OOMYCETES: CHARACTERISTICS, LIFE CYCLE, NUTRITION, REPRODUCTION - BIOLOGY - 2025

The oomycetes or water molds (Oomycetes or Oomycota) are a group of organisms traditionally classified among the fungi. Among the characteristics shared by both groups of organisms (fungi and oomycetes) are the type of growth, the form of nutrition and the use of spores during reproduction. However, molecular studies have shown that oomycetes are not related to true fungi.

Some species are parasites of plants, being among the most devastating pathogens of crops. Diseases they cause are seedling blight, root rot, leaf blight, and downy mildews.

Phytophthora infestans. Direct germination of a sporangium by germ tube. Photograph by HD Thurston. Taken and edited from apsnet.org/edcenter/intropp/LabExercises/Pages/Oomycetes.aspx

The Great Famine, or Irish Potato Famine, was caused by an oomycete named Phytophthora infestans. The pathogen wiped out Irish potato crops in the 1840s.

At that time, about half of the population depended exclusively on this crop for their survival. The loss of crops caused nearly a million people to starve to death and a similar number to flee the island in search of better living conditions.

characteristics

Oomycetes are a group of organisms, mainly aquatic, that have a cell wall composed of ß-glucans, proline and cellulose. Its life cycle is predominantly diploid.

The hyphae are multinucleated or coenocytic and asept. The mycelium produces septa solely to separate the thallus from the reproductive structures.

Asexual reproduction is by means of biflagellate spores (zoospores) produced in zoosporangia. Sexual reproduction is heterogamous and occurs by direct injection of the male nuclei (= sperm) of the antheridium into the eggs contained in the oogonia.

The typical genome size of oomycetes is 50 to 250 Megabases (Mb), very large compared to that of fungi, which is 10 to 40 Mb.

Taxonomy

Traditionally the oomycetes had been classified within the kingdom of fungi (Fungi). However, molecular and biochemical studies have led to them being relocated to the Protista Kingdom. They belong to the phylum Heterokontophyta, Class Oomycota. The class contains to date 15 orders.

Lifecycle

During the epidemic phase, oomycetes are dispersed by wind or water, by means of asexual sporangia. These sporangia can germinate directly, forming invasive hyphae.

The germination of the sporangium can also be indirect, releasing mobile zoospores. Zoospores are attracted to the surface of future hosts. In some species, the direct or indirect germination of the sporangium will depend on the environmental temperature.

When germinating, the sporangia and zoospores form germ tubes, which will infect through the formation of appressoria and penetration structures.

After penetrating, the hyphae will grow both inter and intracellularly in the host. After at least 3 days of growth, the hyphae can form new sporangia that will spread to infect new organisms.

Sexual reproduction occurs through the production of gametangia: oogonia and antheridia. Each individual generally produces both antheridia and oogonia. In some species, reproduction must be crossed (heterothallic), in others there may be self-fertilization (homothalic).

Within the gametangia, meiotic division occurs. One or more oospheres are produced in the oogonia. Flagellated sperm are absent in oomycetes. In the antheridium, haploid nuclei are formed. The antheridium grows into the oogonia and forms the fertilization tubes. The fertilization tubes penetrate the oospheres, transferring the haploid nuclei.

These nuclei fertilize the oospheres, giving rise to a thick-walled diploid oospore. The released oospore can remain in the medium for a long time before germinating and producing a hypha that will rapidly produce a sporangium.

Nutrition

Many oomycetes are saprophytes, others are parasites. Some species combine both lifestyles. Parasitic species have adapted to parasitize different groups of organisms, such as plants, nematodes, vertebrates and crustaceans.

Saprophytic organisms perform an external digestion of their food, secreting enzymes, and subsequently absorbing the dissolved molecules resulting from digestion.

Parasitic oomycetes can be biotrophic, hemibiotrophic, or necrotrophic. Biotrophic species obtain their nutrients from living tissues by means of a specialized hypha called a haustorium.

Hemibiotrophs first feed on living tissue and kill their host at a later stage. Necrotrophs secrete toxins and enzymes that kill host cells and then obtain nutrients from them.

Reproduction

Asexual

Oomycetes reproduce asexually by means of sporangia. The sporangia form biflagellate spores called zoospores. In oomycetes there can be two types of zoospores, primary and secondary.

The primaries have the flagella inserted at the apex. Secondary zoospores, reniform in appearance, have flagella inserted laterally. In some cases, the sporangia do not form spores, but germinate directly. This is considered an adaptation to terrestrial life.

Sexual

Sexual reproduction occurs through oogamy. The production of sex gametes occurs in the gametangia. The female gametangium, or oogonium, is generally large and will, by meiosis, produce several oospheres. The male, or antheridium, will produce haploid nuclei.

