Polyporales

The Polyporales are an order of about 1800 species of fungi in the division Basidiomycota. The order includes some (but not all) polypores as well as many corticioid fungi and a few agarics (mainly in the genus ''Lentinus''). Many species within the order are saprotrophic, most of them wood-rotters. Some genera, such as '' Ganoderma'' and '' Fomes'', contain species that attack living tissues and then continue to degrade the wood of their dead hosts. Those of economic importance include several important pathogens of trees and a few species that cause damage by rotting structural timber. Some of the Polyporales are commercially cultivated and marketed for use as food items or in traditional Chinese medicine.

Taxonomy

History

The order was originally proposed in 1926 by Swiss mycologist Ernst Albert Gäumann to accommodate species within the phylum Basidiomycota producing basidiocarps (fruit bodies) showing a gymnocapous mode of development (forming the spore-bearing surface externally). As such, the order included the ten families Brachybasidiaceae, Corticiaceae, Clavariaceae, Cyphellaceae, Dictyolaceae, Fistulinaceae, Polyporaceae, Radulaceae, Tulasnellaceae, and Vuilleminiaceae, representing a mix of poroid, corticioid, cyphelloid, and clavarioid fungi.

In a series of publications in 1932, E.J.H. Corner explained the occurrence of different types of hyphae in the fruit bodies of polypore fungi. He introduced the concept of hyphal analysis, which later become a fundamental character in polypore taxonomy.

The order Polyporales was not widely adopted by Gäumann's contemporaries; most mycologists and reference works preferring to use the catch-all, artificial order Aphyllophorales for polypores and other "non-gilled fungi". When an attempt was made to introduce a more natural, morphology-based classification of the fungi in the 1980s and 1990s, the order was still overlooked. A standard 1995 reference work placed most polypores and corticioid fungi in the Ganodermatales, Poriales, and Stereales.

Current status

Molecular research, based on cladistic analysis of DNA sequences, has resurrected and redefined the Polyporales (also known as the polyporoid clade). Studies using a combination of rRNA gene sequences, single-copy protein-coding genes,
and genome-based phylogenetic analyses have shown that the Polyporales are a monophyletic group. They are a member of the class Agaricomycetes, but have not been assigned to a subclass. Though the precise boundaries of the order and its constituent families are yet to be resolved, it retains the core group of polypores in the family Polyporaceae, with additional species in the Fomitopsidaceae and Meripilaceae. It also includes polypores in the Ganodermataceae, which were previously assigned to their own separate order, the Ganodermatales, based on their distinctive basidiospore morphology. Corticioid fungi belonging to the Cystostereaceae, Meruliaceae, Phanerochaetaceae, and Xenasmataceae are also included, as are the cauliflower fungi in the Sparassidaceae.

In an extensive molecular analysis, Manfred Binder and colleagues analyzed 6 genes from 373 species and confirmed the existence of four previously recognized lineages of Polyporales: the antrodia, core polyporoid, phlebioid, and residual polyporoid clades. Extending this work, Alfredo Justo and colleagues proposed a phylogenetic overview of the Polyporales that included a new family-level classification. They assigned family names to 18 clades and four informal unranked clades. The families are listed below, followed by their taxonomic authorities and year of publication:
* Phanerochaetaceae Jülich (1981)
* Irpicaceae Spirin & Zmitr. (2003)
* Meruliaceae Rea (1922)
* Steccherinaceae Parmasto (1968)
* Cerrenaceae Miettinen, Justo & Hibbett (2017)
* Panaceae Miettinen, Justo & Hibbett (2017)
* Hyphodermataceae Jülich (1981)
* Meripilaceae Jülich (1981)
* Podoscyphaceae D.A.Reid (1965)
* Polyporaceae Corda (1939)
* Fomitopsidaceae Jülich (1981)
* Laetiporaceae Jülich (1981)
* Dacryobolaceae Jülich (1981)
* Sparassidaceae Jülich (1981)
* Grifolaceae Jülich (1981)
* Gelatoporiaceae Miettinen, Justo & Hibbett (2017)
* Incrustoporiaceae Jülich (1981)
* Ischnodermataceae Jülich (1981)

Other families that putatively belong to the Polyporales, but for which molecular confirmation is absent or lacking, include Diachanthodaceae Jülich, (1981); Fragiliporiaceae Y.C.Dai, B.K.Cui & C.L.Zhao (2015); Hymenogrammaceae Jülich (1981); and Phaeotrametaceae Popoff ex Piątek (2005). The Nigrofomitaceae, formerly placed in the Polyporales, was shown to be nested as a distinct lineage within the Hymenochaetales.

The family Steccherinaceae was redefined in 2012 to contain most species of the poroid and hydnoid genera '' Antrodiella'', '' Junghuhnia'', and '' Steccherinum'', as well as members of 12 other hydnoid and poroid genera that had been traditionally classified in the families Phanerochaetaceae, Polyporaceae, and Meruliaceae. Several new genera were added to the Steccherinaceae in 2016–17.

Ecology

The order is cosmopolitan and contains around 1800 species of fungi worldwide—about 1.5% of all known fungus species. All species in the Polyporales are saprotrophs, most of them wood-rotters. Their fruit bodies are therefore typically found on living or moribund trees or on dead attached or fallen wood. Polyporales species that fruit on the ground are either root rot species–such as '' Laetiporus cincinnatus'' and '' Grifola frondosa'', or are fruiting from buried pieces of substrate–such as '' Polyporus radicatus'' and '' P. melanopus''.

Wood-decay Polyporales reduce the volume of dead wood in the forest and are an important component of the carbon cycle. Wood is composed of primarily three types of tissue: lignin, cellulose, and hemicelluloses. White rot species of Polyporales are efficient degraders of the decay-resistant polymer lignin, leaving partially degraded cellulose as a residue. Brown rot species break down the cellulose fibres, leaving a brittle, brown lignin residue. Brown-rot residues such as humus can remain in the soil for hundreds of years, increasing aeration and water-holding capacity.

Peroxidase enzymes that degrade lignin, such as lignin peroxidase, manganese peroxidase, or versatile peroxidase, are present in all white-rot members of the Polyporales, but absent in brown-rot species. Oxidase enzymes, including members of the glucose-methanol-choline oxidoreductase family, play a key role in the breakdown of plant polymers because they generate hydrogen peroxide, which acts as the ultimate oxidizer in both white-rot and brown-rot decay.

Two species of Polyporales, ''Daedalea quercina'' and ''Fomitopsis pinicola'', use paralysing toxins to destroy and colonize nematodes that feed on their fruit bodies.

Importance

Many wood-decay fungi in the genera '' Fomes'', '' Fomitopsis'' and '' Ganoderma'' are pathogenic, causing butt and root rot of living trees and consequent losses in forestry plantations. Several species, such as the mine fungus ''Fibroporia vaillantii

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