One of the most serious fungal diseases of wheat is Septoria tritici Blotch caused by Zymoseptoria tritici. This disease is typified by an asymptomatic latent phase, followed by a rapid onset on host-cell necrosis. There has long been debate about whether the necrosis is due to production of secondary metabolite toxins or other effects of the fungus such as use of small protein effectors thereby providing nutrients for fungal growth. Secondary metabolites are mainly classified into polyketides (PKs), non-ribosomal peptides (NRPs), alkaloids and terpenes. Genome analysis of Z. tritici identified nine PKS and two hybrid PKS NRPS pathways in addition to the NRPS believed to be responsible for siderophore biosynthesis. Promoter: GFP fusions were made for all of these candidate toxin synthase genes and their expression followed in vitro and through the disease cycle by confocal microscopy, however only pPKS3 gave any apparent GFP expression in planta. Disruption of PKS3 did not generate any mutant phenotype. In fungi, the phosphopantetheine transferase (PPTase) is needed not only for the PKS and NRPS activation, but also both lysine and siderophore synthesis and its disruption in Z. tritici is reported to have very reduced virulence. The Z. tritici alpha-aminoadipate reductase (Lys2) gene was disrupted and the ZtΔlys2 mutant was auxotrophic for lysine. Furthermore, the pathogenicity testing of ZtΔlys2 on wheat showed a significant reduction in symptom development and the pycnidia and spore number compared with a wild type. In a parallel experiment, ornithine N hydroxylase (SidA) was disrupted to prevent hydroxamate siderophore synthesis. The resulting ZtΔlys2 mutant was unable to synthesise ferrichrome and required supplementation with a high concentration of iron. It was more sensitive to oxidative stress compared with wild type and showed reduction of pycnidia and spore numbers. Whilst L1PPTstrain shows reduced virulence, the impacts of lysine and iron uptake are sufficient to account for the reduction in virulence so given that these genes also had very low if any expression in planta, it is unlikely that PKS and hybrid PKS NRPS play a major role in disease of Z. tritici.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:687598 |
| Date | January 2015 |
| Creators | Ali, Solaf Jawhar |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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