The antheridium will grow towards the oogonium and will introduce, through fertilization tubes, the haploid nuclei into the oogonium. The way the antheridium is attached to the oogonium can vary.

In some cases, the antheridium joins the oogonium laterally, being called the paragyne. In others, the male gamentagium surrounds the base of the oogonium (amphiginum). Fusion of the male haploid nucleus with the nucleus of the oosphere to give rise to a diploid oospore occurs in the oogonium.

Diseases

In plants

Some of the better known diseases caused by oomycetes in plants include late blight of potatoes, downy mildew of grapes, sudden death of oak, and root and stem rot of soybeans.

During infection, these pathogens achieve colonization of their hosts, modulating plant defenses through a series of disease effector proteins.

These effectors are classified into two classes based on their target sites. Apoplastic effectors are secreted into the extracellular space of the plant. Cytoplasmics, on the other hand, are introduced into the plant cell through the haustoria of the oomycete.

The genus Phytopthora includes hemibiotrophic (eg P. infestans, P. sojae) and necrotrophic (eg P. cinnamomi) phytopathogens. Species of this genus have had a severe impact on agriculture, Phytophora infestans, the cause of late blight in potatoes and responsible for the Great Famine of the 1940s, can infect a variety of plant species other than potatoes, such as tomatoes and soybeans. This species can infect the entire plant, tubers, roots or leaves, leading to the death of the plant.

Phytophthora ramorum, for its part, produces the infection called sudden oak death, which affects these and other trees and shrubs causing rapid death.

Other phytopathogens

Plasmopara viticola, the cause of vine downy mildew, was introduced from North America to Europe in the late 19th century. It is characterized by attacking foliage and clusters.

The symptoms on the leaves are yellow lesions with fuzzy edges, 1 to 3 cm in diameter. As the disease progresses, it can produce necrosis of the leaves and even complete defoliation of the plant.

Plasmopara vitícola. Causative of downy mildew on the vine. Taken and edited from

Aphanomyces euteiches causes root rot in many legumes. It is considered the pathogen that most limits the yield of pea crops in some parts of the world. Other species of this genus affect animals, both terrestrial and aquatic habitats.

In animals

Aphanomyces astaci is a specific parasite of the crayfish, highly pathogenic for European species. It has caused the disappearance of a large part of the European populations of crustaceans of the family Astacidae.

Oomycete zoospores are attracted to chemical signals from the crustacean and encyst on the crab cuticle. The cysts germinate and produce a mycelium that rapidly grows in the cuticle, until it reaches the internal body cavity. Once the internal tissues are reached, the crustacean dies within 6 to 10 days.

Members of the genus s aprolegnia cause the group of diseases called saprolegniosis that attack fish or their eggs. Among them, ulcerative dermal necrosis is one of the most important diseases that affect salmonid species. This disease greatly affected salmon populations in British rivers in the late 19th century.

Saprolegnioses are characterized by white or gray spots of filamentous mycelium on the fish. The infection begins in the epidermal tissue and can spread inward.

It can also parasitize eggs and is often visible as a cottony white mass on the surface of eggs or fish in home aquariums. Recently, s aprolegnia ferax was related to declining amphibian populations.

Pythiosis is a disease caused by the oomycete Pythium insidiosum. This disease is characterized by granulomatous lesions on the skin, gastrointestinal tract, or in various organs.

Oomycete zoospores develop in stagnant waters in the tropics and subtropics and enter the host through skin wounds. Once they have reached the host, the zoospores encyst and invade the host tissue. It affects horses, cats, dogs, and occasionally humans.

References

1. GW Beakes, S. Sekimoto (2009). The evolutionary phylogeny of oomycetes-insights gained from studies of holocarpic parasites of algae and invertebrates. In: K. Lamour, S. Kamoun (Eds.), Oomycete genetics and genomics: diversity, interactions, and research tools. John Wiley & Sons, Inc.
2. HS Judelson (2009) Sexual reproduction in oomycetes: biology, diversity, and contributions to fitness. In: K. Lamour, S. Kamoun (Eds.), Oomycetegenetics and genomics: diversity, interactions, and research tools. John Wiley & Sons, Inc.
3. S. Kamoun (2003). Molecular genetics of pathogenic Oomycetes. Eukaryotic Cell.
4. J. Makkonen (2013). The crayfish plague pathogen Aphanomyces astaci. Genetic diversity and adaptation to the host species. Publications of the University of Eastern Finland. Dissertations in Forestry and Natural Sciences No 105
5. S.-K. Oh, S. Kamoun, D. Choi. (2010). Oomycetes RXLR effectors function as both activator and suppressor of plant immunity. The Plant Pathology Journal.
6. B. Paula, MM Steciow (2004). Saprolegnia multispora, a new oomycete isolated from water samples taken in a river in the Burgundian region of France. FEMS Microbiology Letters.