Arbutoid mycorrhiza are found in two families in the order Ericales, the structural features of the mycorrhizas and the fungal species involved are distinct from those of other members of this order.
In certain aspects, they resemble ectomycorrhizas and ectendo-mycorrhizas and some researchers have included them under the category of ectendomycorrhiza; however, because of structural differences and the fungi involved in the association, it is best to consider them as a distinct category.
Structurally, arbutoid mycorrhizas have a mantle, a Hartig net, and intracellular hyphae forming hyphal complexes, the latter two being confined to the epidermis.
2. Plant species involved
Two genera in the Ericaceae (Arbutus and Arctostaphylos) and several genera in t
he Pyrolaceae (including Pyrola) form typical arbutoid mycorrhizas. Although there are 50 species in the genus Arctos-taphylos, few have been examined for the presence of mycorrhizas.
The two genera Arbutus and Pyrola have fewer species than the genus Arctosta-phylos
, but again a limited number has been studied in terms of their mycorrhizal associations. Little is known about
the mycorrhizal status of other genera in the Pyrolaceae (e.g., Ramischia, Moneses, Orthilia and Chimaphila).
Although the commercial importance of these genera is mostly as ornamentals, several species form important components of terrestrial ecosystems.
3. Fungal species involved
A number of fungal species that form typical ectomycorrhizas with both gymnos-perm and angiosperm tree species also colonize roots of Arbutus and Arctostaphylos to form arbutoid mycorrhizas under laboratory conditions (Molina and Trappe 1982).
Most of these are broad host range fungi, meaning that they show little specificity as to the hosts that they colonize. Narrow host range fungi such as Alpova diplophloeus, known to only associate with Alnus species, do not form mycorrhizas with either Arbutus or Arctostaphylos.
Roots of Arbutus menziesii and Arctostaphylos manzanita collected from the field in Northern California were colonized by various species of ectomycorrhizal fungi but formed typical arbutoid mycorrhizas (Acsai and Largent 1983a,b).
Studies reported in the literature suggest that genera of mycorrhizal fungi are mostly non-specific towards Arbutus and Arctostaphylos as host plants.
A field experiment in the chaparral on the central coast of California, showed that Pseudotsuga menziesii seedlings established only in patches of Arctostaphylos and not in patches of Adenostoma (an arbuscular mycorrhiza host);
Identification of ectomycorrhizal fungal species by the presence of sporocarps and by molecular methods led to the conclusion that ectomycorrhizal fungi associated with Arctostaphylos contributed to the success of Pseudotsuga seedling establishment (Horton et al. 1999).
Most information concerning the fungi forming mycorrhizas with Pyrola is based on the hyphal characteristics from roots processed for microscopy.
Hyphae with either dolipore septa or simple septa with Woronin bodies have been described, indicating the presence of basidiomycete and ascomycete fungi, respectively. More research, particularly using molecular methods, is needed to determine the range of fungal species able to form arbutoid mycorr-hizas.
B. Development and structure(Arbutoid Mycorrhiza)
1. Arbutus and Arctostaphylos
Laboratory experiments have shown that the fungal genome may influence the type of root tip branc-hing induced in Arbutus menziesii (Pacific madrone) (Massicotte et al. 1993) and Arctostaphylos uva-ursi (bearberry) (Zak 1976).
In A. menziesii, the fungus Pisolithus tinctorius induces lateral root formation that results in complex root clusters , whereas the fungus Piloderma bicolor induces only a few laterals and no root clusters.In A. uva-ursi, depending on the fungal species, either simple root systems with few unbranched laterals form or root systems with clusters of lateral roots develop.
Field-collected roots of Arbutus menziesii and Arbutus unedo also show variable root branching patterns, presumably due to differences in fungal species forming the mycorrhizal association.
In both Arbutus and Arctostaphylos, the mantle, depending on the fungal species, can be either compact and several cell layers thick or loosely-organized with only a few cell layers . A mantle is present during all stages of root cluster formation.
Rhizomorphs, when present, extend from the mantle surface into the substrate. Azospirillum like bacteria have been isolated from the mantle of Arbutus unedo mycorrhizas and transmission electron microscopy has shown that they can occur within the mantle as well as along external hyphae (Filippi et al. 1995).
