliae is mediated by MAPK signaling pathways, but the role of scaffold proteins in insulation of these pathways was yet unstudied in plant pathogens. Ham5 is a MAPK scaffold in different ascomycetes. Here it was shown, that MAPK signaling pathways in V. dahliae mediate differentiation, stress response, and virulence independent from this scaffold protein.
Plant invasion and suppression of the plant defense responses rely on the secretion of certain tools which can be regulated by the UPR pathway. Therefore, the V. dahliae bZIP transcription factor Hac1 and its role in virulence was characterized in this study. The UPR regulator has conserved as well as species-specific impacts on differentiation of V. dahliae and is an important factor for virulence.
Virulence can be connected to fungal differentiation. Important regulators of fungal differentiation are lipid metabolites which can act as signaling molecules.
Characterization of the fungal oleate ∆12-fatty acid desaturase Ode1, catalyzing the synthesis of linoleic acid, revealed an important contribution to fungal growth with only a minor impact on the induction of disease symptoms.
Figure 29 represents an overview about the findings of this thesis, which will be discussed in detail in the following.
Discussion
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V. longisporum isolates with their plant hosts studied in this thesis.