Glomeromycetes

Almost all glomeromycetes are obligate symbiotrophs and form endomycorrhiza (arbuscular mycorrhiza) with more than 80% of the studied species of land plants ^[1] . This type of mycorrhiza predominates in savannas , deserts and tropical forests , and in temperate latitudes it is found in 8 out of 10 species of herbaceous plants. Glomeromycetes form mycorrhiza with such economically important plants as cereals . They form mycorrhiza not only with many species of flowering plants, but also with some gymnosperms , bryophytes, and vascular spore plants ^{[2] [3]} .

The mycelium haploid , non-septate ( ) or with rare septa, develops in the soil and penetrates into the tissue of the roots of plants, where it spreads through the intercellular spaces of the parenchyma of the cortex . Hyphae narrow or wide (2-10 microns, sometimes up to 20 microns), often nodular. In some representatives, hyphae may form anastomoses , including end-to-end anastomoses, which connect the destroyed hyphal joints. Septa can be formed in the aging parts of the mycelium or after the formation of spores ^[4] .

Unlike ectomycorrhiza , which form the outer sheath around the roots, glomeromycetes mycorrhiza practically do not change the morphology of the roots and are not accompanied by loss of ^[5] . Only sometimes on the surface of the root is found a network of hyphae connecting the interstitial mycelium with the fungus located in the soil. Glomeromycete hyphae penetrate the cell wall of the root cells and cause the formation of plasmama leaks formation, in which they form tree-branching formations - arbuscules, which provide physiological contacts of plants and fungi at the intercellular level, thus, they perform the feeding function. In addition to arbuskul, round swellings - vesicles are often found under the cell wall of a symbiotic plant or outside it. In some cases, the fungus concentrates in special nodules. The nutrition of plants, carried out during symbiosis, occurs when the arbusculus fungus, and sometimes vesicles and hyphae, are digested by them. The most important substances that plants receive from fungi in arbuscular mycorrhiza are phosphorus compounds ^{[6] [3]} . It is still not possible to isolate glomeromycetes in pure culture without symbiotic plants ^{[7] [8]} .

The glomeromycetes include , within which the hyphae of the hyphae are intracellular symbionts - filamentous cyanobacteria of the genus Nostoc ^{[3] [9]} (as a rule, ^[10] ).

Most glomeromycetes have lost their ability to reproduce . The sexual process in the form of zygogamy is described only in two types. At the same time, Glomus has a set of 51 genes encoding proteins necessary for meiosis , and it is assumed that Glomus species may have a cryptic sexual process ^{[11] [12] [13]} . Reproduction of hyphal fragments remaining in the soil is possible ^[4] .

Most of the representatives reproduce by simple spores that form at the ends of the hyphae, with a diameter of 80–500 microns (glomospores) ^[14] , and more complex azigospores formed in terminal sporangia ^[15] . Azigospores have a complex six-layer wall containing chitin and cellulose , and serve not only to reproduce, but also to endure adverse conditions. The spores of glomeromycetes are always multi-core; they can contain from less than 50 to several thousand nuclei . It is not known whether these nuclei are identical genetically or whether they represent a mixed set of genotypes . They also contain lipid and protein globules. They are formed outside the roots, less often - inside them ^{[2] [3]} . Spores can form singly, loose clusters, dense masses or in sporocarpes . Sporocarpes are agglomerations of several hundred thousand spores, and their size varies from less than 500 microns to more than 4 cm. Sporocars are sometimes covered with an external peridium and are most often formed on the surface of the earth. Spores can be immersed in the mycelium or located radially in the interlacing of the hyphae ^[16] .

The spore germination paths differ in different taxa: the growth tube can pass through the spore wall or through the site of attachment to the hyphae; special film structures can take part in the germination process. The germination of spores can increase under the influence of factors produced by plants. shown to induce spore germination in the vicinity of a possible host root ^[4] .

The penetration of glomeromycetes into new roots can be carried out by means of spores or directly through hyphae leaving the colonized root. On the surface of the root, the fungus forms appressoria (hyphopodia), which allow the hyphae to enter the rhizoderm cells . Penetration of hyphae through the integumentary cells and cortical cells is directed and facilitated through the formation of a special penetration apparatus by a plant ^[4] .

