Monotropoid mycorrhizae, in common with arbutoid mycorrhizas, share features typical of ectomycorrhizas and ectendomycorhizas. Monotropoid mycorrhizas have a mantle which is sometimes very thick, and a Hartig net confined to the epidermis (para epidermal).
Although the plant species that form this category of mycorrhiza also belong to the large order Ericales, structural features of the mycorrhiza separate it from ericoid and arbutoid mycorrhizas (Duddridge and Read 1982). They also possess a unique feature, the invasion of epidermal cells by short hyphae originating from the Hartig net or inner mantle.
These structures, referred to as fungal pegs (Figures 1,2), form along the outer tangential wall of epidermal cells in Monotropa species but at the base of the radial wall of epidermal cell Pterospora and Sarcodes sanguinea (Robertson and Robertson 1982). Host cells respond by depositing additional cell wall material, in finger-like projections, around each peg.
A second feature of monotropoid mycorrhizas is that plants forming this type of mycorrhiza are all achlorophyllous, heterotrophic (non photosynthetic) species; they depend on symbiotic fungal associations that act as linkages to neighbouring autotrophic (photosynthetic) trees or shrubs for their carbon acquisition.
In an extensive review of heterotrophic plants and their fungal associations, Leake (1994) suggests the term myco-heterotrophic best describes these associations. Others refer to these plants as epi-parasites, because they indirectly “parasitize” surrounding trees. It is generally accepted that the term saprophyte should be avoided.
Plant species involved
Depending on the taxonomic system used, all genera are found either in the family Monotropaceae or in the clade Monotropoideae, in the family Ericaceae. Regardless, ten genera (Allotropa, Cheilotheca, Hemitomes, Monotropa, Monotropantham, Monotropsis, Pityopus, Pleuricospora, Pterospora, Sarcodes) are presently recognized (Leake 1994), all but three of these genera being mono typic (i.e., with only one species).
The Asian genus Cheilotheca has four recognized species, and the genera Monotropa and Pleuricospora each have two species. The distribution of the group is primarily north temperate with the largest number of species in western North America. Most of the research, in terms of mycorrhizal associations, has been with the genera Monot-ropa, Sarcodes and Pterospora.
Fungal species involved (Monotropoid Mycorrhizae)
Over the years, a number of studies have documented the fungi possibly associating with members of the Monotropoideae; with the advancement of molecular approaches, the identities of some fungi have been confirmed (Bidartondo and Bruns 2001).
The plant genera examined are associated with members of five families of basidiomycetes, fungi also known to form ectomyco-rrhizas with numerous tree species. The interesting trend is that each plant genus or species, where there is more than one species in the genus, has become highly specialized in terms of its fungal associates.Figure 194. Diagram of main features of a monotropoid mycorrhiza as found in a Monotropa sp. A mantle (M), Hartig net (arrowheads) and fungal pegs (arrows) develop. Each peg forms from an inner mantle hypha that enters the cell through the outer tangential wall. Each peg is surrounded by finger-like projections of host-derived wall material. Figure 195. Diagram of a monotropoid mycorrhiza as found in the genus Pterospora. A mantle (M), Hartig net (arrowheads), and fungal pegs surrounded by host-derived wall material (arrows). In this genus, a hypha penetrates the epidermal cell along the base of a radial wall.
For example, Monotropa hypopitys appears to form monotropoid mycorrhizas only with species in the fungal genus Tricholoma, whereas Monotropa uniflora forms mycorrhizas with Russula species and other members in the family Russulaceae (Bidartondo and Bruns 2001; Young et al. 2002).
This specificity is true regardless of the geographical distribution of the plant species. The monotypic plant species, Sarcodes sanguinea and Pterospora andromedea, shown to be closely related phylogenetically, form mycorrhizas with fungal species in the genus Rhizopogon, with each plant species showing preference for particular Rhizopogon species.
Bruns and Read (2000) isolated Rhizopogon species from mature plants of both S. sanguinea and P. andromedea and used these, as well as a number of other ectomycorrhizal fungal species, in germination trials with both plant species.
Of the fungi tested, only Rhizopogon isolates were effective in promoting seed germination; isolates originating from either plant species were effective in stimulating germination in both plant species. Seeds without a Rhizopogon isolate did not germinate.
This study shows the importance of the presence of the appropriate fungus for seed germination in these myco-heterotrophic species.
Development and structure(Monotropoid Mycorrhizae)
The most detailed observations of the develop -ment and structure of monotropoid mycorrhizas have been obtained from studies on the genus Monotropa. In M. uniflora, and M.hypopitys, clusters of roots form, each root becoming ensheathed with fungal hyphae resulting in a well-developed mantle.
Details of the mantle can best be seen using scanning electron micro-scopy.Frequently, large calcium oxalate crystals are deposited among the outer mantle hyphae (Figure 204) and along the surface of hyphae within the mantle.
Cystidia may be present, depending on the fungal species. Rhizo-morphs are sometimes present (Figures 206, 207), and have been traced in vivo from the surface of M. hypopitys mycorrhizas to the roots of neighbouring pine trees (Duddridge and Read 1982).Figure 199. Large root cluster (arrows) and new shoots (arrowheads) of Monotropa uniflora. All root tips appear to be mycorrhizal, probably with a fungal symbiont in the family Russulaceae. Figure 200. Mycorrhizal root tip of Monotropa uniflora showing compact mantle (*) and bristle- like cystidia (arrowheads); these features are common to some fungal members of the Russulaceae. Photo courtesy of B. Young. Figures 201–205. Scanning electron micrographs of Monotropa uniflora mycorrhizas showing features of the mantle. Figure 201. A main root with several developing lateral roots (arrowheads), all covered by mantle hyphae. Figure 202. Surface features of a root apex showing abundant cystidia (arrowheads) and crystals (arrows). Figure 203. Higher magnification of portion of a mycorrhiza showing large numbers of awl- shaped cystidia (arrowheads). Figure 204. A large crystal of calcium oxalate and fusiform to flask-shaped cystidia (arrow- heads), some having small apical knobs. Figure 205. Sectional view of mycorrhiza showing two types of cystidia, awl-shaped (black arrowheads) and flask-shaped (white arrowheads). Figures 206, 207. Features of rhizomorphs found on Pterospora mycorrhizas. Scanning electron microscopy.
