Magnaporthe grisea, rice blast

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Taxonomic lineage

cellular organisms - Eukaryota - Fungi/Metazoa group - Fungi - Dikarya - Ascomycota - Pezizomycotina - Sordariomycetes - Sordariomycetes incertae sedis - Magnaporthaceae - Magnaporthe - Magnaporthe grisea

Brief facts

• Magnaporthe grisea is a filamentous ascomycete, which causes rice blast disease.
• Rice blast is the most serious disease of cultivated rice and therefore poses a threat to the world's most important food security crop. Losses of US$55 billions are estimated in South and Southeast Asia each year due to this pathogen.
• In addition to rice, M. grisea causes disease on a wide variety of alternative hosts, including barley, wheat and millet (Eleusine coracana).
• Blast was first reported in Asia more than three centuries ago and is now present in over 85 countries. It is highly adaptable to environmental conditions and can be found in irrigated lowland, rain-fed upland, or deepwater rice fields - virtually everywhere the rice is grown.
• Blast can survive on seeds and can easily move over borders if proper safety checks are not in place. In 1996 in California, USA, despite the enforcement of strict safety measures to prevent the entry of blast, the disease managed to find its way into the state’s paddy fields for the first time.

Developmental stages

• Sexual Sexual reproduction occurs when two strains of opposite mating types meet and form a perithecium in which ascospores develop.
• Asexual Conidiogenesis or sporulation. The appearance of necrotic disease lesions is accompanied by the development of aerial conidiophores. Conidia are arrayed at the tips of these aerial hyphae. Mitotic division of a single progenitor nucleus occur in the conidiophore, leading to the production of the first three-cell conidium. Thereafter, the hyphal tip moves to the side of the conidium and produces a second spore until three to five conidia are produced in a whorl at the conidiophore tip.
• Appressorium
  formation A stage in fungus development characterized by development of appressoria that infect aerial tissues of the host.
• Vegetative Vegetative growth of the fungus mycelium (hyphae) in tissues of the host or on the laboratory medium. Short period of vegetative growth is followed by asexual reproduction (conidiogenesis or sporulation).

Tissues

• Perithecium A small flask-shaped fruiting body that develops during sexual cycle of the fungus and contains ascospores. M. grisea is a Pyrenomycete, producing fusiform, curved ascospores in unordered asci within perithecia.
• Appressorium A specialized cell produced from the germ tube of the conidium (appressorium formation stage). Development of the appressorium requires a hard, hydrophobic surface and/or the absence of exogenous nutrients. Once formed the appressorium is a dome-shaped cell with a highly differentiated cell wall structure. The cell wall is rich in chitin and contains a layer of melanin on the inner side of the wall. The melanin is a virulence characteristic of the fungus. It is well known that the appressorium is able to generate an enormous turgor pressure and physical force, allowing the fungus to breach the host cuticle and invade plant tissue; it is also known that melanin-deficient mutants cannot generate the pressure of the magnitute required for penetration of the plant cuticle. It is believed that melanin provides effective means of preventing solute efflux and allows appressoria to accumulate the substantial turgor. The region of the appressorium adjacent to the substatum, appressorium pore, lacks both, chitin and melanin. Folowing appressorium expansion, melanization and some quiescence, the pore develops a new cell wall. The penetration peg emerges through this new wall, perforates the host surface and conveys the content of the appressorium into cells of the leaf epidermis.
• Mycelium Body (biomass) of the fungus consisting of hyphae which can grow in tissues of the plant or on a suitable laboratory medium.
  • Germinated conidia Onset of appressorium formation stage. Spore germination requires the presence of free water and is very rapid: within 2-4 hours of landing on the leaf, a polarized germ tube extends for a short distance from one of the apical cells of the conidium. Then, it swells at its tip and changes direction while becoming flattened against the leaf surface. This process, known as hooking, preceeds formation of the appresorium and is thought to constitute a recofnition phase.
  • Hypha Microscopic threadlike filaments that forms mycelium of the fungus. Hyphae grow inttracellularly, invading adjacent epidermal cells as well as underlying mesophyl cells. The pathogen hyphae can account for up to 10% of the biomass of infected leaf tips by three days after inoculation. After penetration of the plant cuticle and cell wall, the penetration peg enlarges to form a primary infection hypha that subsequently differentiates into a branched and bulbous secondary hypha, which enables proliferation throughout the host tissue.
• Conidium Asexual spore of the fungus produced on conidiophore by hyphae fragmentation. Mature conidia are three-celled, pear-shaped, and bear a basal appendage at the point of attachment to the conidiophore. As spores reach maturity, the apical cell wall of the conidium breaks open, and a droplet of viscous sticky material is produced while the conidium is still attached to conidiophore. This material serves to attach the conidium to the wax-covered host surface upon contact in a nonspecific manner. Once attached, even vigorous attempts to dislodge adherent conidia from surfaces are typically unsuccessful.

Mating types

M. grisea is heterophallic, i. e., two mating types of the fungus are present. When fertile isolates carrying opposite mating types are paired on an appropriate growth medium, they will form sexual fruiting bodies, perithecia within 21 days. Rice pathogenic strains are very rarely fertile.

• Mat1-1
• Mat1-2 A commonly studied strain, Guy11, is a Mat1-2 strain from French Guiana and has proved very valuable in a large number of genetic studies.

Strains

• Guy11 Mat1-2 mating type
• 70-15 Mat1-1 mating type
• P2 could not find corresponding mating type
• K261 Mat1-2 mating type
• K364 Mat1-2 mating type
• 4454-R-1 wild type; Mat1-1 mating type

References

• Dean RA et al. The genome sequence of the rice blast fungus Magnaporthe grisea. Nature. 2005 Apr 21;434(7036):980-6.
• Talbot NJ. On the trail of a cereal killer: Exploring the biology of Magnaporthe grisea. Annu Rev Microbiol. 2003;57:177-202.
• Howard RJ, Valent B. Breaking and entering: host penetration by the fungal rice blast pathogen Magnaporthe grisea. Annu Rev Microbiol. 1996;50:491-512.
• Major topic Magnaporthe: Free full text articles in PubMed

Websites

• Wikipedia: Magnaporthe grisea
• Rice Blast
• Blast, biotech and big business: implications of corporate strategies on rice research in Asia