arbuscular mycorrhizal fungi

An arbuscular mycorrhiza (AM) (plural ''mycorrhizae'') is a type of mycorrhiza in which the symbiont fungus (''Arbuscular mycorrhizal fungi'', or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. Arbuscular mycorrhiza is a type of endomycorrhiza along with ericoid mycorrhiza and orchid mycorrhiza (not to be confused with ectomycorrhiza). They are characterized by the formation of unique tree-like structures, the arbuscules. In addition, globular storage structures called vesicles are often encountered.

Arbuscular mycorrhizae are formed by fungi in the subphylum Glomeromycotina. This subphylum, along with the Mortierellomycotina, and Mucoromycotina, form the phylum Mucoromycota, a sister clade of the more well-known and diverse dikaryan fungi.

AM fungi help plants to capture nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil. It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonisation of land by plants and in the evolution of the vascular plants.
It has been said that it is quicker to list the plants that do not form endomycorrhizae than those that do. This symbiosis is a highly evolved mutualistic relationship found between fungi and plants, the most prevalent plant symbiosis known, and AMF is found in 80% of vascular plant families in existence today.

Previously this type of mycorrhizal associations were called 'Vesicular arbuscular mycorrhiza (VAM)', but since some members of these fungi do not produce any vesicles, such as the members of Gigasporaceae; the term has been changed to 'Arbuscular Mycorrhizae' to include them.

Advances in research on mycorrhizal physiology and ecology since the 1970s have led to a greater understanding of the multiple roles of AMF in the ecosystem. An example is the important contribution of the glue-like protein glomalin to soil structure (see below). This knowledge is applicable to human endeavors of ecosystem management, ecosystem restoration, and agriculture.

Evolution of mycorrhizal symbiosis

Paleobiology

Both paleobiological and molecular evidence indicate that AM is an ancient symbiosis that originated at least 460 million years ago. AM symbiosis is ubiquitous among land plants, which suggests that mycorrhizas were present in the early ancestors of extant land plants. This positive association with plants may have facilitated the development of land plants.

The Rhynie chert of the lower Devonian has yielded fossils of the earliest land plants in which AM fungi have been observed. The fossilized plants containing mycorrhizal fungi were preserved in silica.

The Early Devonian saw the development of terrestrial flora. Plants of the Rhynie chert from the Lower Devonian (400 m.yrs ago) were found to contain structures resembling vesicles and spores of present ''Glomus'' species. Colonized fossil roots have been observed in ''Aglaophyton major'' and ''Rhynia'', which are ancient plants possessing characteristics of vascular plants and bryophytes with primitive protostelic rhizomes.

Intraradical mycelium was observed in root intracellular spaces, and arbuscules were observed in the layer thin wall cells similar to palisade parenchyma. The fossil arbuscules appear very similar to those of existing AMF. The cells containing arbuscules have thickened walls, which are also observed in extant colonized cells.

Mycorrhizas from the Miocene exhibit a vesicular morphology closely resembling that of present Glomerales. This conserved morphology may reflect the ready availability of nutrients provided by the plant hosts in both modern and Miocene mutualisms. However, it can be argued that the efficacy of signaling processes is likely to have evolved since the Miocene, and this can not be detected in the fossil record. A finetuning of the signaling processes would improve coordination and nutrient exchange between symbionts while increasing the fitness of both the fungi and the plant symbionts.

The nature of the relationship between plants and the ancestors of arbuscular mycorrhizal fungi is contentious. Two hypotheses are:
* Mycorrhizal symbiosis evolved from a parasitic interaction that developed into a mutually beneficial relationship.
* Mycorrhizal fungi developed from saprobic fungi that became endosymbiotic.
Both saprotrophs and biotrophs were found in the Rhynie Chert, but there is little evidence to support either hypothesis.

There is some fossil evidence that suggests that the parasitic fungi did not kill the host cells immediately upon invasion, although a response to the invasion was observed in the host cells. This response may have evolved into the chemical signaling processes required for symbiosis.

