Oömycota or oömycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms (Protists). They are filamentous, microscopic, absorptive organisms that reproduce both sexually and asexually. Oomycetes occupy both saprophytic and pathogenic lifestyles – and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death

Sudden oak death

Sudden Oak Death is the common name of a disease caused by the oomycete plant pathogen Phytophthora ramorum. The disease kills oak and other species of tree and has had devastating effects on the oak populations in California and Oregon as well as also being present in Europe...

. They are also often referred to as water molds (or water moulds), although the water-loving nature which led to that name is not true of most species, which are terrestrial pathogens.

Morphology

The oomycetes rarely have septa (see hypha

Hypha

A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...

), and if they do, they are scarce, appearing at the bases of sporangia, and sometimes in older parts of the filaments. Some are unicellular, but others are filamentous and branching.

Taxonomy

The group is arranged into six orders. Briefly:

• The Lagenidiales are the most primitive; some are filamentous, others unicellular; they are generally parasitic.
• The Leptomitales
  Leptomitales
  Leptomitales are an order of water moulds within the class Oomycetes that contains the genus Apodachlya, belonging to the kingdom Protista....
  
   have wall thickenings that give their continuous cell body the appearance of septation. They bear chitin and often reproduce asexually.
• The Rhipidiales use rhizoids to attach their thallus to the bed of stagnant or polluted water bodies.
• The Saprolegniales
  Saprolegniales
  Saprolegniales is an order of freshwater mould that includes the Saprolegniaceae and other families....
  
   are the most widespread, lack cell walls, and branch a lot. Many break down decaying matter; others are parasites.
• The Peronosporales
  Peronosporales
  The Peronosporales are an order of water moulds which can be pathogenic.Many diseases of plants are sometimes classified under this order, but are sometimes considered members of order Pythiales. Some of these pathogenic protists include the organisms responsible for potato blight, eucalyptus...
  
   too are mainly saprophytic or parasitic on plants, and have an aseptate, branching form. Many of the most damaging agricultural parasites belong to this order.
• The Albuginales are considered by some authors to be a family (Albuginaceae) within the Paronosporales, although it has been shown that they are phylogenetically distinct from this order.

(The above after ).

Reproduction

Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s called zoospore

Zoospore

A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...

s, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind. They also produce sexual spores, called oospore

Oospore

An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi. Also the result of plasmogamy/karyogamy in oomycetes, which in turn leads to the development of hyphae, then mycelium....

s, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.

Pathogenicity

Many oomycetes are economically important because they are aggressive plant pathogens. Some species can cause disease in fish. The majority of the plant pathogenic species can be classified into four groups, although more exist.

• The Phytophthora
  Phytophthora
  Phytophthora is a genus of plant-damaging Oomycetes , whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The genus was first described by Heinrich Anton de Bary in 1875...
  
  group is a paraphyletic genus
  Genus
  In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
  
   that causes diseases such as dieback
  Phytophthora cinnamomi
  Phytophthora cinnamomi is a soil-borne water mould that produces an infection which causes a condition in plants called root rot or dieback. The plant pathogen is one of the world's most invasive species and is present in over 70 countries from around the world.- Life cycle and effects on plants :P...
  
  , late blight in potato
  Potato
  The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
  
  es (the cause of the Great Hunger or Potato Famine of the 1840s in Ireland
  Ireland
  Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
  
   and other parts of Europe), sudden oak death
  Sudden oak death
  Sudden Oak Death is the common name of a disease caused by the oomycete plant pathogen Phytophthora ramorum. The disease kills oak and other species of tree and has had devastating effects on the oak populations in California and Oregon as well as also being present in Europe...
  
  , rhododendron root rot, and ink disease
  Phytophthora cambivora
  Phytophthora cambivora is a plant pathogen that causes Ink disease in European Chestnut trees . Ink disease, also caused by Phytophthora cinnamomi, is thought to have been present in Europe since the 18th century and causes chestnut trees to wilt and die, major epidemics occurred during the 19th...
  
   in the American chestnut
  American Chestnut
  The American Chestnut is a large, deciduous tree of the beech family native to eastern North America. Before the species was devastated by the chestnut blight, a fungal disease, it was one of the most important forest trees throughout its range...
  
  .