Glomeromycetes, apparently, are not distinguished by their narrow specificity for host plants. The almost complete compatibility of various plant species and glomeromycetes has been shown experimentally. However, this conclusion was made on the basis of experiments conducted in greenhouses, and in nature the situation may be different. Molecular methods of analysis also confirm the absence of strict specificity. Most plants can form symbiosis with several species of glomeromycetes at the same time, and most species of glomeromycetes can form mycorrhiza with various plant species. However, some studies show the presence of some degree of preference of the host ^[17] .

The connection of glomeromycetes with plants is secretive, and therefore there is practically no data on the geographical distribution of glomeromycetes. Some species were found only in one place and may be endemic , while others are widespread. The natural way of spreading glomeromycetes is the spread by means of hyphal fragments and spores carried along with soil particles. In addition, there is evidence of the distribution of glomeromycete spores by earthworms and mammals . Some sporocarpic species may spread with rodent droppings ^[18] .

Glomeromycetes can be of great economic importance, since the formation of symbiosis with them can increase the yield of cultivated plants . For example, it has been shown that glomeromycete , while inoculated into a plant with Trichoderma atroviride, acts as a biostimulator, enhancing growth, absorption of nutrients and yield in vegetables ^[19] .

For a long time, the fungi forming arbuscular mycorrhiza were considered as part of the genus in the zygomycete department. Then several genera were distinguished, and in 1990 the order of Glomerales consisting of three families was described ^[3] . According to the phylogenetic classification of zygomycete fungi of 2016, the fungi forming the arbuscular mycorrhiza belong to the Mucoromycota section, in which they are allocated to the Glomeromycotina subdivision with a single Glomeromycetes class ^[20] . This class includes four orders: Archaeosporales , Glomerales , Diversisporales and Paraglomerales ^[21] . The largest order is Glomerales ^[2] , and there are about 230 species in the Glomeromycotina subdivision ^[22] .

Some researchers, on the basis of molecular and morphological data, propose to divide the Glomeromycota division into three classes: Glomeromycetes, Archaeosporomycetes and Paraglomeromycetes. In the first class, it is proposed to include the Glomerales, Diversisporales, and the new Gigasporales order, in the other two - the Archaeosporales and Paraglomales, respectively ^[24] .

Sequence analysis of the rRNA genes of the small ribosomal subunit (SSU) showed that glomeromycetes have a common ancestor with higher fungi (Dikarya) ^{[25] [15]} and are a sister group in relation to ascomycetes and basidiomycetes . However, data from the analysis of protein-encoding genes indicate that glomeromycetes are one of the groups in the paraphyletic group of zygomycetes. If glomeromycetes and Dikarya are sister taxa, their unifying feature is the ability to enter into mutualistic relations with plants and algae , which is very rare among representatives of other treasures . With the zygomycetes of glomeromycetes, the presence of an unseparated (coenocytic) mycelium and a number of structural features of the spores and sporocarpia, which both groups could inherit from a common ancestor, brings together ^[22] .

Studies using molecular markers have shown that the diversity of fungi that form arbuscular mycorrhiza is significantly underestimated, possibly due to the fact that many species form spores very rarely or do not form them at all ^[22] .

The oldest fossil remains, reliably belonging to the glomeromycetes, are glomoid spores and hyphae aged 460 Ma, found in Ordovician limestone. Thus, glomeromycetes appeared on Earth before vascular plants . Perhaps these ancient glomeromycetes came into symbiosis with mosses or with cyanobacteria like modern Geosiphon ; perhaps some of them led a saprotrophic lifestyle ^[26] . The oldest and most well-preserved arbuscules are 400–412 Ma old; they were found in the rhizomes of Devonian plants, for example, . Although these plants already had a conducting system , they did not yet have real roots. Thus, in the process of evolution, mycorrhiza appeared before the true roots ^[27] . It is worth noting that the ancient plants entered into a symbiosis not only with glomeromycetes, but also with the mushrooms of the group, and the simultaneous formation of symbiosis with fungi from both groups is also found in modern plants ^[28] .

• G. Belyakova, Yu. T. Dyakov, and K. L. Botanik Tarasov : in 4 volumes. T. 1. Algae and mushrooms. - M .: Publ. Center "Academy", 2006. - 320 p. - ISBN 5-7695-2731-5 .
• Garibova L.V., Lekomtseva S.N. Fundamentals of Mycology: Morphology and Systematics of Fungi and Mushroom-like Organisms. - M .: Fellowship of scientific publications KMK, 2005. - ISBN 5-87317-265-X .
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• The Mycota / Edited by Karl Esser. - Springer, 2014. - Vol. VIIA: Systematics and Evolution. - ISBN 978-3-642-55317-2 .