Phytophthora infestans

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Taxonomy

cellular organisms - Eukaryota - stramenopiles - Oomycetes - Peronosporales - Phytophthora - Phytophthora infestans

Phytophthora infestans is heterothallic (sexes reside in different individuals) oomyctete (water mold). For a long time oomycetes were considered as close relatives of Fungi such as ascomyctetes (various molds, yeasts) and basidiomycetes (mushrooms, rusts and others). However, studies of metabolism, cell wall composition, and rRNA sequence indicate that oomycetes are more properly grouped with chrysophytes, diatoms, and brown algae.

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Brief facts

• Phytophthora is a large genus of over sixty species causing a range of economically important plant diseases worldwide. Phytophthora infestans is most infamous species of the genus. It causes late blight of potato and tomato. Unlike most Phytophthora species, which produce soil-borne root-rotting diseases, late blight is primarily a disease of foliage, stems, potato tubers and tomato fruits.
• The potato blight caused the Irish Potato Famine in 1845-49 which, it is estimated, caused over 1,000,000 people to starve to death and forced a further 2,000,000 to emigrate. Even now, more than 150 years after it was first associated with potato late blight disease in Europe and North America, P. infestans remains a major problem with direct costs of control efforts and lost production estimated at billions of dollars worldwide.
• For a long time Phytophthora infestans had been under control in North America because represented by only one mating type the pathogen was not able to quickly develop resistance to pesticides used against it. However in late 80's, the second mating type of the Phytophthora was first noticed in Wisconsin, and had probably already spread throughout of North America. The presence of the second mating type allows sexual reproduction and quicker gene recombination, which, in turn, leads to more rapid development of resistance to pesticides.
• The disease also affect other members of Solanaceae family such as tomato and eggplant.

Hosts of Phytophthora infestans

• Solanum tuberosum (potato, papas): life cycle, tuber anatomy at GeoChemBio (link opens in new window)
• Solanum lycopersicum (Lycopersicon esculentum, Lycopersicon lycopersicum), tomato: life cycle, flower and fruit anatomy at GeoChemBio (link opens in new window)

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Developmental stages

The organism is diploid on all stages of its life cycle, with the exception of haploid nuclei formed within gametangia during sexual reproduction.

• Sexual in response to the hormones, male and female gametangia (antheridia and oogonia are formed within a mating zone in which normal vegetative growth and asexual sporulation are inhibited
  • Gametogenesis Haploid nuclei are generated within the gametangia.
  • Oospore formation Gametes fuse and a diploid oospore containing one viable nucleus is generated; progeny of the A¹ or A² type develop from the germinated oospore.
• Asexual Under conditions of high humidity the asexual cycle of the pathogen normally takes 5 – 7 days but may be shorter under higher temperatures; four to six cycles may be necessary to progress from a few initial, unseen infections to significant areas of foliage destruction; significant visual evidence of progressing infection in a field crop may not appear until 4 – 6 weeks after its initial infection.
  • Sporulation The pathogen generate sac-like structures, sporangia born on specialized branches called sporangiophores; airborn sporangia may infect plants over several kilometres if they are not desiccated en route; at temperatures lower than 15, one sporangium may produce 10-12 motile biflagellated spores called zoospores.
  • Germination The sporangia or zoospore produces a germ tube; plant penetration occurs when the tip of the germ tube differentiates into an appresorium which enables invasion of the underlying host cell; penetration can take only 2 hours. The availability of free moisture for at least short period of time on plant surfaces is essential to sporangium germination.
    • Direct germination At temperatures above 15°C sporangia can germinate directly
    • Indirect germination When the zoospores are released; after short motile period Often less than 60 minutes), they encyst and produce germ tubes; zoospores can be washed down the stems and reach daughter tubers.
  • Vegetative growth Hyphae grow intercellularly invading the host tissue and forming biotrophic relationships with plant cells through haustoria; macroscopically, there are generally no visible symptomps for at least 2 days (characteristic of a hemibiotroph); the host cells eventually die and areas of necrosis become visible; as the infestation proceeds new sporangia are formed on the plant surface.

 

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Tissues

• Sporangium Sac-like structure containing asexual spores; sporangia are hyaline (clear) and lemon-shaped; they are formed on specialized branches called sporangiophores; sporangia are relatively short-lived; they are seen as a white halo on underside of the infected leaf. Up to 300,000 of sporangia per lesion are produced.
  • Zoospore Zoospores emerge from sporangia under favorable conditions.
• Cyst Encysted zoospore; overwintering form of zoospores which retract or shed their flagella and secrete a wall that protects them from harsh environments.
• Mycelium Mycelium constitutes a main biomass of vegetative stage of Phytophthora infestans; the mycelium is coenocytic, i. e. it represents a multinucleate cytoplasmic mass enclosed by a single cell wall.
• Chlamydospore A thick-walled, asexual fungal spore that is derived from a hyphal cell and can function as a resting spore.

 

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Mating types

P. infestans is heterophalic, i.e., two mating types are present.

• Mating type A¹ Prevalent mating type.
• Mating type A² Rarely occuring mating type of the organism, however, recent distribution of the A² mating type has had significant impacts on disease severity and incidence.

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References

• Nicholls H. Stopping the Rot. PLoS Biol. 2004 July; 2(7): e213.

  P. infestans on potato field.

  

  Reproductive Structures of Phytophthora: the asexual (A) sporangia, (B) zoospores, and (C) chlamydospores, and the sexual (D) oospores.

  
  Photo credit: Matteo Garbelotto, UC Berkeley [A, D], and Edwin R. Florance, Lewis & Clark College [Portland, Oregon, United States] and the USDA Forest Service Pacific Southwest Research Station in Albany, California [B, C].

• Thines M. Bridging the gulf: Phytophthora and downy mildews are connected by rare grass parasites. PLoS One. 2009;4(3):e4790.

  Apophyses in Phytophthora: Sporangiophores as seen in phase contrast light microscopy. Small pictures: Close-up of the apophyses as seen in scanning electron microscopy. Arrows point to apophyses on the ultimate branchlets, which are typical for Phytophthora.

  

• Judelson HS. The genetics and biology of Phytophthora infestans: modern approaches to a historical challenge. Fungal Genet Biol. 1997 Oct;22(2):65-76.
• Fry W. Phytophthora infestans: the plant (and R gene) destroyer. Mol Plant Pathol. 2008 May;9(3):385-402.
• Shattock RC. Phytophthora infestans: populations, pathogenicity and phenylamides. Pest Manag Sci. 2002 Sep;58(9):944-50.
• Latijnhouwers M, de Wit PJ, Govers F. Oomycetes and fungi: similar weaponry to attack plants. Trends Microbiol. 2003 Oct;11(10):462-9.
• Thines M. Bridging the gulf: Phytophthora and downy mildews are connected by rare grass parasites. PLoS One. 2009;4(3):e4790
• Major topic Phytophthora: Free full text articles in PubMed

Websites

• Wikipedia: Phytophtora infestans
• Return of the Potato Blight
• Potato Late Blight: The Disease and its Control (guide, .pdf)

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