In 1991, a mummified body was discovered in the Val Senales glacier in Italy. The man (named Ӧtzi the Iceman), who lived 5300 years ago, carried two fragments of a fruiting body of Fomitopsis betulina (formerly Piptoporus betulinus). Some scientists believe that Ӧtzi might have used the fungus for medical purposes (Capasso 1998) and, although the idea arouses some controversy (Pöder 2005), the long tradition of the use of F. betulina in folk medicine is a fact (Reshetnikov et al. 2001; Wasser 2010). Infusion from F. betulina fruiting bodies was popular, especially in Russia, Baltic countries, Hungary, Romania for its nutritional and calming properties. Fungal tea was used against various cancer types, as an immunoenhancing, anti-parasitic agent, and a remedy for gastrointestinal disorders (Grienke et al. 2014; Lucas 1960; Peintner and Pöder 2000; Semerdžieva and Veselský 1986; Shamtsyan et al. 2004). Antiseptic and anti-bleeding dressings made from fresh F. betulina fruiting body were applied to wounds and the powder obtained from dried ones was used as a painkiller (Grienke et al. 2014; Papp et al. 2015; Rutalek 2002).

In the present paper, we have shown the current knowledge of the fungus F. betulina, including its lifestyle, chemical composition, and potential in biotechnology.

Piptoporus betulinus (Bull.) P. Karst. (known as birch polypore, birch bracket, or razor strop) is a common Basidiomycota brown rot macrofungus growing on decaying birch wood. Homobasidiomycetes were divided into eight clades. The family Polyporaceae with the genus Piptoporus was classified to the polyporoid clade, and then the antrodia clade—the Fomitopsis-Daedalea-Piptoporus group comprising brown rot fungi was identified within this clade (Hibbett and Donoghue 2001; Hibbett and Thorn 2001). Further studies of the phylogenetic relationships among members of the antrodia clade revealed polyphyly of the Fomitopsis genus and suggested that P. betulinus was phylogenetically closer to Fomitopsis than to Piptoporus (Kim et al. 2005; Ortiz-Santana et al. 2013). Recently, P. betulinus (Bull.) P. Karst. has been transferred to Fomitopsis (Han et al. 2016) and, according to Index Fungorum (2016), is classified in the genus Fomitopsis, family Fomitopsidaceae, order Polyporales, class Agaricomycetes, division Basidiomycota, kingdom Fungi, with the current name Fomitopsis betulina (Bull.) B.K. Cui, M.L. Han and Y.C. Dai, comb.nov. (MycoBank no.: MB 812646).

Fomitopsis betulina is characterized by annual, sessile to effused-reflexed, tough to woody hard basidiocarps, white to tan or pinkish-colored pore surface with mostly small and regular pores. Fruiting bodies grow singly or in small groups, are covered with a laccate, glabrous crust, never zonate, young cream to white, later ochraceous-brown to greyish brown (Fig. 1a). The mycelium of F. betulina developing on agar media is white, relatively homogeneous, downy-felt, with regular colony edges (Fig. 1b). The hyphae develop radially. The hyphal system is mostly dimitic. The clamped generative hyphae, 1.5–3.5 µm in diameter, are branched and hyaline whereas the skeletal hyphae with the diameter of 3– 4 µm, are less branched and have thicker walls. No primordia or fruiting bodies of this species were found in vitro (Petre and Tanase 2013). Basidiospores are smooth, hyaline, thin-walled, and cylindrical (Han and Cui 2015; Han et al. 2016; Kim et al. 2005; Schwarze 1993).

The birch polypore grows mainly as a saprophyte on dead trees and occasionally as a parasite of living trees. It occurs in northern temperate forests and parks in Europe, North America, and Asia. The host range of the fungus is restricted exclusively to birch species, e.g. Betula pendula Roth., B. pubescens Ehrh., B. papyrifera Marsh., and B. obscura Kotula (Schwarze 1993; Žižka et al. 2010).

Wood rotting fungi are traditionally divided into white and brown rot species based on the structure and composition of residual wood. Brown rot fungi extensively degrade the carbohydrate fraction of lignocellulose but, in contrast to white rot fungi, leave lignin, although in a modified form. In these fungi, chemical depolymerization of cellulose, which precedes and supports its enzymatic degradation, is very important. They lack ligninolytic peroxidases and usually some other enzymes such as processive cellobiohydrolases used for degradation of crystalline cellulose, but contain H₂O₂-generating oxidases and Fe³⁺- and quinone-reducing enzymes used for non-enzymatic depolymerization of polysaccharides (Arantes and Goodell 2014; Baldrian and Valášková 2008; Hori et al. 2013). Modern phylogenetic evidence suggest, however, that there is no sharp distinction between the two groups of fungi (Hori et al. 2013; Riley et al. 2014).

Fomitopsis betulina is one of the most common brown rot species but its wood-decaying mechanism has been tested only fragmentarily (Meng et al. 2012) and is still poorly understood. As other fungi of this type, it degrades wood to yield brown, cubical cracks easily broken down. Many factors, including microflora or compounds present in wood, contribute to this complex process (Przybył and Żłobińska-Podejma 2000; Song et al. 2016; Zarzyński 2009). Shang et al. (2013) showed that wood samples decayed by F. betulina lost 57% of dry weight (dw) and 74% of holocellulose after 30 days, whereas the fungus growing on wheat straw causes 65% loss of dw within 98 days of culture (Valášková and Baldrian 2006a). A set of enzymes of F. betulina involved in the degradation of lignocellulose was characterized in detail by Valášková and Baldrian (2006a, b). The fungus growing on straw produced enzymes with wide substrate specificities: (1→4)-β-endoglucanase, β-glucosidase, (1→4)-β-endoxylanase, (1→4)-β-endomannanase, (1→4)-β-xylosidase, and (1→4)-β-mannosidase. The activities of ligninolytic enzymes and cellobiose dehydrogenase for oxidoreductive cleavage of cellulose were not detected. Similar results were obtained in liquid cultures by Vĕtrovský et al. (2013). When F. betulina grew in nature, β-glucosidase and β-mannosidase activity was associated with the fruiting bodies while endopolysaccharidases were detected in colonized wood (Valášková and Baldrian 2006a).

