Oomycetes

7 Key excerpts on "Oomycetes"

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  Annual Plant Reviews, Molecular Aspects of Plant Disease Resistance

  • Jane Parker(Author)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Here we present some of the latest discoveries and insights into oomycete biology and pathology. The first part gives a brief overview of oomycete pathogens of plants and highlights the species that currently fea-ture as model organisms. We then describe the genomic resources available for oomycete research and how genomics has accelerated gene discovery. The last part is devoted to various types of secreted proteins and the potential roles these proteins play in host–pathogen interactions. 5.2 Biology and pathology of Oomycetes 5.2.1 Branches in the tree of life Oomycetes, also known as water molds, resemble fungi in many ways. Like fungi, Oomycetes have a global distribution and prosper in quite diverse en-vironments. They can live as (hemi-) biotrophic or necrotrophic pathogens in association with plants (see Section 5.2.3; Box 5.1), animals or other microbes, but also as saprophytes feeding on decaying matter. Currently, at least 800 oomycete species are known, but depending on the definition of a species this number might actually reach 1500 (Dick, 2001). Nevertheless, the species richness seems low when compared to the number of fungal species known to date: 30 000 basidiomycete species have been described and ascomycetes reach a similar number (Kirk et al ., 2001; James et al ., 2006). It is, however, likely that there are many Oomycetes out there yet to be discovered. In this respect, the genus Phytophthora is illustrative. In the last 10 years, at least 18 new species have been described, expanding the genus to nearly 95 members (Blair et al ., 2008; http://www.PhytophthoraDB.org, accessed June 2008). 104 Molecular Aspects of Plant Disease Resistance Box 5.1 Lifestyles of plant pathogenic Oomycetes Plant pathogens can be divided into groups based on the different strategies they employ to colonize plants (Agrios, 2004). So-called obligate biotrophs grow and reproduce in living plant tissue and obtain energy by uptake of plant nutrients.

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  Fungicides in Practice

  • Richard P. Oliver, Janna L Beckerman(Authors)
  • 2022(Publication Date)
  • CAB International

    (Publisher)

  This group includes several highly destructive and historically significant pathogens. The most famous example is the potato late blight pathogen Phytophthora infestans that triggered the great 1845 Irish potato famine; it still causes major losses today and is a major target for fungicide development. The other two groups are Pythiales and Peronosporales. Pythium species are the cause of seedling damping-off diseases, whereas the Peronospora cause the downy mildews. The diseases caused by Oomycota include many that can be described as biotrophic, such as the downy mildews, as well as hemibiotrophic interactions, such as those caused by the Phytophthora group. At first glance, these three groups share many of the features of fungi. They are eukaryotic, heterotrophic, acquire nutrients only by adsorption and grow by filamentous expansion. They cause diseases with mildew, blight or rot symptoms just like fungi. However, there are also obvious differences. They have motile spores that use flagella. They lack chitin and ergosterol and instead have cellulose-reinforced cell walls with phytosterols in their cell membranes. And importantly, most of the fungicides that work against Oomycetes do not control fungi and vice versa. These differences were resolved once molecular phylogenetic data were applied to eukaryotic taxa (Forster et al., 1990). These data clearly showed that Oomycetes were completely unrelated to fungi. Indeed, fungi share a common ancestor with animals and if anything, Oomycetes share more common features with plants. Plasmodiophora A common root disease of brassica crops called clubroot is caused by Plasmodiophora brassicae. This organism has been placed into a distant taxon, the Rhizaria. It was previously known as a slime mould and placed with the ‘protists’

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  Principles of Horticulture

  • C.R. Adams, K.M. Bamford, M.P. Early(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  11 Fungi, Bacteria and Viruses In this chapter the three groups of organisms are described, and emphasis is placed on the damage they cause, the important aspects of their life cycles, and the control measures which are available to reduce their infection and spread. Within each group, the diseases are classified according to the part of the plant attacked. Because of the microscopic size of the fungal spore, bacterial cell or virus particle, it may be difficuh for the grower to relate the causitive organism to the disease as he sees it. For example, Phytophthora infest-ans, a fungal organism, travels through the air as a spore to infect potato leaves. When the leaves start to turn black, the plant is said to show signs or symptoms of the potato blight disease. Only when the plant's growth and tuber production are decreased by the organism is yield loss said to occur. This chapter emphasizes the fact that symptoms of a disease are the horticulturist's main guide to the presence of fungi, bacteria and viruses. FUNGI Structure and biology These organisms, commonly called moulds, cause serious losses in all areas of horticulture. They are thought to have common ancestors with the fila-mentous algae, a group including the present-day green slime in ponds. The fungus is composed in most species of micro-scopic strands (hyphae), which may occur together in a loose structure (mycelium), form dense resting bodies (sclerotia), (Figure 11.1) or produce com-plex undergound strands {see rhizomorphs). The hyphae in most fungi are capable of pro-ducing spores. Wind-borne spores are generally very small (about 0.01 mm), not sticky, and often borne by hyphae protruding above the leaf sur-face, e.g. grey mould, so that they catch turbulent wind currents. Water or rain-borne spores are often sticky, e.g. damping-off. Asexual spores pro-duced without fusion of two hyphae commonly occur in seasons favourable for disease increase, e.g.

