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Hartig net
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<h2 id="structure-and-development">Structure and Development</h2> <p>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 <a href="/facts/Apoplast/AeYBwkea">apoplastic</a> space between cells in the root. This network extends between the <a href="/facts/Epidermis_(botany)/ldKXCLRl">epidermal</a> cells near the root surface, and may also extend between cells in the root <a href="/facts/Cortex_(botany)/xePNxtUo">cortex</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> The hyphae in the Hartig net formed by some ECM fungi are described as having <a href="/facts/Transfer_cell/4vgU75W6">transfer-cell</a> like structures, with highly folded membranes that increase surface area and facilitate secretion and uptake of resources exchanged in the mutualistic symbiosis.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>The initiation of hyphal growth into the intercellular space between roots often begins between 2–4 days following the establishment of the hyphal mantle in contact with the root surface.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> The initial development of the Hartig net likely involves a regulated decrease of plant defense responses, thus allowing fungal infection. Studies carried out with the model ectomycorrhizal fungus <i><a href="/facts/Laccaria_bicolor/IIfW12bX">Laccaria bicolor</a></i> have shown that the fungus secretes a small effector protein (MISSP7) that may regulate plant defense mechanisms by controlling plant response to <a href="/facts/Plant_hormone/h3noouev">phytohormones</a>.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> Unlike some plant root <a href="/facts/Pathogenic_fungus/t33r087W">pathogenic fungi</a>, ectomycorrhizal fungi are largely unable to produce many plant cell-wall-degrading enzymes, but increased pectin modification enzymes released by <i><a href="/facts/Laccaria_bicolor/IIfW12bX">Laccaria bicolor</a></i> during fungal infection and Hartig net development indicate that pectin degradation may function to loosen the adhesion between neighboring plant cells and allow room for hyphal growth between cells<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a><a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>This Hartig net 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 <a href="/facts/Pinaceae/Ry0GUN97">Pinaceae</a> form a robust Hartig net that penetrates between cells deep into the root cortex, while the Hartig net formation in ectomycorrhizal symbioses with many <a href="/facts/Angiosperm/E5eGYIvY">angiosperms</a> may not extend beyond the root epidermis.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> 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 <i><a href="/facts/Paxillus_involutus/xVjPyIgT">Paxillus involutus</a></i>, one of which only developed a loose mantle at the root surface and no developed Hartig net in poplar roots, showed that plant nitrate uptake was still improved by the symbiosis regardless of the presence of internal hyphal structure.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> As an additional caveat some fungal species such as <i><a href="/facts/Tuber_melanosporum/Ge42MdM5">Tuber melanosporum</a></i> can form <a href="/facts/Arbutoid_mycorrhiza/bAqxvRwm">arbutoid</a> mycorrhizae, involving some intracellular penetration into plant root cells by fungal hyphae in addition to developing a shallow Hartig-net-like structure between epidermal cells.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> </p> <h2 id="function">Function</h2> <p>The Hartig net supplies the plant root with chemical elements required for plant growth, such as <a href="/facts/Nitrogen_deficiency/YoCAyNQf">nitrogen</a> and <a href="/facts/Phosphorus_deficiency/FwyaA7Kb">phosphorus</a>,<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> <a href="/facts/Potassium_deficiency_(plants)/ceY9X8GI">potassium</a>,<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a><a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> and <a href="/facts/Micronutrient/nwzKuSMQ">micronutrients</a><a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> in addition to water supplied to the roots through hyphal transport.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> Essential nutrients acquired from surrounding <a href="/facts/Soil/5Ddpr3QM">soil</a> by the extramatrical mycelium are transported into the hyphae in the Hartig net, where they are released into the apoplastic space for direct uptake directly by plant root cells.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> </p><p>In exchange for the nutrients provided by the fungal partner, the plant provides a portion of its photosynthetically fixed carbon to the fungal partner as sugars. Sugars are released into the apoplastic space and made available for uptake by Hartig net hyphae. Although <a href="/facts/Sucrose/7y1ke94L">sucrose</a> was long considered to be an important form of carbon provided by the plant to the fungus, many ectomycorrhizal fungi lack sucrose uptake transporters. Therefore, the fungal symbiont may depend on plant production of <a href="/facts/Invertase/hNCIhAK6">invertases</a> to degrade sucrose into useable <a href="/facts/Monosaccharide/yzs2d6Eq">monosaccharides</a> for fungal uptake.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a><a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> In the Hartig net of <i><a href="/facts/Amanita_muscaria/2VybE2L2">Amanita muscaria</a></i> within <a href="/facts/Populus/SSgurBpF">poplar</a> roots, expression of important fungal enzymes for <a href="/facts/Trehalose/1N3XTfxV">trehalose</a> biosynthesis was higher than in the extrametrical mycelium, indicating that trehalose production may function as a <a href="/facts/Carbohydrate/Geuuzxsf">carbohydrate</a> sink, increasing fungal demand of plant <a href="/facts/Photosynthesis/TMV7w693">photosynthesized</a> <a href="/facts/Carbohydrate/Geuuzxsf">carbon</a> compounds through the symbiotic exchange.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> The plant regulatory mechanisms that influence the nutrient supply by the Hartig net are not fully understood, but it is thought that upregulation of plant <a href="/facts/Plant_disease_resistance/7KbAxbGm">defense</a> mechanisms in response to decreased nitrogen transport by ECM fungi, rather than reductions in carbon allocation to ECM roots, suggesting that the regulation of symbiotic resource exchange for ECM symbiosis is not a simple reciprocal response.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p><p>In addition to the exchange of essential nutrients, the Hartig net may play an important role in plant strategies for tolerance of abiotic stressors, such as regulating bioaccumulation of metals<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> or mediating plant stress responses to salinity.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a> </p> <h2 id="name">Name</h2> <p>The Hartig net is named after <a href="/facts/Theodor_Hartig/WnG4guEx">Theodor Hartig</a>,<a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a><a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> a 19th-century German forest biologist and botanist. He reported research in 1842 on the <a href="/facts/Anatomy/p8TCWJuV">anatomy</a> of the interface between ectomycorrhizal fungi and tree roots. </p> <h2 id="see-also">See also</h2> <ul> <li>Fungi portal</li></ul> <ul><li><a href="/facts/Mycorrhiza/bAqxvRwm">Mycorrhiza</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Smith, Sally, Read, David (2002). "Chapter 6: Structure and Development of Ectomycorrhizal roots". Mycorrhizal Symbiosis. IV–V: 163–232. doi:10.1016/B978-012652840-4/50007-3.{{cite journal}}: CS1 maint: multiple names: authors list (link) <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Brundrett, Mark C.; Tedersoo, Leho (December 2018). "Evolutionary history of mycorrhizal symbioses and global host plant diversity". New Phytologist. 220 (4): 1108–1115. doi:10.1111/nph.14976. ISSN 0028-646X. 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