Truffle ##VERIFIED##
Tiziana Betance
A truffle is the fruiting body of a subterranean ascomycete fungus, predominantly one of the many species of the genus Tuber. In addition to Tuber, over one hundred other genera of fungi are classified as truffles including Geopora, Peziza, Choiromyces, and Leucangium.[1] These genera belong to the class Pezizomycetes and the Pezizales order. Several truffle-like basidiomycetes are excluded from Pezizales, including Rhizopogon and Glomus. Truffles are ectomycorrhizal fungi, so they are usually found in close association with tree roots. Spore dispersal is accomplished through fungivores, animals that eat fungi.[2] These fungi have significant ecological roles in nutrient cycling and drought tolerance.
Some truffle species are highly prized as food. French gastronome Jean Anthelme Brillat-Savarin called truffles "the diamond of the kitchen".[3] Edible truffles are used in Italian, French[4] and numerous other national haute cuisines. Truffles are cultivated and harvested from natural environments.
The black truffle or black Périgord truffle (Tuber melanosporum), the second-most commercially valuable species, is named after the Périgord region in France.[5] Black truffles associate with oaks, hazelnut, cherry, and other deciduous trees and are harvested in late autumn and winter.[5][6] The genome sequence of the black truffle was published in March 2010.[7]
The black summer truffle (Tuber aestivum) is found across Europe and is prized for its culinary value.[8] Burgundy truffles (designated Tuber uncinatum, but the same species) are harvested in autumn until December and have aromatic flesh of a darker colour. These are associated with various trees and shrubs.[8]
Tuber magnatum, the high-value white truffle (tartufo bianco d'Alba in Italian) is found mainly in the Langhe and Montferrat areas[9] of the Piedmont region in northern Italy, and most famously, in the countryside around the cities of Alba and Asti.[10] A large percentage of Italy's white truffles also come from Molise.
The "whitish truffle" (Tuber borchii) is a similar species native to Tuscany, Abruzzo, Romagna, Umbria, the Marche, and Molise. It is reportedly not as aromatic as those from Piedmont, although those from Città di Castello are said to come quite close.[6]
In the U.S. Pacific Northwest, several species of truffle are harvested both recreationally and commercially, most notably, the Leucangium carthusianum, Oregon black truffle; Tuber gibbosum, Oregon spring white truffle; and Tuber oregonense, Oregon winter white truffle. Kalapuya brunnea, the Oregon brown truffle, has also been commercially harvested and is of culinary note. The Oregon white truffle is increasingly harvested due to its high quality and also exported to other countries. Oregon celebrates its traditional truffle harvesting with a 'truffle festival', combined with culinary shows and wine tastings.[11]
The pecan truffle (Tuber lyonii)[12] syn. texense[13] is found in the Southern United States, usually associated with pecan trees. Chefs who have experimented with them agree "they are very good and have potential as a food commodity".[14] Although pecan farmers used to find them along with pecans and discard them, considering them a nuisance, they sell for about $160 a pound and have been used in some gourmet restaurants.[15]
The term "truffle" has been applied to several other genera of similar underground fungi. The genera Terfezia and Tirmania of the family Terfeziaceae are known as the "desert truffles" of Africa and the Middle East. Pisolithus tinctorius, which was historically eaten in parts of Germany, is sometimes called "Bohemian truffle".[16]
Geopora spp. are important ectomycorrhizal partners of trees in woodlands and forests throughout the world.[1] Pinus edulis, a widespread pine species of the Southwest US, is dependent on Geopora for nutrient and water acquisition in arid environments.[17] Like other truffle fungi, Geopora produces subterranean sporocarps as a means of sexual reproduction.[17] Geopora cooperi, also known as pine truffle or fuzzy truffle, is an edible species of this genus.[1]
Rhizopogon spp. are ectomycorrhizal members of the Basidiomycota and the order Boletales, a group of fungi that typically form mushrooms.[18] Like their ascomycete counterparts, these fungi can create truffle-like fruiting bodies.[18] Rhizopogon spp. are ecologically important in coniferous forests where they associate with various pines, firs, and Douglas fir.[19] In addition to their ecological importance, these fungi hold economic value, as well. Rhizopogon spp. are commonly used to inoculate coniferous seedlings in nurseries and during reforestation.[18]
Hysterangium spp. are ectomycorrhizal members of the Basidiomycota and the order Hysterangiales that form sporocarps similar to true truffles.[20] These fungi form mycelial mats of vegetative hyphae that may cover 25-40% of the forest floor in Douglas fir forests, thereby contributing to a significant portion of the biomass present in soils.[20] Like other ectomycorrhizal fungi, Hysterangium spp. play a role in nutrient exchange in the nitrogen cycle by accessing nitrogen unavailable to host plants and acting as nitrogen sinks in forests.[19]
