14.3 The Rice Blast fungus Magnaporthe oryzae (Ascomycota)

14.3 The Rice Blast fungus Magnaporthe oryzae (Ascomycota)

Magnaporthe oryzae is the most destructive pathogen of rice worldwide, destroying enough rice to feed 60 million people each year. The fungus infects its host by producing an appressorium from the germinating conidium. This has become an experimental system of choice for studying spore attachment, germination and plant surface recognition, the formation of infection structures, and their penetration of host cells and tissues (Knogge, 1998; Martin-Urdiroz et al., 2016; Yan  & Talbot, 2016).

 Magnaporthe oryzae, which causes the blast disease of rice (Oryza sativa), is morphologically indistinguishable from Magnaporthe grisea, a species that affects the crabgrass Digitaria; but the two species of Magnaporthe are taxonomically distinct (Couch & Kohn, 2002). Unfortunately, you will often find rice blast referred to as ‘Magnaporthe grisea’; as we do not consider this to be correct we will use the name Magnaporthe oryzae for the rice blast fungus.

Magnaporthe oryzae has also become the main ‘model organism’ for studying the molecular aspects of fungal plant disease. The draft sequence of the M. oryzae genome was published in 2005 (Dean et al., 2005) and subsequently a functional genomics study of pathogenicity revealed many new gene functions required for rice blast disease, improving understanding of the adaptations required by a fungus to cause disease. Research into the molecular genetics of fungal pathogenesis has been stimulated by the availability of increasingly sophisticated molecular analytical methods such as insertional mutagenesis and whole genome, transcriptome, proteome, secretome and metabolome studies (Brown & Holden, 1996; Lorenz, 2002; Jeon et al., 2007; Talbot, 2007).

Jeon et al. (2007) used Agrobacterium tumefaciens–mediated transformation (AMT; see Section 18.13) to create more than 20,000 mutant strains of the rice blast fungus Magnaporthe oryzae. This fungus infects plants by forming specialised infection structures, appressoria, that penetrate the rice cuticle, leading to the invasion of epidermal and cortical cells and the development of disease symptoms. The insertional mutants (a mutant characterised by the insertion of one or a few nucleotide base pairs to a chromosome) were grown in multiwell plates and analysed for growth, conidiation, appressorium development and pathogenicity in a high-throughput plant infection assay. The resulting data were stored in a relational database and mutants were selected for in-depth phenotyping and molecular characterisation of the mutation; 202 new pathogenicity genes were identified in this study. The fungus is responsible for the most serious disease of rice and is a continuing threat to global food security so there is a large community of researchers trying to develop new disease control strategies.

Of special interest is the remarkable ability of the rice blast fungus to invade plant tissue and manipulate the host plant using a battery of secreted effector proteins that suppress plant immunity, altering the organisation of the host cells and enabling rapid fungal growth (Martin-Urdiroz et al., 2016; Yan & Talbot, 2016).

Resources Box 14.1

Where to find more information about crop diseases, crop losses, plant pathogens and food and agriculture statistics

We have a page giving references to scientific papers and hyperlinks to online resources.

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Updated July, 2018