In both cases, the symbiotic plant-fungi interaction is thought to have evolved from a relationship in which the fungi was taking nutrients from the plant into a symbiotic relationship where the plant and fungi exchange nutrients.

The ancient plants did not have true roots. Strullu-Derrien and Strullu proposed the term 'Paramycorrhizae' for the mycorrhizae that infected the rhizome or shoot or thalli, and 'Eumycorrhizae' that infects true roots. These structures were reported in both sporophytes and gametophytes of the early land plants.

Molecular evidence

Increased interest in mycorrhizal symbiosis and the development of sophisticated molecular techniques has led to the rapid development of genetic evidence. Wang ''et al.'' (2010) investigated plant genes involved in communication with order Glomales fungal partners (DMI1, DMI3, IPD3). These three genes could be sequenced from all major clades of modern land plants, including liverworts, the most basal group, and phylogeny of the three genes proved to agree with then current land plant phylogenies. This implies that mycorrhizal genes must have been present in the common ancestor of land plants, and that they must have been vertically inherited since plants colonized land.

AM fungi and cyanobacteria symbiosis

It was revealed that AM fungi have the bacterial type core enzyme (ribonuclease III) of the sRNA processing mechanism, probably by the process of horizontal gene transfer from a cyanobacterial ancestor, and possibly related to symbiosis,. This finding of a genetic fossil inside AM fungi raises the possibility of an intimate relationship between AM fungi and cyanobacterial ancestors. A similar ''Geosiphon''–''Nostoc'' symbiosis was previously reported.

Circadian clock evolution in AM fungi

Surprisingly, despite their long evolution as an underground partner of plant roots, whose environment is far from light or temperature fluctuation, AMF still have a conserved circadian clock whose fungal circadian oscillator (''frq'') is activated by the blue light, similar to the model circadian fungus ''Neurospora crassa''.Lee, SJ., Kong, M., Morse, D. et al. Expression of putative circadian clock components in the arbuscular mycorrhizal fungus Rhizoglomus irregulare. Mycorrhiza (2018) https://doi.org/10.1007/s00572-018-0843-y The proven conservation of a circadian clock and output genes in ''R. irregulare'' opens the door to the study of circadian clocks in the fungal partner of AM symbiosis. The same research characterized the AMF ''frq'' gene, which is the first ''frq'' gene identified outgroup of Dikarya, and suggests the ''frq'' gene evolution in the fungal kingdom is much older than previously thought.

Physiology

Presymbiosis

The development of the AM fungi prior to root colonization, known as presymbiosis, consists of three stages: spore germination, hyphal growth, host recognition and appressorium formation.

Spore germination

Spores of the AM fungi are thick-walled multi-nucleate resting structures. The germination of the spore does not depend on the plant, as spores have been germinated under experimental conditions in the absence of plants both ''in vitro'' and in soil. However, the rate of germination can be increased by host root exudates. AM fungal spores germinate given suitable conditions of the soil matrix, temperature, carbon dioxide concentration, pH, and phosphorus concentration.

Hyphal growth

The growth of AM hyphae through the soil is controlled by host root exudates known as strigolactones, and the soil phosphorus concentration. Low-phosphorus concentrations in the soil increase hyphal growth and branching as well as induce plant exudation of compounds that control hyphal branching intensity.Douds, D.D. and Nagahashi, G. 2000. Signalling and Recognition Events Prior to Colonisation of Roots by Arbuscular Mycorrhizal Fungi. In Current Advances in Mycorrhizae Research. Ed. Podila, G.K., Douds, D.D. Minnesota: APS Press. Pp 11–18.

The branching of AM fungal hyphae grown in phosphorus media of 1 mM is significantly reduced, but the length of the germ tube and total hyphal growth were not affected. A concentration of 10 mM phosphorus inhibited both hyphal growth and branching. This phosphorus concentration occurs in natural soil conditions and could thus contribute to reduced mycorrhizal colonization.

