Investigating the role of the Crh gene family in Magnaporthe oryzae on cell wall integrity and fungal virulence

Thesis

The rice blast disease caused by the fungus Magnaporthe oryzae causes significant annual crop losses of up to 30%. The fungal specific cell wall is a potential antifungal target due to the external location and importance to fungal survival and morphogenesis. The skeletal framework of the cell wall is made from glucan chains cross-linked with chitin, facilitated by the Congo Red Hypersensitivity (Crh) enzyme. In Saccharomyces cerevisiae, ΔCrh1/ΔUtr2 mutants exhibit reduced wall integrity and the same is observed in Candida albicans, with reduced virulence in a mammalian host. Little is known about the cell wall in fungal plant pathogens and nothing of the Crh orthologs.

Bioinformatics revealed five M. oryzae Crh-like genes lying in two clades. Gene expression analyses revealed up to four-fold elevated expression of MGG_Crh2 and MGG_Utr2 when grown under cell wall stress. Consistently, the corresponding deletant strains showed hypersensitivity towards cell wall perturbants and was attributed to a loss in cell wall rigidity. Cell wall analysis of the mutant ΔΔMGG_Crh2_Utr2 revealed changes in the cell wall composition consistent with the reduction of Crhp activity and activation of cell wall compensatory mechanism often observed in a compromised cell wall. Furthermore, eGFP-tagged MGG_Crh2p showed localisation to the lateral cell wall with high accumulation at septum and polarised growth sites, consistent with sites of chitin accumulation. These mutants are, however, able to form normal infective structures and are fully pathogenic on susceptible rice cultivars.

In conclusion, the Crh gene family in M. oryzae is important for cell wall integrity by the cross-linking of chitin to glucan, but dispensable for virulence. Future work should explore the compensatory mechanism of other cell wall proteins in the ΔΔMGG_Crh2_Utr2 as this could shed light on the potential for targeting multiple cell wall components in a synergistic approach to confer effective disease control.

Authors: Omar, S

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: glucan; fungal cell wall; chitin; fungi; congo red; rice blast; rice disease; GH 16; GPI; cell wall; crh; MGG_Crh2; magnaporthe oryzae; phytopathogen; cell wall protein
• Subjects: Fungal diseases of plants