The mantle is usually multi-layered with a compact inner mantle (Figure 210). A para epidermal Hartig net develops and hyphae originating from this or from inner mantle hyphae penetrate into epidermal cells to form fungal pegs .
Figure 210. Light microscopy of a paradermal section of Monotropa uniflora mycorrhiza showing the compact inner mantle (M), and sectional views of hyphal pegs (arrowheads) in epidermal cells.
Transmission electron micrograph showing details of a hyphal peg . Finger-likedepositions of host cell wall material (arrowheads) are evident. Monotropoid Mycorrhizae
These fungal pegs, one per epidermal cell, are encased by wall material synthesized by the host plant. The wall is laid down unevenly so that each peg has wall projections enveloped by host plasma membrane (Figure 211), a structure similar to that in transfer cells described in many plant tissues.
The formation of plasma membrane around. these wall ingrowths increase the surface area of this membrane; increased surface area may be important in nutrient exchange at this interface.
Mitochondria and profiles of endoplasmic reticulum, often suggestive of increased metabolic activity, are always found in the vicinity of these wall in growths. The pegs form along the outer tangential wall of epidermal cells in Monotropa. In any cross section of a root, most ep-idermal cells show the development of these structures.
With time, each peg undergoes changes in that the host wall surroun-ding the tip breaks down; the fungal peg eventually degrades and, concomitantly, changes occur in the surrounding plasma membrane to form a sac-like structure.
The contents of the fungal peg apparently move into this membranous structure.In one study of M. hypopitys, certain stages in the development of mycorrhizas were documented to correlate with stages in plant development (Duddridge and Read 1982).
Mantle and Hartig net formation and the first stages of peg formation in epidermal cells occurred before the shoot emerged above ground. At this time, glycogen was stored in the Hartig net and mantle hyphae.
As shoots emerged, the number of fungal pegs increased dramatically and, with time, the number of pegs with degraded tips dominated the population. Following maturation of flowers and during seed set, degeneration of the fungal pegs, Hartig net and mantle hyphae occurred. Monotropoid Mycorrhizae
Roots of Pterospora andromedea also occur in coralloid clusters (Figure 212) with each root forming several laterals (Figure 213).The mantle covers the main root and lateral roots (Figure 214); hyphae of the external mantle are very irregular in shape and small crystals are present on their surface (Figures 215, 216).
The mantle is multi-layered and the Hartig net is para epidermal. Hyphae from the Hartig net penetrate epidermal cells, usually towards the base of radial walls, to form fungal pegs.
General descriptions of field-collected mycorrhizas of Sarcodes sanguinea were included as part of a study on the transport of radioactive compounds from autotrophic trees to this species (Vreeland et al. 1981).
S.sanguinea mycorrhizal roots developed in clusters or coralloid masses, within which each mycorrhizal root tip formed a mantle and para epidermal Hartig net.Figures 212. Coralloid root mass of Pterospora andromedea with fungi closely resembling the Rhizopogon subcaerulescens group, collected from British Columbia, Canada. Figure 213. Enlarged view of Pterospora andromedea mycorrhiza showing a main root apex and several lateral roots (arrows). A mantle (*) covers all roots. Photo courtesy of J. Catherall. Figures 214–216. Scanning electron micrographs of mantle surface of Pterospora andromedea mycorrhizas. Figure 214. Mantle surface of main and lateral roots. Figure 215. Higher magnification of mantle surface showing irregular features of hyphae. Figure 216. Mantle surface showing irregular hyphae and abundant small crystals (arrowheads) of variable shapes. Figures 217–220. Light microscopy of sections of Pterospora andromedea mycorrhizas. Figure 217. Portions of two lateral roots showing that they are covered by a mantle (arrow- heads). Figure 218. The thick, layered mantle (*) and paraepidermal Hartig net (arrowheads). Figure 219. Inner mantle (M), paraepidermal Hartig net (arrowheads), and evidence of hyphal pegs (arrow). Figure 220. Hyphal pegs (arrowheads) that enter epidermal cells along a radial wall.
In a more detailed ultrastructural study of this species, as well as Pterospora and romedea, Robertson (1982) illustrated the coralloid clusters of roots for both species and provided details of the interface between fungus and root cells.
All plant species having monotropoid mycorrhizas are non-photosynthetic and myco-heterotrophic. Increasing evidence suggests that these specialized plants have evolved a mechanism to obtain photosynthates by forming mycorrhizas with fungi that are also able to associate with neighbouring photosynthetic trees or shrubs.
Using radioisotopes, it has been demonstrated that photosynthates (Björkman 1960) can move from trees to Monotropa hypopitys plants and that phosphorus (Vreeland et al. 1981) can move from trees to Sarcodes sanguinea. Monotropoid Mycorrhizae
Themechanism of transport of nutrients from the mycorrhizal fungus into roots of monotropoid species remains uncertain.
The elaborate wall ingrowths and the concomitant development of the plasma membrane around the fungal pegs is circumstantial evidence that this region may be the site of transfer of compounds from the fungus to epidermal cells.
The role of the Hartig net in transferring metabolites has not been explored. Monotropoid Mycorrhizae
Mycorrhizal Roots Staining and study