BIOLOGY AND USES OF THE KING TUBER MUSHROOM
Okoli, Ifeanyi Charles
Introduction
The king tuber mushroom, scientifically known as Pleurotus tuber-regium is a tropical and subtropical mushroom that grows mostly on dead wood and produces an oyster mushroom-like fruit body and large storage ovoid or globular underground tuber or sclerotium which are both edible. It is native to tropical African countries, Asia, and Australia where traditionally it has economic value, especially as food and medicine. As a saprophyte, it consumes the dead wood on which it grows and produces the sclerotium either within the decaying wood or in the underlying soil. It is a primary decomposer and can be cultivated on lignocellulosic waste similar to other edible and medicinal mushrooms. Its growth was originally associated with the Daniellia tree in Africa, however recent studies have shown that it can be cultivated in an array of substrates such as sawdust, rice husk, corn cob, banana leaves, and loam soil among others. It is a slow-growing basidiomycete or club fungus that is a distinct species incapable of cross-breeding and phylogenetically removed from other species of Pleurotus. P. tuber-regium is also nematophagous, and therefore able to catch nematodes by paralyzing them with a toxin.
Botanically, Pleurotus tuber-regium is currently classified as follows;
Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Pleurotaceae
Genus: Pleurotus
Species: P. tuber-regium
It has also been known as Pachyma tuber-regium and Lentinus tuber-regium
P. tuber-regium is the only Pleurotus species in which the fruiting bodies arise from a sclerotium.
King tuber mushroom is one of the most heat-tolerant mushrooms. According to Oso, the temperature range for mycelial growth is 15 to 40OC with 35OC being optimal. The mushroom has gilled fruiting bodies and white spore print and grows up to 10 cm in height, resembling the oyster mushroom (Pleurotus ostreatus) at the early stage of development. In its natural habitat, P. tuber-regium first forms a thick bundle of mycelium underground, representing the sclerotium, then if the conditions are favourable, the fruiting bodies start to appear from the sclerotium above ground as light ochre mushrooms with funnel-shaped caps. These caps turn flat and then wavy after the mushrooms have reached maturity.
Formation of Sclerotium
Sclerotium structure is a dense aggregation of mycelia, which is usually formed in response to environmental stressors such as dehydration, microbial attack, or the long-term absence of a host. The formation and development of sclerotium have been divided into three stages: initiation, development, and maturation; and are accompanied by morphological and biochemical differentiation under strong genetic control. Sclerotial initiation is usually induced by several endogenous and exogenous factors such as the onset of starvation or depletion of nutrients and other conditions unfavourable to mycelial growth. When conditions improve, growth resumes, often with the formation of fruiting bodies. Okhuoya and Okogbo reported that under experimental conditions, it may take at least 65 days to produce sclerotium. In nature, however, the sclerotium typically forms in response to adverse growing conditions as a method of carrying the life of the fungus through difficult conditions. The sclerotium is usually dark brown on the outside and white on the inside and can be as large as 30 cm or more in diameter. It can be harvested and used to grow new mushrooms by simply planting it inside the soil or any other appropriate substrate. Okhuoya and Etugo reported that a sclerotium can be peeled and both the inner, edible white portion and outer peeling are used independently to produce mushrooms.
Uses and Economic Value
Both the fruiting bodies and the sclerotium of the king tuber mushroom have been used traditionally in West Africa and some parts of Asia for many centuries in the preparation of various food recipes and also in folk medicine. In Nigeria, it is known by different names among the different language groups in the country. It is called ero-usu in Igbo, orlu in Yoruba, katala in Hausa, and awu in Igala. Usu is used as a thickener in local soups and may be milled with melon seed and used to make special melon seed balls, which is an exotic delicacy of the Igbos. In China, P. tuber-regium is called hunai (literally, tiger’s milk) and grows mostly in the southern part of the country. Usually, the dark brown colour outermost part of the sclerotium is peeled to reveal the inner white part which is then used in food preparation. Oso in his early studies on the mushroom documented that traditionally, the fruiting bodies and the sclerotia may be simply chopped or ground and used in soups or broths or dried and used as additive flour or stored for future use. When mature, the fruiting body develops a tough, leathery consistency due to the presence of numerous thick-walled skeletal hyphae and therefore makes the mushroom unpalatable. Recent research findings have however shown that the non-digestible macro-molecules in the cell wall are rich sources of dietary fibre with biological functions that are beneficial to human health. In addition, Bamigboye and colleagues reported that the sclerotia powder has been used as a tablet disintegrant, and was successfully incorporated into pork sausage in earlier studies. Again, the sclerotium has been stored for up to seven years without losing its nutritional quality as a foodstuff or even its ability to produce fruiting bodies.