Host recognition

Root exudates from AMF host plants grown in a liquid medium with and without phosphorus have been shown to affect hyphal growth. Spores of '' Gigaspora margarita'' were grown in host plant exudates. Hyphae of fungi grown in the exudates from roots starved of phosphorus grew more and produced tertiary branches compared to those grown in exudates from plants given adequate phosphorus. When the growth-promoting root exudates were added in low concentration, the AM fungi produced scattered long branches. As the concentration of exudates was increased, the fungi produced more tightly clustered branches. At the highest-concentration arbuscules, the AMF structures of phosphorus exchange were formed.

This chemotaxic fungal response to the host plants exudates is thought to increase the efficacy of host root colonization in low-phosphorus soils. It is an adaptation for fungi to efficiently explore the soil in search of a suitable plant host.

Further evidence that arbuscular mycorrhizal fungi exhibit host-specific chemotaxis, that enable hyphal growth toward the roots of a potential host plant: Spores of '' Glomus mosseae'' were separated from the roots of a host plant, nonhost plants, and dead host plant by a membrane permeable only to hyphae. In the treatment with the host plant, the fungi crossed the membrane and always emerged within 800 μm of the root, but not in the treatments with nonhost plants and dead plants.

Molecular techniques have been used to understand the signaling pathways between arbuscular mycorrhizae and plant roots. In 2003 it was shown how the AM undergoes physiological changes in the presence of exudates from potential host plant roots, to colonize it. Host plant root exudates trigger and turn on AM fungal genes required for the respiration of spore carbon compounds. In experiments, transcription rate of 10 genes increased half-hour after exposure and at an even greater rate after 1 hour. after 4 hours exposure AM respond with morphological growth. Genes isolated from that time are involved in mitochondrial activity and enzyme production. The fungal respiration rate, measured by O₂ consumption rate, increased by 30% 3 hours after exposure to root exudates, indicating that host plant root exudates stimulate AMF spore mitochondrial activity. It may be part of a fungal regulatory mechanism that conserves spore energy for efficient growth and the hyphal branching upon receiving signals from a potential host plant.

Appressorium

When arbuscular mycorrhizal fungal hyphae encounter the root of a host plant, an appressorium or 'infection structure' forms on the root epidermis. From this structure hyphae can penetrate into the host's parenchyma cortex. AM need no chemical signals from the plant to form the appressoria. AM fungi could form appressoria on the cell walls of "ghost" cells in which the protoplast had been removed to eliminate signaling between the fungi and the plant host. However, the hyphae did not further penetrate the cells and grow in toward the root cortex, which indicates that signaling between symbionts is required for further growth once appressoria are formed.

Symbiosis

Once inside the parenchyma, the fungus forms highly branched structures for nutrient exchange with the plant called arbuscules. These are the distinguishing structures of arbuscular mycorrhizal fungus. Arbuscules are the sites of exchange for phosphorus, carbon, water, and other nutrients. There are two forms: ''Paris'' type is characterized by the growth of hyphae from one cell to the next; and ''Arum'' type is characterized by the growth of hyphae in the space between plant cells. The choice between ''Paris'' type and ''Arum'' type is primarily determined by the host plant family, although some families or species are capable of either type.

The host plant exerts a control over the intercellular hyphal proliferation and arbuscule formation. There is a decondensation of the plant's chromatin, which indicates increased transcription of the plant's DNA in arbuscule-containing cells. Major modifications are required in the plant host cell to accommodate the arbuscules. The vacuoles shrink and other cellular organelles proliferate. The plant cell cytoskeleton is reorganized around the arbuscules.

There are two other types of hyphae that originate from the colonized host plant root. Once colonization has occurred, short-lived runner hyphae grow from the plant root into the soil. These are the hyphae that take up phosphorus and micronutrients, which are conferred to the plant. AM fungal hyphae have a high surface-to-volume ratio, making their absorptive ability greater than that of plant roots. AMF hyphae are also finer than roots and can enter into pores of the soil that are inaccessible to roots. The fourth type of AMF hyphae grows from the roots and colonizes other host plant roots. The four types of hyphae are morphologically distinct.

Nutrient uptake and exchange

AM fungi are obligate symbiont

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