• The paraphyletic Pythium
  Pythium
  Pythium is a genus of parasitic oomycete. Most species are plant parasites, but Pythium insidiosum is an important pathogen of animals...
  
  group is even more prevalent than Phytophthora and individual species have larger host ranges, usually causing less damage. Pythium damping off is a very common problem in greenhouses, where the organism kills newly emerged seedlings. Mycoparasitic members of this group (e.g. P. oligandrum
  Pythium oligandrum
  Pythium oligandrum is an Oomycete. It is a parasite of many fungi and other oomycetes including Botrytis, Fusarium and Phytophthora. It has been licensed as a biocontrol agent in the form of an oospore soil treatment, which reduces pathogen load and concomitant plant disease.P. oligandrum can grow...
  
  ) parasitize other oomycetes and fungi, and have been employed as biocontrol agents. One Pythium species, Pythium insidiosum, is also known to infect mammals.

• The third group are the downy mildew
  Downy mildew
  Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines...
  
  s, which are easily identifiable by the appearance of white, brownish or olive "mildew" on the lower leaf surfaces (although this group can be confused with the unrelated fungal powdery mildew
  Powdery mildew
  Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the...
  
  s).

• The fourth group are the white blister rusts (Albuginales, which cause white blister disease on a variety of flowering plants. White blister rusts sporulate beneath the epidermis
  Epidermis (botany)
  The epidermis is a single-layered group of cells that covers plants' leaves, flowers, roots and stems. It forms a boundary between the plant and the external environment. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds,...
  
   of their hosts, causing spore-filled blisters on stems, leaves and the inflorescence
  Inflorescence
  An inflorescence is a group or cluster of flowers arranged on a stem that is composed of a main branch or a complicated arrangement of branches. Strictly, it is the part of the shoot of seed plants where flowers are formed and which is accordingly modified...
  
  . The Albuginales are currently divided into three genera, Albugo
  Albugo
  Albugo is a genus of oomycetes which are not true fungi , although many discussions of this organism still treat it as a fungus. The taxonomy of this genus is incomplete, but several species are plant pathogens...
  
  parastic predominantly to Brassicales
  Brassicales
  The Brassicales are an order of flowering plants, belonging to the eurosids II group of dicotyledons under the APG II system. One character common to many members of the order is the production of glucosinolate compounds...
  
  , Pustula, parasitic predominantly to Asterales
  Asterales
  Asterales is an order of dicotyledonous flowering plants that includes the composite family and its related families.The order is a cosmopolite, and includes mostly herbaceous species, although a small number of trees and shrubs are also present.The Asterales can be characterized on the...
  
  , and Wilsoniana, predominantly parasitic to Caryophyllales
  Caryophyllales
  Caryophyllales is an order of flowering plants that includes the cacti, carnations, amaranths, ice plants, and many carnivorous plants. Many members are succulent, having fleshy stems or leaves.-Description:...
  
  . Like the downy mildews, the white blister rusts are obligate biotrophs, which means that they are unable to survive without the presence of a living host.

Phylogenetic relationships and classification

Although this group was traditionally classified as protists based on general morphology and lifestyle, a cladistic classification based on modern insights supports a relatively close relationship with the photosynthetic organisms such as brown algae and diatom

Diatom

Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

s, within the heterokont

Heterokont

The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...

s.

This relationship is supported by a number of observed differences in the characteristics of oomycetes and fungi. For instance, the cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

s of oomycetes are composed of cellulose

Cellulose

Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

 rather than chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

 and generally do not have septations

Septum

In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Most oomycetes produce self-motile zoospores with two flagella. One flagellum has a "whiplash" morphology, and the other a branched "tinsel" morphology. Spores of the few fungal groups which retain flagella (such as the Chytridiomycetes

Chytridiomycetes

Chytridiomycetes is a class of fungi. Members are found in soil, fresh water, and saline estuaries. They are primitive fungi, closely related to the true fungi, and are first known from the Rhynie chert. It has recently been redefined to exclude the taxa Neocallimastigomycota and...

) have only one whiplash flagellum. Oomycota and fungi have different metabolic pathways for synthesizing lysine

Lysine

Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....

 and have a number of enzymes which differ. The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.
In spite of this, many species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.

Etymology

"Oomycota" means "egg fungi", referring to the large round oogonia, structures containing the female gametes, that are characteristic of the oomycetes.

The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water, which is characteristic for the basal taxa of the oomycetes.

External links

• Introduction to the Oomycota - University of California Museum of Paleontology (UCMP)
• DESCRIPTION OF THE PHYLUM OOMYCOTA - Systematic Biology