RHIZOPUS STOLONIFER: CHARACTERISTICS, MORPHOLOGY AND LIFE CYCLE - BIOLOGY - 2024

Rhizopus stolonifer is a cosmopolitan phytopathogenic fungus belonging to the Mucoraceae family, capable of growing and developing in a wide variety of environments. It is a fast growing species, which favors the colonization and decomposition of stored food or agricultural products.

This species constitutes one of the most common fungi, distributed globally, although it is more frequent in tropical and subtropical areas. In fact, it is a saprophytic organism that intervenes in the colonization of the soil substrate and acts as a parasite, invading decomposing plant tissues.

Rhizopus stolonifer. Source: WDKeeper

Commonly known as "black bread mold", it is characterized by a dense branched mycelium, composed of three types of hyphae: stolons, sporangiophores and rhizoids. The sporangia generate numerous multinucleated spores that are involved in asexual reproduction, unlike sexual reproduction that requires compatible but physiologically different mycelia.

At an industrial level the fungi of the genus Rhizopus, including R. stolonifer, are used for the production of ethyl alcohol product of fermentation. In addition, this species is used commercially to obtain high purity lactic acid and fumaric acid used in the chemical, food and pharmaceutical industries.

General characteristics

Distribution and habitat

The zygomycota fungus Rhizopus stolonifer is one of the most common mucorals, since it is widely distributed throughout the planet. Indeed, it is one of the first fungi to appear on stale bread, rotting vegetables and fruits, being a careful organism in perishable foods.

Rhizopus stolonifer is one of the first fungi present in bread. Source: Matt Wharton

It thrives effectively in a temperature range between 12 and 32º C, the optimum growth temperature being 25º C. In fact, its spores are rare in fresh air environments, but they are abundant in humid environments and around decomposing biological products.

They are usually located on moist soil, in compost or manure, on disaggregated plant material or in accumulated dust. Likewise, on wood pulp, bird nests, honeycombs, or on various seeds and fruits.

Indeed, this species is capable of colonizing a wide variety of natural substrates, as it adapts to different concentrations of essential nutrients. In fact, you can use nitrogen and carbon simply or in combination with different nutrients.

At the laboratory level, Rhizopus stolonifer grows on various culture media, including those containing amino compounds and ammonium salts. However, it does not grow on media with high nitrate content, such as Czapek Dox Agar, which has nitrate as the only nitrogen source.

In in vitro culture, the zygospores of Rhizopus stolonifer germinate after 8-20 days at an average temperature of 21º C. In fact, the spores of this fungus require a period of rest prior to the germination process and mycelial development.

Pathologies in vegetables and food

Rhizopus stolonifer is the causal agent of the disease known as "soft rot of fruits", mainly in strawberry, peach and melon. These fruits with a high sugar and water content are infected by this fungus from physical damage or injuries caused during post-harvest handling.

During storage, the tissues of infected fruits soften, releasing a liquid with a strong acid odor. In fact, high temperature and humidity conditions favor the development of mycelium, which quickly covers the surface of the fruits.

Rhizopus stolonifer on a strawberry showing black sporangia. Source: Eric McKenzie

Sporangia and spores develop from the mycelium, which upon germination generate various esterase and cutinase enzymes that disintegrate the cell wall of adjacent tissues. In this way, the disease spreads to neighboring healthy fruits, with the intervention of insects or the wind.

Laboratory tests have made it possible to determine that the pathogenic action of Rhizopus stolonifer is due to tissue maceration of stored products. Being the enzymatic complexes of polygalacturonase (PG) and pectinmethylesterase (PME) responsible for said maceration in decomposing fruits, vegetables or tubers.

In this way, the affected foods acquire a dark brown hue, due to the extensive accumulation of dark sporangia on the mycelium. On many occasions this symptom is often confused with the attack induced by Monilia fructicola, the causal agent of "brown rot."

Pathologies in man

For humans, the high environmental concentration of Rhizopus spores causes a type of respiratory tract allergy. This disease is known as hypersensitivity pneumonitis or extrinsic allergic alveolitis; similarly, some people may experience skin reactions.

Taxonomy

The Rhizopus stolonifer species is a cosmopolitan fungus taxonomically located as:

- Fungi kingdom

- Phylum: Zygomycota

- Division: Mycota

- Subdivision: Mycotina

- Class: Zygomycetes

- Order: Mucorales

- Family: Mucoraceae

- Genus: Rhizopus

- Species: Rhizopus stolonifer Vuillemin (1902)

Synonyms

- Rhizopus artocarpi

- Rhizopus nigricans Ehrenberg (1820)

- Mucor stolonifer Ehrenberg (1818)

- Rhizopus artocarpi var. luxurians Schroet (1886)

- Mucor niger Geodoelst (1902)

- Rhizopus niger Ciaglinski & Hewelke (1893)

Morphology

Rhizopus stolonifer is a filamentous fungus that presents unbranched sporangiophores, 2 mm long by 20 μm thick. This aerial hypha, peduncle or sporangiophore emerges from well-developed rhizoids and is dark brown in color.