Phylogenetic analysis has demonstrated the convergent evolution of the ectomycorrhizal trophic mode in diverse fungi. The subphylum, Pezizomycotina, containing the order Pezizales, is approximately 400 million years old.[22] Within the order Pezizales, subterranean fungi evolved independently at least fifteen times.[22] Contained within Pezizales are the families Tuberaceae, Pezizaceae, Pyronematacae, and Morchellaceae. All of these families contain lineages of subterranean or truffle fungi.[1]
The evolution of subterranean fruiting bodies has occurred numerous times within the Ascomycota, Basidiomycota, and Glomeromycota.[1] For example, the genera Rhizopogon and Hysterangium of Basidiomycota both form subterranean fruiting bodies and play similar ecological roles as truffle forming ascomycetes. The ancestors of the Ascomycota genera Geopora, Tuber, and Leucangium originated in Laurasia during the Paleozoic era.[21]
The truffle habit has evolved independently among several basidiomycete genera.[27][28][29] Phylogenetic analysis has revealed that basidiomycete subterranean fruiting bodies, like their ascomycete counterparts, evolved from above ground mushrooms. For example, Rhizopogon species likely arose from an ancestor shared with Suillus, a mushroom-forming genus.[27] Studies have suggested that selection for subterranean fruiting bodies among ascomycetes and basidiomycetes occurred in water-limited environments.[21][27]
Most sources agree that the term "truffle" derives from the Latin term tūber or the Vulgar Latin tufera, meaning "swelling" or "lump".[30][31][32][33] This then entered other languages through Old French dialects.
The mycelia of truffles form symbiotic, mycorrhizal relationships with the roots of several tree species, including beech, birch, hazel, hornbeam, oak, pine, and poplar.[34] Mutualistic ectomycorrhizal fungi such as truffles provide valuable nutrients to plants in exchange for carbohydrates.[35] Ectomycorrhizal fungi cannot survive in the soil without their plant hosts.[22] In fact, many of these fungi have lost the enzymes necessary for obtaining carbon through other means. For example, truffle fungi have lost their ability to degrade the cell walls of plants, limiting their capacity to decompose plant litter.[22] Plant hosts can also depend on their associated truffle fungi. Geopora, Peziza, and Tuber spp. are vital in the establishment of oak communities.[36]
Tuber species prefer argillaceous or calcareous soils that are well drained and neutral or alkaline.[37][38][39] Tuber truffles fruit throughout the year, depending on the species, and can be found buried between the leaf litter and the soil. Most fungal biomass is found in the humus and litter layers of soil.[19]
Most truffle fungi produce both asexual spores (mitospores or conidia) and sexual spores (meiospores or ascospores/basidiospores).[40] Conidia can be produced more readily and with less energy than ascospores and can disperse during disturbance events. Production of ascospores is energy intensive because the fungus must allocate resources to the production of large sporocarps.[40] Ascospores are borne within sac-like structures called asci, which are contained within the sporocarp.
Because truffle fungi produce their sexual fruiting bodies underground, spores cannot be spread by wind and water. Therefore, nearly all truffles depend on mycophagous animal vectors for spore dispersal.[1] This is analogous to the dispersal of seeds in fruit of angiosperms. When the ascospores are fully developed, the truffle exudes volatile compounds that attract animal vectors.[1] For successful dispersal, these spores must survive passage through the digestive tracts of animals. Ascospores have thick walls composed of chitin to help them endure the environment of animal guts.[40]
Some mycophagous animals depend on truffles as their dominant food source. Flying squirrels, Glaucomys sabrinus, of North America play a three-way symbiosis with truffles and their associated plants.[1] G. sabrinus is particularly adapted to finding truffles using its refined sense of smell, visual clues, and long-term memory of prosperous populations of truffles.[1] This intimacy between animals and truffles indirectly influences the success of mycorrhizal plant species.
After ascospores are dispersed, they remain dormant until germination is initiated by exudates excreted from host plant roots.[42] Following germination, hyphae form and seek out the roots of host plants. Arriving at roots, hyphae begin to form a mantle or sheath on the outer surface of root tips. Hyphae then enter the root cortex intercellularly to form the Hartig net for nutrient exchange. Hyphae can spread to other root tips colonising the entire root system of the host.[42] Over time, the truffle fungus accumulates sufficient resources to form fruiting bodies.[42][36] Rate of growth is correlated with increasing photosynthetic rates in the spring as trees leaf out.[36]
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