3.4.2  Fungus–root interfaces

Printer-friendly version


Figure 3.13 Mature arbuscule of Glomus mosseae which has formed in a root cortical cell of leek (Allium parvum). A large surface area between fungus and host cell enables exchange of organic solutes and mineral nutrients across the interface formed by the two membrane systems. Roots were cleared, stained with Chlorazol black and viewed with interference contrast microscopy (Photograph courtesy M. Brundett; reproduced with permission of ACIAR and CSIRO Forestry and Forest Products)

Arbuscular mycorrhizal associations are formed by fungi from the family Glomales (Zygomycetes). During the infection process, fungal hyphae penetrate the epidermal cell layer before forming highly branched structures called arbuscules within individual cells of the cortex (Figure 3.13). Hyphae do not penetrate the endodermis or enter the stele. Arbuscules represent the major interface across which nutrient and carbon exchange takes place in arbuscular mycorrhizal symbioses. Although the plant cell wall is penetrated during infection, the host plasma membrane remains intact and apparently functional, proliferating to surround the arbuscular branches as they develop. The interface between fungus and plant thus comprises membranes of both partners separated by a region of apoplasm, a feature common to all types of mycorrhizal association (Smith et al. 1994). The highly branched nature of arbuscules is thought to increase the surface area to volume ratio of the host plant by up to 20-fold, providing an extensive interface across which nutrient exchange can take place. Ericoid mycorrhizal fungi (largely ascomycetes) form a similar interface between cell walls and membranes in the fine hair roots of the Ericales. Fungi penetrate the epidermal cells, forming dense hyphal coils which are surrounded by host plasma membrane, although the extent to which this increases the surface area of host plasma membrane has not yet been determined (Figure 3.14).


Figure 3.14 (a) Hair root of Lysinema ciliatum, from the Australian Epacridaceae, stained with Trypan blue showing enlarged epidermal cells containing intracellular hyphal coils. Hyphae only penetrate single epidermal cells (× 450). (b) Scanning electron micrograph of L. ciliatum showing enlarged ‘balloon-like’ epidermal cells and loose hyphal weft (Photographs courtesy K. Dixon)


Figure 3.15 Transverse section of an ectonqycorrhiza showing a Hartig net (H) in roots of Eucalyptus globulus. Fungal hyphae are structurally modified, making intimate contact with root epidermal cells (E) and enabling exchange of resources through the interface between fungus and host (× 300) (Photograph courtesy I. Tommerup and M. Brundett; reproduced with permission of CSIRO Forestry and Forest Products)

Ectomycorrhizal symbioses are formed largely by higher fungi in the Basidiomycotina and Ascomycotina, which form mycorrhizas with the short lateral roots of trees. Unlike arbuscular mycorrhizas and ericoid mycorrhizas, ectomycorrhizal fungi do not normally penetrate host cell walls. Rather, they form an entirely intercellular interface, with extensively branched hyphae penetrating between epidermal and cortical cells, forming a network known as the Hartig net (Figure 3.15). Depending upon the tree species involved, the Hartig net may extend as far as the endodermis, the highly branched hyphae providing a potentially vast surface area for nutrient exchange between the partners. Ectomycorrhizas are further differentiated from the other two main mycorrhizal types by the fact that the fungus forms a dense hyphal mantle around each short lateral root, sealing the normal absorptive surface of lateral roots from the soil environment to a greater or lesser extent. In such cases, the short lateral root will become entirely dependent upon the fungal symbiont for its nutrient supply.