Lentinus tuber-regium (Fr.) Fr. 1836

Pleurotus tuberregium (Fr.) Singer, commonly known as the King Tuber Mushroom, is a basidiomycete belonging to the Pleurotaceae family.¹¹ It is found in tropical regions including equatorial Africa, Australia, Asia, and other areas in the south Pacific.⁶ This white rot fungus produces ligninolytic, or wood decaying enzymes, which also give the mushroom potential for the mycoremediation of soils contaminated with crude oil or diesel fuel.^3,5,9 A character of this mushroom which is unique to the Pleurotaceae family, and very few other fungi, is the production of nematotoxins to paralyze and digest nematodes.⁶ Pleurotus tuberregium is also unique in that is produces a sclerotium, an underground mass of mycelia used to store nutrients.¹⁰ Research examining this fungus has been done predominantly in Nigeria where it is utilized in many tribes as a significant food source as well as a remedy for various ailments.¹⁰

Basidiocarps are characterized by a scaled, infundibuliform pileus, decurrent gills, and a well-developed stipe.³ Tissues are dimitic, composed of both generative and thick-walled skeletal hyphae.² Spores produced are cylindrical, smooth, and hyaline in appearance.¹ Pleurotus tuberregium is a unique member of the genus Pleurotus in that it forms an underground mass of mycelium called a sclerotium. This tuber-like structure is used for the storage of food products and also gives rise to new fruiting bodies.³ Sclerotia are dark brown with a white interior, spherical to ovoid in form, and may be found upwards of 30 cm in diameter.³ These structures allow P. tuberregium to be more resistant to periods of drought, and support the fruiting of multiple basidiocarps over several growing seasons.²

The fact that P. tuberregium grows easily when kept in a warm, moist environment, and does not require advanced knowledge or training to produce, may make it an ideal fungus to be cultivated on a larger scale in underdeveloped countries.¹ However, enzyme activity is found to vary depending on the substrate on which the mushroom grows.¹ For example, some enzymes are produced in higher quantities when grown on cotton waste rather than a sawdust substrate.¹ This is an important consideration when choosing a substrate for cultivation, because enzyme composition and concentration affects the nutrient and flavor profiles of the fungus.¹

Pleurotus tuberregium is found in tropical countries or regions, including equatorial Africa, Asia, northern Australia, and south Pacific regions, preferring a warm and humid environment for growth.² Primarily a saprotroph, it is a white rot fungus that can be found on dead or decaying wood, where enzymes break down the darker colored lignin, leaving behind white stringy wood debris. Basidiocarps also develop from underground sclerotia and emerge from the soil.³ Pleurotus tuberregium additionally derives some nutrition through carnivory. These fungi are nematophagous, and therefore have evolved methods of trapping and digesting nematodes. In the case of P. tuberregium, secretory processes are formed that discharge nematotoxic droplets, paralyzing nematodes.¹ Hyphae then colonize and digest the organism, which provides an excellent source of nitrogen. Growth for this fungus occurs between 15˚ and 40˚ Celsius (C), with optimal growth at 35˚ C.³

Pleurotus tuberregium, like other white rot fungi, produces enzymes that allow them to degrade not only wood, but a wide range of compounds including some harmful environmental contaminants. Some of these ligninolytic enzymes include laccase and manganese peroxidase, although P. tuberregium produces many non-ligninolytic enzymes as well.^1,2 Attributed to these enzymatic properties, studies support the ability of P. tuberregium to remediate soils contaminated with toxic hydrocarbons from crude oil and diesel fuel.^1,3 As an indigenous fungus to Nigeria, an oil-producing country, P. tuberregium may provide an inexpensive and accessible method of remediating contaminated land for use in farming.³

This fungus, particularly the sclerotium, is an important dietary component for the people of Nigeria due to its nutritional value. The composition of the sclerotium was determined to be high in protein, about 64% of the total mass, with a 100g serving providing up to 141% of the recommended daily nutritional value for protein.¹ It is also a carbohydrate source (20% of total mass), and provides crude fiber (2.89%), with a 100g serving providing up to 13% of the daily recommended value.¹ Compositional analyses have also shown P. tuberregium sclerotia to be rich in essential nutrients including K, Mg, Ca, and Na.³ However, they were also found to contain low concentrations of potentially toxic elements such as Ba, Rb, Sr, Cd, Cu, Ni, Pb, and Hg.³ These were not found in harmful amounts, although some samples varied in Hg and Pb concentrations, suggesting some anthropogenic sources may affect sclerotial composition based on location.³ It has also been shown that the pileus and stipe do not inherently pose any health risks in terms of Hg, and the sclerotium is not an efficient accumulator of this element.²

Pleurotus tuberregium (Fr.) Singer, commonly known as the King Tuber Mushroom, was first collected in Africa and described by Elias Magnus Fries. It was later described and reclassified by Rolf Singer in 1951.³ This fungus has been previously classified under the genera Lentinus and Panus based on the presence of intercalary skeletal hyphae in basidiocarps.¹ However, molecular studies, as well as the characteristic production of nematotoxins within basidiocarps to paralyze nematodes for consumption, support the placement of this fungus within the genus Pleurotus.² This character is useful for classification as it is found in other species of the Pleurotaceae family as well.¹ Mating is not compatible between P. tuberregium and other members of Pleurotus, further supporting its classification as a unique species.² Mating is, however, supported between isolates of P. tuberregium from different geographic lineages.² Phylogenetic studies indicate sequence divergence supporting at least two evolutionary and geographic lineages of African and “Australasian” (region including Australia, Asia, and other south Pacific localities) members of this species.²

Pleurotus tuberregium is found throughout parts of Africa where both the sclerotium and the basidiocarp are economically important to many tribes, as a significant food source as well as for medicinal uses.² The sclerotium is often cut up or ground and made into soup.² The mushrooms are generally eaten when young, as older mushrooms develop skeletal hyphae which create a tough and leathery consistency.¹ Mushrooms are cut up and added to soups and other dishes.² Tribes in Nigeria refer to P. tuberregium with distinct culturally-influenced names, such as Ohu, referring to its growth habit, Utun weromon, meaning “mushroom soup”, and Ofuako, meaning “teeth-cleaner”.² This fungus is used tribally to treat many ailments, from headache, fever, chest pain, and small pox, to spiritual protection against evil charms.² Pleurotus tuberregium also has some spiritual significance within the tribes, believed to be originally introduced to the land by a divine individual.²