Descriptions

Outline

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Phytophthora parasitica

 
Phytophthora parasitica is a devastating pathogen with one of the widest host ranges of any described Phytophthora species, infecting more than 100 genera of plants. Economically important hosts include solanaceous crops (tomato, pepper, eggplant, potato, tobacco), many orchard tree crops (citrus, walnut, almond, pistachio, papaya, peach, plum, apricot, apple, pear, macadamia, avocado, guava, pomegranate), tropical fruits (pineapple, banana, durian, passionfruit), oilseeds (sesame, safflower), vegetables (cucurbits and Brassicas) and many ornamental plants [1]. P.parasitica is especially a problem in Mediterranean climates where soil temperatures are warm and moisture levels are high due to summer irrigation, for example the California central valley. Phytophthora infection of tree crops is particularly damaging because crop rotation is not an option and re-planted trees may take up to 5 years to bear fruit or nuts. P. parasitica causes extensive damage in nurseries, greenhouses and hydroponic systems [1]. Recirculation of irrigation water in these systems exacerbates Phytophthora diseases due to the release of aquatic zoospores by these pathogens [2, 3]. P. parasitica attacks roots as well as stems, leaves and fruits of its hosts. The most important economic losses in the United States are to the tomato, citrus, tobacco and nursery industries. P. parasitica also damages natural communities (e.g. Eucalypts) worldwide [1], and it is now frequently found as a prominent Phytophthora species in forest soils. Furthermore, it has been recently found to outcross with other Phytophthora species, such a P. cactorum, creating hybrids that possess an extended host range [4].

Phytophthora parasitica INRA-310 was isolated from tobacco in Australia [5], and is accessioned in the UC Riverside Phytophthora collection as P1753. INRA-310 can infect tomato, pepper, potato, tobacco, carnation, periwinkle, Arabidopsis thaliana, Medicago truncatula [6] and probably (because it infects tobacco, but it has not been tested) a third model plant host, Nicotiana benthamiana.

As with many Phytophthora species, infection by P. parasitica is hemibiotrophic, beginning with biotrophic colonization then switching to necrotrophic tissue destruction. P. parasitica undergoes both sexual and asexual reproduction. The pathogen can persist in soils through abundant production of thick walled chlamydospores, as well as through sexual oospores. Asexual swimming zoospores are effective at dispersing the pathogen through irrigation water. While P. parasitica as a species has a very broad host range, many isolates are pathogenic to a single, or a restricted number of hosts (tobacco, citrus, tomato etc) [1]. Recently however, collaborator F. Panabieres' team has identified several P. parasitica strains that can attack a very large array of hosts, including the model plant species Arabidopsis thaliana [7] and Medicago truncatula (unpublished). This species therefore offers a unique opportunity to investigate on a genome-wide basis the relationship between mechanisms underlying general pathogenicity and those governing host specificity. Within the genus Phytophthora, P. parasitica falls in the same phylogenetic clade as the important narrow host range pathogen P. infestans [7] (refer to figure above), but has a much smaller genome size (71 Mb versus 240 Mb), providing a unique opportunity to investigate the evolution of pathogenicity, host range and effector protein repertoires.

References
  1. Erwin, D.C. and Ribiero, O.K. (1996) Phytophthora Diseases Worldwide. APS Press, St. Paul,Minnesota.
  2. Hong, C. X.; Richardson, P. A. and Kong, P. (2008) Pathogenicity to ornamental plants of some existing species and new taxa of Phytophthora from irrigation water. Plant Disease 92:1201-1207.
  3. Kong, P.; Hong, C.; Jeffers, S. N. and Richardson, P. A. (2003) A Species-Specific Polymerase Chain Reaction Assay for Rapid Detection of Phytophthora nicotianae in Irrigation Water. Phytopathology 93:822-31.
  4. Bonants, P. J.; Hagenaar-de Weerdt, M.; Man In 't Veld, W. A. and Baayen, R. P. (2000) Molecular Characterization of Natural Hybrids of Phytophthora nicotianae and P. cactorum. Phytopathology 90:867-74.
  5. Colas, V.; Lacourt, I.; Ricci, P.; Vanlerberghe-Masutti, F.; Poupet, A. and Panabieres, F. (1998) Diversity of virulence in Phytophthora parasitica on tobacco, as reflected by nuclear RFLPs. Phytopathology 88:205-12.
  6. Attard, A. and Keller, H. (2008) Dissecting events that govern the compatible interaction between Phytophthora parasitica and Arabidopsis thaliana. J. Plant Pathology 90:ICPP 2008 abstract book S2-211.
  7. Cooke, D. E. L.; Drenth, A.; Duncan, J. M.; Wagels, G. and Brasier, C. M. (2000) A molecular phylogeny of Phytophthora and related oomycetes. Fungal Genet. Biol. 30:17?32.