User:Forestfungi/Hartig net

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The Hartig net is a network of inward-growing hyphae that extends into the plant host root, penetrating between plant cells in the root epidermis and cortex of plants in ectomycorrhizal symbiosis with fungi. This network is the internal component '''of fungal morphology in ectomycorrhizal symbiotic structures formed with host plant roots, in addition to a hyphal mantle or sheath on the root surface, and extramatrical mycelium extending from the mantle into the surrounding soil. The Hartig net is the site of mutualistic resource exchange between the fungus and the host plant. Essential nutrients essential for plant growth are acquired from the soil by exploration and foraging of the extramatrical mycelium, then transported through the hyphal network across the mantle and into the Hartig net, where they are released by the fungi into the root apoplastic space for uptake by the plant. The hyphae in the Hartig net acquire sugars from the plant root, which are transported to the external mycelium to provide a carbon source to sustain fungal growth.'''

review: overview of mycorrhizas. Useful for foundational information and updating references

The Hartig net supplies chemical elements required for plant growth, such as potassium, and provides compounds, such as phosphate and nitrate,  used in combination with the ectomycorrhizal symbiosis for farmable crops, as well as certain kinds of lichens. Part of its role in mutualistic interactions is based on the chemicals it provides, as well as it being essential for bi-directional nutritional uptake, which has shown to help defend the fungi from heavy metal damage,  amongst other benefits.

Structure and Development
The Hartig net is a lattice-like network of hyphae that grow into the plant root from the hyphal mantle at the plant root surface. The hyphae of ectomycorrhizal fungi do not penetrate the plant cells, but occupy the apoplastic space between cells in the root. This network extends between the epidermal cells near the root surface, and may also penetrate between cells in the root cortex. The hyphae in the Hartig net formed by some ECM fungi are described as having transfer-cell like structures, with highly folded membranes that increase surface area and facilitate secretion and uptake of resources exchanged in the mutualistic symbiosis.

This structure is common among ectomycorrhizal fungi, although the depth and thickness of the hyphal network can vary considerably depending on the host species. Fungi associating with plants in the Pinaceae form robust Hartig net structures that penetrate between cells deep into the root cortex, while the Hartig net formation in ectomycorrhizal symbioses with many Angiosperms may not extend beyond the root epidermis. It has also been demonstrated that the depth and development of the Hartig net can vary among different fungi, even among isolates of the same species. Interestingly, an experiment using two isolates of Paxillus involutus, one of which only developed a loose mantle at the root surface and no Hartig net in poplar roots, showed that plant nitrate uptake was improved by the symbiosis regardless of the presence of internal hyphal structure.

Hartig net formation in Picea abies

The Hartig net formed within Picea abies  roots by Piloderma bicolor and an unknown ECM fungus in the Asocomycota did not disrupt the symplastic connection between cortical root cells

Nylund 1980

Morphology of the hartig net may vary among ECM fungal species(Tedersoo), and can also vary among isolates of the same species, as described between two isolates of Paxillus involutus, one of which was able to form a mantle, but did not develop a Hartig net.

However, the improvement of NO3 uptake by ECM colonization was not dependent on Hartig net formation between these two isolates

However, a study by Sa et al. demonstrated that nitrate uptake in poplar trees was improved when colonized by either of two different isolates of the ECM fungal species Paxillus involutus, despite the fact that one of these isolates did not form a Hartig net, but only a loose hyphal mantle on the root

Although some fungal species such as Tuber melanosporum form arbutoid mycorrhizae with plant roots, including intracellular penetration of plant root cells by fungal hyphae. These fungi develop a shallow Hartig net, often only between epidermal cells.

Colonization of Pinus massoniana by Suillus bovinus (**last resort example)

The discussion incudes references to studies showing the induction of changes in root morphology by exposure to fungal VOCs and exudates prior to contact.

The initiation of Hartig net development

Hartig net development of Paxillus involutus in Betula pendula began on the third day following initial fungal adhesion on the root surface. The Hartig net development began after the mantle was formed.

Quéré et al. 2005

Time sequence of ECm development in eucalyptus with Pisolithus and with Paxillus also shows beginning of Hartig net development after the mantle formation, and after three days of infection.

Horan et al

The possible role of plant hormones in ECM development between poplar and Laccaria bicolor.

The presence of the fungal symbiosis influenced the plant sensitivity to phytohormone exposure

Basso et al., 2019

In Laccaria bicolor colonizing trembling aspen seedlings, the aquaporin LaAQP1 was more highly expressed when the fungus made contact with the root surface. This channel may be important for the exudation of MISSP7  proteins by the fungus during initiation of symbiosis. When the LbAQP1 gene was knocked down in Laccaria bicolor, Hartig net development was inhibited

Effector that may regulate plant defense mechanisms by controlling plant response to phytohormones

Daguerre et al., 2020

Increased pectin methylesterases were released by Laccaria bicolor during the fungal infection and Hartig net development

Pectin degradation at the ECM interface

Loosen the adhesion between neighboring plant cells by pectin modification enzymes released by plant or fungus to mediate the pectin degradation

Pathogens use plant cell wall degrading enzymes to break down the cell wall barrier (cite*) but eCM fungi contain relatively low PCDWE genes compared to plant pathogens.

and a secreted β-1,4 endoglucanase that may also play a role in loosening of plant root cell wall adhesion.

The hyphae that extend into the plant root to make up the Hartig net have been described as having transfer cell-like structures, with multiple

Function
Specialized phosphate transporters highly expressed in the Hartig ne compared to other fungal tissues

Several K  transport proteins have been identified as putatively playing an important role in delivering K to the Hartig net, including the high affinity K transport protein (HAK),  TRK, and the TOK channel, which may be voltage gated or driven by cation concentration oon either side of the membrane.

The identity of plant K transporters involved in plant acquisition of K released from the Hartig net into the intracellular apoplastic space in the root remains unknown,

Complexation of metals related to Hartig net

In exchange for the nutrients provided by the fungal partner, the plant provides a portion of its photosynthetically fixed carbon to the fungus as sugars. Sugars are released into the apoplastic space and make available for fungal uptake by the Hartig net hyphae.

SWEET transporters (Sugars Will Eventually Be Transported) in plant root cell membrane have been identified as important for releasing sugars from root cells into the apoplastic space for uptake by fungi.

Although sucrose was long considered to be an important form of carbon provided by the plant to the fungus, most ECM fungi lack sucrose uptake transporters. Therefore, the fungal symbiont may depend on plant production of invertases to degrade sucrose into useable monosaccharaides?? For fungal uptake.

In the Hartig net of Amanita muscaria within poplar roots, expression of important fungal enzymes for trehalose biosynthesis was higher than in the extrametrical mycelium, indicating that trehalose production may function as a carbohydrate sink, increasing demand of plant carbohydrates through the symbiotic exchange (Lopez et al. 2007)

Name
The Hartig net is named after Theodor Hartig, a 19th-century German forest biologist and botanist. He reported research in 1842 on the anatomy of the interface between ectomycorrhizal fungi and tree roots.