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FOR 546:  Analysis of Systemic Effects of Infections of Selected Brassicaceae by Soil-borne Microorganisms Considering Especially Multitrophic Interactions with Insects and Microbial Pathogens

Subject Area Biology
Agriculture, Forestry and Veterinary Medicine
Term from 2004 to 2011
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 5471048
 
Final Report Year 2012

Final Report Abstract

Verticillium longisporum (VL) is an interesting model system for understanding the biology of a vascular pathogen both in its natural host (Brassica napus) and in the model plant Arabidopsis thaliana. The ultimate goal of research unit FOR546 was to understand the interaction between both organisms at the molecular level, both in the roots, where the fungus invades the plant, and the xylem, where the fungus proliferates. Basal methods and tools (growth conditions, infections, quantification of fungal DNA, cDNA-AFLP, metabolomics, transgenic fungi and plants) were established. Interaction-induced responses of the plant and the fungus were described, covering physiological parameters like water status and photosynthesis, anatomical analyses and the identification of interaction-induced proteins, transcripts and metabolites. Arabidopsis thaliana. VL invades unwounded Arabidopsis roots, which in turn synthesize antimicrobial secondary metabolites that derive from tryptophan. The fungus overcomes this layer of defense and colonizes the xylem. Later, proliferation and formation of microsclerotia in the shoot depends on the induction of premature senescence which leads to the mobilization of nutrients and their subsequent transport into the vascular bundle. The water status and the efficiency of photosynthesis are not significantly altered in infected plants demonstrating that plant responses are rather due to regulatory cues than to occlusion of vessels. Induction of senescence requires the jasmonate receptor COI1 in roots. Unexpectedly, this senescencepromoting function of COI1 is independent from any jasmonate-related ligand. Possible mechanisms how these root-borne processes promote senescence are degradation of cytokinins or up-regulation of the ethylene pathway. In addition, VL synthesizes toxins and harmful proteins that might contribute to the leakage of metabolites into the xylem. Fungal infections induce transdifferentiation of bundle sheath and phloem cells into xylem vessels which help the plant to persist drought stress. The formation of more lignified cells is associated with the transcriptional activation of the phenylpropanoid pathway. Although the plant defense hormones salicylic acid and jasmonic acid are induced after fungal infection, the corresponding defense responses are not effective. An anti-fungal Germin-Like Protein (GLP3) accumulates in the apoplast . Mapping studies using the susceptible ecotype Landsberg erecta and the more resistant ecotype Burren pointed at several QTLs and the ERECTA locus to confer resistance. Brassica napus. The xylem sap of B. napus contains antifungal proteins, the phytoalexin cyclobrassinin as well as the defense hormone salicylic acid (SA) and its glucoside (SAG). Preliminary studies indicate that the SA pathway confers some level of resistance, which restricts fungal propagation in the hypocotyl. Moreover, the xylem sap contains regulatory signals that promote fungal growth. Verticillium longisporum. The fungus has to adapt to the nutrient-poor and anti-fungal conditions in the xylem. In planta up-regulation of anti-oxidative enzymes, regulators of amino acid synthesis, necrosis and ethylene-inducing peptides (NEPs) have been shown to support fungal performance in planta. Moreover, other proteins of unknown function were detected. Candidate regulatory factors which might control fungal adhesion to the plant surface were identified.

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