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  Fish Diseases (2 Vols.)

  • Jorge Eiras, Jorge Eiras, Helmut Segner, Thomas Wahli(Authors)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  In conventional terms, fungi have the following morphological characteristics. They are eukaryotic and multicellular, many consist of hyphae that grow apically and, although the majority are saprophytic, many are symbiotes that form permanent associations with plants (mycorrhiza) and algae (lichen) and some have developed a parasitic mode of life and are major parasites of plants and animals. In the last ten years or so, advances in molecular taxonomy have given us a better view of the relationships between the various groups of organisms that have conventionally been included as fungi and included in overviews of this type. The fungi sensu lato are now regarded as having only morphological and not taxonomic affinities and falling into three separate kingdoms. The first of these, the true fungi, the Eumycota includes the Ascomycetes and Basidiomycetes; the second, the Chromista (Stramenopiles), includes the Oomycetes or water moulds as well as diatoms and brown algae. Finally, there is what is now recognized on a molecular basis to be a group of organisms which, while appearing to lie at the base of the Eukaryote tree, seem to be best placed in the Kingdom Protoctista in a clade currently referred to as the DRIPs (Ragan et al., 1996). DRIP includes Dermocystidium, the Rosette agent, Ichthyophonus and Psorospermium. The remaining 'fungal' fish parasite, Branchiomyces, has never been validly described; even its morphology in fish is inadequately described and it will not be considered further in this review. By far the most important of these groups of'fungal' pathogens offish are the Chromistan Oomycetes, which include the water moulds Saprolegnia and Aphanomyces. These differ from the Eumycota in numerous ways, including the presence of motile biflagellate zoospores as the primary means of asexual reproduction and the absence of chitin from their cell walls which instead are basically cellulosic.

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  Wood Microbiology

  Decay and Its Prevention

  • Robert A. Zabel, Jeffrey J. Morrell(Authors)
  • 2020(Publication Date)
  • Academic Press

    (Publisher)

  Early classifications of wood-decay fungi were based on convenient macroscopic characters such as the size and shape of the fruiting structures and the nature of the surfaces bearing the hymenium (tubes, gills, spines, smooth, etc.). The limited fossil record of fungi made it difficult to suggest phylogenetic relationships. Another great difficulty was the evolutionary phenomena of convergence, where macroscopically similar fungi sometimes have very different origins. The result was a classification system based upon fruiting body features such as smooth, poroid, toothed or other arrangements of the hymenium that placed many similar looking, but phylogenetically distant organisms into the same groups. Techniques for assessing differences in DNA and RNA have markedly enhanced the ability to delineate between direct genetic connections and convergence.

  Major revisions in the classifications of many groups will continue as new information on the microscopic, ultrastructural, physiologic, genetic and biochemical features of fungi accumulates. Similar changes are occurring in the classifications of the bacteria.

  The general classification scheme we will use for fungi is in the current edition of the “Ainsworth & Bisby’s Dictionary of Fungi” (Kerk et al., 2009). Emphasis in this section will be placed on the groups of fungi that inhabit, modify, or destroy wood. Drastic taxonomic revisions have occurred in all groups of fungi and will continue to occur as researchers examine the genetic basis for many groups.

  Fungi are placed in a single group, the Eumycota, and are considered to have a common ancestor. One current classification scheme lists 7 Phyla within the Fungi: Microsporidia, Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Glomeromycota, Ascomycota, and Basidiomycota. Mycosporidia are internal parasites of other organisms and will not be considered here.

  Chytridiomycota are often called chytrids are distinctive because they produce motile zoospores with a single flagellum. They are not important in wood degradation, but they have recently been implicated in a number of high-profile animal diseases notably with bats and many amphibians. Like the chytrids, Blastocladiomycota and Neocallimastigomycota produce motile zoospores, but primarily feed on decaying organic matter. The latter group lives in anaerobic environments including the digestive systems of many herbivores or other extreme environments where oxygen is limited. These groups, while important in other applications are not considered to be wood degraders.

