Magnaporthe

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A Microbial Biorealm page on the genus Magnaporthe

Magnaporthe grisea A: Conidium with a drop of spore tip mucilage (STM) adhesive. B: Attachment of conidium by means fo STM. Howard and Valent, "Breaking and Entering: Host Penetration by the Fungal Rice Blast Pathogen Magnaporthe grisea."

Classification

Higher order taxa

Eukaryota; Fungi/Metazoa group; Fungi; Ascomycotoa; Pezizomycotina; Sordariomycetes; Sordariomycetes incertae sedis; Magnaporthaceae

Species

Magnaporthe grisea
Magnaporthe oryzae
Magnaporthe rhizophila

NCBI: Taxonomy Genome

Description and Significance

Magnaporthe is a genus within Ascomycota. This genus is most well-known for pathogenic species such as Magnaporteh grisea, which causes rice blast disease.

Genome Structure

Dean et. al. published the draft sequence of the Magnaporthe grisea genome in 2005. The genome of M. grisea contains more genes than the genomes of its cousins, Neurospora crassa and Aspergillus nidulans. M. grisea has a high degree of genetic variablity.

Cell Structure and Metabolism

Rice blast lesions on rice plant leaves. Photo by R.K. Webster.

Magnaporthe are filamentous fungi. The species Maganporthe griseia is equipped with adhesives that are released from the tips of spores. This adhesive is called spore tip mucilage (STM) adhesive. It allows M griseia to bind to host organisms, beginning the infection process. The appressoria of M. griseia (described below) contains melanin. Appressoria also have thickened cell walls.

Like other members of Ascomycota, Magnaporthe are heterotrophic organisms. They can obtain nutrients by decomposing organisms. Infectious species such as M. grisea and M. oryzae obtain nutrients from the hosts they infect.

Magnaporthe are haploid organisms. However, they can reproduce both sexually and asexually. Because it is pathogenic, Magnaporthe grisea also has an infectious life cycle, which is asexual. The infectious form of M. griseia is called the conidium. After binding to its host, M. griseia developes a specialized dome-shaped infection cell called the appressorium. This cell generates turgor pressure which ruptures the host's leaf cuticle, allowing M. grisea to infect the leaf tissue. Once M. grisea colonizes its host, lesions form. These lesions are sites for fungus sporulation, allowing M. grisea to spread to more plants.



Ecology

The species Magnaporthe grisea causes rice blast disease. Upon infection, this fungus defeats or suppresses the plant's immune system, as well as affecting metabolism and cell signaling. Rice blast disease is so powerful that it kills enough rice to feed 60 million people annually, infecting plants all over the world. The Centers for Disease Control and Prevention consider this disease a major biological weapon. This disease adapts so well that it is able to defeat a rice cultivar, which is bred specifically to resist this fungus. In addition to rice, it attacks wheat, barley, millet, and turf grass. Severity of infection depends on what part of the plant becomes infected. If rice blast attacks young seedlings, a whole plant can die. However, if the disease attacks the stems, nodes, or panicle, the effects are much more severe. In these infections, the result is almost total loss of the rice grain. Rice blast disease also has the capacity to infect plant roots.

Magnaporthe grisea is also considered a model organism for the study of fungal phytopathogenicity and host-parasite interactions.

References

Broad Institute. "Magnaporthe Information." 24 June 2005. Accessed 21 July 2005.

Dalke, Kate. "Fighting blast disease: Rice pathogen sequenced." Genome News Network. 13 September 2002. Accessed 21 July 2005.

Dean, Ralph A. et. al. "The genome sequence of the rice blast fungus Magnaporthe grisea." Nature. 21 April 2005;434:980-986.

Howard, Richard J. and Barbara Valent. "Breaking and Entering: Host Penetration by the Fungal Rice Blast Pathogen Magnaporthe grisea." Annu. Rev. Microbiol. 1996. 50:491–512.

Kershaw, Michael J., Gavin Wakley, and Nicholas J. Talbot. "Complementation of the Mpg1 mutant phenotype in Magnaporthe grisea reveals functional relationships between fungal hydrophobins." The EMBO Journal Vol.17 No.14 pp.3838–3849, 1998.

Kubo, Yasuyuki, Yoshitaka Takano, Noriko Edno, Nobuku Yasuda, Sohkichi Tajima, and Iwao Furusawa. "Cloning and Structural Analysis of the Melanin Biosynthesis Gene SCD1 Encoding Scytalone Dehydratase in Colletotrichum lagenarium." Appl Environ Microbiol. 1996 Dec;62(12):4340-4.

Peabody, Erin. "Newly Exporter Rice Gene Could Help 'Blast' Killer Fungus." Agricultural Research Service. 12 August 2005.