How does Epichloë festucae avoid the host defence response? : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Genetics at Massey University, Palmerston North, New Zealand
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Date
2018
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Massey University
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Abstract
Epichloë festucae is a filamentous fungus, which forms symbiotic associations with aerial
tissues of Lolium and Festuca grass species. Chitin, a polymer of N-acetyl-Dglucosamine,
is an important component of the fungal cell wall and a well-known
pathogen associated molecular pattern (PAMP). Chitin promotes pathogen-triggered
immunity (PTI) upon hydrolysis with plant chitinases and release of chitin oligomers.
Therefore, to establish a stable and successful symbiosis, the endophyte needs to remain
‘hidden’ from the host immune system or actively suppress it. Confocal laser scanning
microscopy (CLSM)-based analysis of leaf tissue infected with the E. festucae wild type
strain and infiltrated with the chitin-specific molecular probe, WGA-Alexa Fluor-488,
showed that only the septa of endophytic hyphae bound this probe while the entire cell
wall was labelled in epiphyllous hyphae confirming previous observations that hyphal
cell wall chitin is either masked or remodelled in endophytic hyphae. The aims of this
project were (i) to test whether E. festucae LysM-containing proteins have a role in
binding to or sequestering cell wall chitin oligomers and thereby preventing PAMPtriggered
immunity and (ii) to analyse the composition of the cell wall of endophytic and
epiphytic hyphae. An analysis of the E. festucae genome identified seven genes encoding
proteins with LysM domains. Expression of two of these genes, lymA and lymB, increased
in planta compared to in culture. Interestingly, both are divergently transcribed from
chitinase encoding genes (chiA and chiB respectively), which also have increased
expression in planta. Single gene deletion mutants of lymA, lymB, chiA and chiB as well
as a double gene deletion ΔlymA/B were generated, and their plant interaction phenotype
analysed. Plants infected with DlymA, DlymB or DchiA had the same plant-interaction
phenotype as wild type whereas ΔchiB and ΔlymA/B mutants had defects in hyphal
growth within the leaves. Analysis of hyphal cell wall structure using Chitin Binding
Protein (CBP) and chitosan (CAP (Chitosan Affinity Protein) and OGA-488)-specific
eGFP-based biosensors suggest that cell wall chitin is converted to chitosan in endophytic
hyphae. This structural change is consistent with a lack of a defence response when E.
festucae forms a mutualistic symbiotic association with L. perenne. Three E. festucae
chitin deacetylase genes were identified (cdaA, cdaB and cdaC), and gene expression
analysis showed cdaA expression is significantly increased in planta compare to in culture.
Functional analysis of cdaA revealed that although plants infected with the ΔcdaA mutant
had a similar whole plant interaction phenotype as wild type, they had an abnormal cellular phenotype. Patches of chitin were exposed along the endophytic hyphae
confirming this mutant was unable to convert chitin to chitosan. However, hyphae in these
plants still labelled with the chitosan biosensor OGA-488 demonstrating that despite the
deletion of the cdaA, the hyphal cell wall of endophytic hyphae still contain chitosan
suggesting that another chitin deacetylase, possibly CdaB has a redundant function in E.
festucae. Collectively these results show that lymA, lymB and chiB are required for
establishment of the symbiosis between E. festucae and L. perenne. In addition, this study
shows that chitin is converted to chitosan in the hyphal cell wall of endophytic hyphae
during the infection and colonisation of the host. The E. festucae chitin deacetylase gene
cdaA is also essential for proper hyphal growth in planta and the symbiotic interaction.
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Keywords
Epichloë, Genetics, Endophytic fungi, Genetics, Lolium perenne, Plant defenses, Immune response, Molecular aspects