The sporangia are spherical in shape and shiny black in color, measure 250-280 μm in diameter and have a structure called columella. The sporangiospores or asexual spores typical of the fungi of the genus Rhizopus are black in color and measure 15-18 μm.

Rhizopus stolonifer is a species of fungus in the Mucoraceae family. Source: Trakunsukharat, P., Beasley, DR, Shivas, RG (2011)

This species has a very broad rhizoid, and abundant zygospores or sexual part of the fungus. Indeed, zygospores are spherical structures, thick and smooth cell walls, approximately 200 μm in diameter.

The colonies of Rhizopus stolonifer present a rapid growth, its culture in the laboratory completely covers the Petri dish in just three days at 25º C. The structure of the colony is firm in appearance, with dense and cottony aerial mycelium, initially white and then later. shades of gray, reddish or brown.

Indeed, its dynamic development is favored by the presence of stolons that provide an area structure that supports the growth of the mycelium. In this way, the fungus quickly occupies large areas, either in a horizontal or vertical position.

Rhizopus stolonifer is a facultative aerobic organism that grows widely on succulent tissues with an abundant concentration of sugars. In this way, the fungus accumulates a reserve for the moment of developing fertile hyphae with external aerobic sporangia.

Life cycle

Rhizopus stolonifer reproduces sexually or asexually. In fact, it is a heterothalic organism, which for its sexual reproduction requires the mating of two thalli of different charges to create a sexed spore.

Sexual reproduction

Sexual reproduction occurs under unfavorable conditions with the mating of compatible strains, ultimately resulting in the zygospore. Indeed, sporangiophores harbor mating strains of the positive "+" or negative "-" type, which facilitates their union.

In Rhizopus stolonifer, sexual reproduction begins when the progametangia or specialized hyphae of two strains of different sign fuse. This attraction between dissimilar progametangia occurs with the intervention of diffused hormones in the form of gases.

Next, two gametangia, or apical cells, arise, each containing abundant "+" nuclei and the other containing "-" nuclei. The gametangia fuse, causing the union of numerous pairs of nuclei "+" and "-", forming diploid nuclei.

In this way, the zygosporangium is produced, a product of multinucleated cells with a firm, pigmented and rough cell wall, which contains a single zygospore. Under favorable environmental conditions, diploid nuclei undergo the meiosis process just before germination occurs.

During germination, the zygosporangium wall breaks, releasing the zygospore, giving rise to the sporangiophore. At the end of the sporangiophore a sporangium is located that will give rise to the spores, which once germinated will form a "+" or "-" mycelium.

The Rhizopus stolonifer fungus behaves like a haploid organism for most of the sexual reproduction cycle. In addition, the mycelium is made up of numerous branched hyphae that fulfill the function of support and nutrition of the fungus.

Sexual and asexual cycles of Rhizopus stolonifer. Source: Pancrat

Asexual reproduction

The asexual cycle occurs under favorable conditions when the production of sporangiophores begins from sporangia containing sexually compatible spores. The dispersal form of the spores is favored by the wind, as the sporangia mature, their thin wall disintegrates releasing the spores.

Aerial hyphae are generated from the internodes and grow to a certain height. The nuclei and cytoplasm are then grouped towards the apical end, promoting the growth of the apex of the aerial hyphae.

This area grows globularly giving rise to a rounded sporangium, formed by the sporoplasm and the columella. The sporoplasm nuclei undergo a rapid division until they transform into spongiospores.

As these structures mature, the columella disintegrates, releasing large amounts of sporangiospores into the atmosphere. In a suitable substrate and under favorable temperature and humidity conditions these spores germinate.

In this way, a new complex of hyphae is formed that will expand to continue the cycle. Under adverse conditions, septa are produced in the intercalary mycelium, giving rise to a plump spore called chlamydiospora.

References

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2. Karki, G. (2018) Rhizopus stolonifer: morphology and reproduction of black bread mold. Recovered at: onlinebiologynotes.com
3. Petruzzello, M. (2016) Rhizopus. Encyclopædia Britannica, inc. Recovered at: britannica.com
4. Pontón, J., Moragues, MD, Gené, J., Guarro, J., & Quindós, G. (2002). Allergenic fungi and actinomycetes. Bilbao: Ibero-American Journal of Mycology.
5. Rhizopus. (2019). Wikipedia, The Free Encyclopedia. Recovered at: es.wikipedia.org
6. Rhizopus stolonifer (Ehrenberg: Fries) Vuillemin (2002) RevIberoamMicol. P. 38.
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