Carpophores of F. betulina from natural habitats or mycelium and culture liquid from submerged cultures were used as raw material to obtain extracts and bioactive substances with medicinal properties (Table 1) (Lomberh et al. 2002). Studies concerning the mycelium growth rate in the presence of various substances (metals, dyes) were conducted mainly on agar media or in liquid cultures (Baldrian and Gabriel 2002; Dresch et al. 2015; Hartikainen et al. 2016). The yield of F. betulina mycelium was established in liquid cultures with addition of some agricultural wastes in the studies of Krupodorova and Barshteyn (2015). The enzymatic activity of F. betulina was studied in laboratory conditions on agar media (Krupodorova et al. 2014), in liquid cultures (Vĕtrovský et al. 2013), on wheat straw (Valášková and Baldrian 2006a, b), and on Betula sp. wood samples (Reh et al. 1986; Shang et al. 2013).

There are limited data on small- or large-scale cultivation of this species in which carpophores could be obtained in controlled conditions. The first such report referring to outdoor log cultivation of F. betulina on Betula davurica Pallas originated from Korea (Ka et al. 2008). Logs with a diameter of 8–18 cm and length of 107–135 cm were inoculated and then cultured in natural conditions. The yield obtained was in the range from 212 to 1298 g fresh weight (1–2 mushrooms per log). Development of fruiting bodies took an average of 18 months. The ratio of log yield was estimated at 2.8–6.1%. The only report on indoor production of F. betulina fruiting bodies was given by Pleszczyńska et al. (2016). In the study, four strains of F. betulina isolated from natural habitats were applied. Their mycelia were inoculated into birch sawdust supplemented with organic additives. Mature fruiting bodies weighing from 50 to 120 g were obtained from only one strain, after 3–4 months of the cultivation in artificial conditions (Fig. 1c). The biological efficiency ranged from 12 to 16%. It was shown that extracts isolated from cultivated and naturally grown F. betulina fruiting bodies had comparable biological activity (Table 1).

With the knowledge of the mechanisms of action of brown rot decay, there are possibilities of new applications of these fungi in biotechnology. The enzymatic and non-enzymatic apparatus for lignocellulose degradation can be used for bioprocessing of biomass towards fuels and chemicals (Arantes et al. 2012; Giles and Parrow 2011; Ray et al. 2010). Brown rot fungi, including F. betulina, were tested for bioleaching of heavy metals (Cu, Cr, and As) from wood preservatives due to accumulation of metal-complexing oxalic acid (Sierra Alvarez 2007). Production of biomass degrading enzymes, for instance cellulases, hemicellulases, amylases, etc., was also studied (Krupodorova et al. 2014; Valášková and Baldrian 2006a, b).

The cell wall of F. betulina can be a source of useful polysaccharides, e.g. water-insoluble, alkali-soluble α-glucans (Grün 2003; Jelsma and Kreger 1979). (1→3)-α-d-glucans whose main chain contains 84.6% of (1→3)-linked α-d-glucopyranose in addition to 6% of (1→4)-linked units were purified and characterized by Wiater et al. (2011). Another polysaccharide, named piptoporane I, was extracted and purified by Olennikov et al. (2012). This α-glucan was built from residues of (1→3)-α-d-glucopyranose with occasional branching by single residues of β-d-glucopyranose at the C₆ position (17.3%). It has been shown that fungal (1→3)-α-d-glucans, including that from F. betulina, effectively induce the production of microbial (1→3)-α-glucanases (mutanases), i.e. enzymes that have potential in dental caries prevention. (1→3),(1→6)-α-d-Glucans (mutans) synthesized by mutans streptococci are key structural and functional constituents of dental plaque matrix; therefore, they seem to be a good target for enzymatic anti-caries strategy (Pleszczyńska et al. 2015). However, streptococcal glucans are difficult to use as inducers of mutanases because of the low yield and structural variation. Birch polypore α-glucan, whose amount in the cell wall of F. betulina reaches even 44–53% dw (Grün 2003), can be used to replace streptococcal glucans (Wiater et al. 2008).

The F. betulina fungus has been widely used and appreciated in folk medicine, and modern pharmacological studies have confirmed its potential indicating significant antimicrobial, anticancer, anti-inflammatory, and neuroprotective activities. The possibility of successful cultivation thereof in artificial conditions additionally promotes the applicability of the fungus. However, compared with other polypore fungi, the research on F. betulina is less developed; for instance, little is known about its lifestyle, including the wood degradation strategy. Moreover, most of the bioactivity studies have been performed using crude extracts; hence, only a few of the effects have been associated with the active substances identified, e.g. antibacterial activities with piptamine or polyporenic acids. With a few exceptions, we still do not know the mechanisms underlying the biological activities. Verification of biological activities in in vivo and clinical studies is also required. The further research could contribute to better exploitation of the F. betulina application potential.