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  Handbook of Soil Sciences

  Properties and Processes, Second Edition

  • Pan Ming Huang, Yuncong Li, Malcolm E. Sumner, Pan Ming Huang, Yuncong Li, Malcolm E. Sumner(Authors)
  • 2011(Publication Date)
  • CRC Press

    (Publisher)

  An outline of the higher classifica-tion of Fungi has been published (Hibbett et al., 2007), and these changes have been included in the latest edition of the Dictionary of the Fungi (Kirk et al., 2008). The latter also has separate sec-tions for fungus-like organisms belonging in the Heterokonta or “Protozoa” (Kirk et al., 2008). It will be some time before these changes make it into textbooks of mycology, let alone those for plant pathology, botany, or general biology. However, as in all fields of biology, a classification that more closely reflects the evolutionary history of the organisms will be more predictive and useful than our historical classifications that were misled by morphological convergences. Secondly, new molecular techniques are allowing a rapid acquisition of knowledge about the composition and function-ing of complex fungal communities that was simply unattain-able even recently. Applied critically, these new methods are rejuvenating mycology and bringing the fundamental impor-tance of fungi to the attention of ecologists and soil biologists. It is time for fungi to be given their due. References Agrios, G.N. 2004. Plant pathology. 5th edn. Academic Press, Amsterdam, the Netherlands. Ahmadjian, V. 1993. The lichen symbiosis. John Wiley, New York. Alexopoulos, C.J., C.W. Mims, and M. Blackwell. 1996. Introductory mycology. 4th edn. John Wiley, New York. Amewowor, D.H.A.K., and M.F. Madelin. 1991. Numbers of myxomycetes and associated microorganisms in the root zones of cabbage ( Brassica oleracea ) and broad bean ( Vicia faba ) in field plots. FEMS Microbiol. Lett. 86:69–82. Anderson, J.P.E., and K.H. Domsch. 1975. Measurement of bacte-rial and fungal contributions to respiration of selected agri-cultural and forest soils. Can. J. Microbiol. 21:314–322. Anderson, A.S., and E.M.H. Wellington. 2001. The taxonomy of Streptomyces and related genera. Int. J. Syst. Evol. Microbiol. 51:797–814. Appoloni, S., Y.

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  Marine Fungi

  and Fungal-like Organisms

  • E. B. Gareth Jones, Ka-Lai Pang, E. B. Gareth Jones, Ka-Lai Pang(Authors)
  • 2012(Publication Date)
  • De Gruyter

    (Publisher)

  Thus, an understanding of host–parasite interactions on all trophic levels is required to properly describe the energy fluxes in marine food webs. Oomycete parasites range in morphology from holocarpic to mycelial and eucarpic forms (Porter 1986). The majority of marine parasites belong to the “lower Oomycetes”, a group of intracellular and holocarpic taxa that are predominantly, but not exclusively, parasites of several marine organisms. Species of Ectrogella, Eurychasma, Haliphthoros, Halodaphnea, Lagenisma, Pontisma and Sirolpidium are exclusively marine pathogens; while species of Haptoglossa, Olpidiopsis and Petersenia are also found in freshwater and terrestrial habitats (Strittmatter et al. 2009). The majority of species are parasites of marine algae and crustaceans, but they are also parasites of fishes, molluscs, nematodes, and rotifers (Table 11.1). Diseases of algae due to fungal-like infection are quite common in the marine habi-tat, both in nature and in mariculture (Raghukumar 1996; Hyde et al. 1998; Murray and 198 Chapter 11 Peeler 2005; Das et al. 2006; Gachon et al. 2010). Many of the taxa of Oomycetes listed in Table 11.1 are parasites of algae (reviewed in Strittmatter et al. 2009). Diseases caused by these organisms appear to be host-specific and show characteristic symptoms, such as changes in color, rot lesions and abnormal growth in the host (Li et al. 2010). Olpidiopsis is a holocarpic genus that parasitizes several species of marine green, brown and red algae (Dick 2001), as listed in Table 11.1. Some species of marine fungal-like organisms are par-asites of economically-important seaweeds, such as Porphyra species. This alga is parasit-ized by Pythium porphyrae (van der Plaats-Niterink 1981), and by Olpidiopsis porphyrae (Sekimoto et al. 2008b). The infection of Porphyra (commonly called nori) by Olpidiopsis porphyrae and O. bostrychiae has been shown to cause significant decreases in nori pro-duction (Fujita 1990; Gachon et al. 2010).

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