Okhuoya and Okogbo reported that in Nigeria P. tuber-regium is used in combination with various herbs and other ingredients in the treatment of ailments such as headache, stomach problems, colds, fever asthma, smallpox, and high blood pressure. In Ghana, it is used for similar purposes in addition to being used to influence weight gain in malnourished children. Other documented health benefits documented in recent studies include anti-diabetic, antioxidant, anti-inflammatory, anti-pathogenic, anti-tumours, and cancer properties as shown in figure 1. Specifically, it has also been explored as a photogenic source of glucosamine for use in the management of various joint, and bone inflammatory conditions. The sclerotium has been shown to contain considerable amounts of polysaccharides, especially beta-glucan, minerals, and phytochemicals such as calcium, potassium, magnesium, sodium, alkaloids, flavonoids, and tannins which are probably responsible for its medicinal properties.
Fig. 1: Propagation, nutritional and phytochemical properties of P. tuber-regium
(Image source: https://nuvedo.com)
Research Prospects
The bulk of the sclerotia sold in the market is presently collected from the wild by rural farmers and hunters, with a very limited effort at commercial cultivation. The sclerotia are usually stored for extended periods under ambient conditions by the farmers and sold during lean seasons to maximize profit. There is however growing research interest in innovative cultivation methods on various agricultural residues, the development of faster-growing strains, and elucidation of its medicinal and dietary fiber properties. Its cultivation on novel substrates could for example serve as an innovative approach to transforming agricultural wastes into useful products that would serve as new income sources for rural farmers.
Of additional interest is the application of P. tuber-regium in the production of animal feeds, organic fertilizers, and biogas. These concepts are currently receiving research attention and in one instance involve dietary incorporation of either the fruiting bodies and the sclerotia or their extracts as additives in the diets of monogastric animals to achieve better growth performance and product characteristics. Through its bio-degradation, P. tuber-regium is also being used to improve the nutritional value of locally abundant lignocellulosic materials such as rice offal, straws, and sawdust for ruminants, rabbits, pigs, and fish. The use of spent substrates from P. tuber-regium either directly as organic fertilizers or after further processing with other white-rot fungi or by vermicomposting is also being conceptualized. The fungus is also being studied for other potential applications such as the use of its sclerotium as a coagulant and disinfectant in natural and wastewater purification. Toxins produced by aerial hyphae of P. tuber-regium cultures on agar have been shown to paralyze nematodes that came in contact with it and then become colonized by hyphae, indicating its pesticide potential in agriculture. The antifungal activity of P. tuber-regium against filamentous fungi is also being exploited in the treatment of mycosis in mammals.
Interesting new studies have revealed several potential applications of P. tuber-regium such as its use in the bioremediation of petroleum-polluted environments and also the biodegradation of polyethylene plastic wastes. Again, being a heat-tolerant mushroom that can thrive at environmental temperatures as high as 40OC, it has been proposed as a viable crop for marginal environments where conventional crops might find it difficult to survive, indicating its potential value in both food security and environmental management. Overall, the mushroom is actively being studied in several research laboratories for its potential application in modern medicine.
Conclusion
P. tuber-regium is traditionally an edible and medicinal mushroom that grows mostly on dead wood to produce an underground sclerotium in many tropical countries. P. tuber-regium is currently receiving research attention on better cultivation methods, as well as potential applications in food, medicine, livestock, organic agriculture, and environmental management. Knowledge about such innovative uses of P. tuber-regium could be transferred to rural farmers to improve their livelihood and food security.
Bibliographic References
Bamigboye, C.O., Oloke, K.K. and Dames, J.F. (2019). Development of a high yielding strain of Pleurotus tuber-regium: fructification, nutritional and phylogenetic studies. J. Food Sci. Technol., 56(8): 3597 – 3608.
Hameed, J. (2016). A culinary and medicinal king– The king tuber oyster mushroom. https://nuvedo.com/2022/06/16/a-culinary-medicinal-king-the-king-tuber-oyster-mushroom/
Okhuoya, J.A. and Etugo, J.E. (1993). Studies of the cultivation of Pleurotus tuber-regium (FR) Sing. an edible mushroom. Bioresource Technology, 44: 1 - 3.
Okhuoya, J.A. and Okogbo, F.O. (1990). Induction of edible sclerotia of Pleurotus tuber-regium (FR) Sing. in the laboratory. Annals of Applied Biology, 117: 295 - 298.
Oso, B.A. (1977). Pleurotus tuber-regium from Nigeria. Mycologia, 69: 271 - 279.
Raman, J., Janga, K., Oha, Y., Oha, M., Ima, J., Lakshmananb, H. and Sabaratnam, V. (2021). Cultivation and nutritional value of prominent Pleurotus spp.: An overview. Mycobiology, 49(1): 1–14. https://doi.org/10.1080/12298093.2020.1835142