Hartig net formation occurs only around epidermal cells resulting in a paraepidermal Hartig net. Hartig net hyphae may branch, penetrate the wall of adjacent epidermal cells, and develop further as branched intracellular hyphae.
The confinement of Hartig net hyphae to the epidermis may be the result of the suberization of the walls of the outer layer of cortical cells .
Intracellular hyphae frequently occupy most of the epidermal cell volume; they are separated from the epidermal cell cytoplasm by the elaboration of a host-derived plasma membrane and an interfacial matrix of undetermined composition (Fusconi and Bonfante-Fasolo 1984).
Colonized epidermal cells retain their cytoplasm as well as the usual organelles of living plant cells.The inner mantle, Hartig net and intracellular hyphae all contain abundant mitochondria, endoplasmic reticulum, and ribosomes.
These hyphae may also store glycogen and poly phosphate (Ling-Lee et al. 1975). Intracellular hyphae appear to degenerate in older root systems. Arbutoid MycorrhizaFigure 180. Clusters of roots of Arbutus menziesii colonized by Pisolithus tinctorius. Numerous extraradical hyphae (arrowheads) are present. Figure 181. Scanning electron micrograph of root clusters of Arbutus menziesii colonized by Pisolithus tinctorius. Each lateral root is covered by a mantle (M) of interwoven hyphae. Rhizomorphs (arrowhead) are present in the extraradical mycelium. Figure 182. Field collected cluster of roots of Arctostaphylos uva-ursi with extraradical hyphae (arrowhead).
The root systems of several Pyrola species have been illustrated by Robertson and Robertson (1985); some are sparsely branched while others have many laterals, often forming clusters. One of the species illustrated (P. secunda) has been renamed Orthilia secunda.
The role of the fungus in the branching patterns has not been determined.The Pyrola mycorrhizas described to date have loosely-organized, thin mantles, a para-epidermal Hartig net that is usually one cell wide, and intracellular hyphae that may fill most of the volume of epidermal cells .
The intracellular hyphae are separated from the epidermal cell cytoplasm by host-derived plasma membrane and an interfacial matrix.The nature of this matrix has not been determined. Mantle hyphae and Hartig net hyphae are highly branched and possess numerous septa .
Figures 188, 189. Transverse sections of Arbutus menziesii roots colonized by Piloderma bicolor showing the mantle (M), the restriction of the Hartig net (arrowheads) to the epidermis (paraepi- dermal), intracellular hyphae (arrows), and the penetration of an epidermal cell by a Hartig net hypha (double arrowhead). Figure 190. Transverse section of an Arbutus menziesii root showing an outer fluorescent layer (the exodermis, arrow), and an inner fluorescent layer (the endodermis, arrowhead). The fluores- cence in both cell layers is due to suberin deposition in the walls. Figure 191. Section of Pyrola mycorrhiza with intracellular hyphae filling an epidermal cell (e). Cortex (c), mantle (m), Hartig net (arrowheads). Figures 192, 193. Confocal laser scanning microscopy of Pyrola mycorrhizas. Figures 192a,b. Sections through the Hartig net showing branching patterns of hyphae. Figure 193. Hartig net (arrowheads) and intracellular hyphae (arrow).
C. Functions of Arbutoid mycorrhiza
Many arbutoid hosts are important components of global ecosystems and may help to maintain the diversity of numerous mycorrhizal fungi. Although they represent few genera, their taxonomic diversity is noteworthy. Estimates are in the order of 50 species for Arctostaphylos; this genus is native to North and Central America with A.uva-ursi being circumpolar (Young and Young 1992).Arbutoid Mycorrhiza
Arbutus species (estimated 10–20 species) are native to the Mediterranean region, Canary Islands and North America (Young and Young 1992), with A. menziesii being a prominent coastal tree species from southwestern British Columbia,Canada, to southern California, U.S.A.
Members in the family Pyrolaceae include such genera as Pyrola, Chimaphila, Moneses and Orthilia (with approximately 40 species overall, of which Pyrola spp. represent about half), and are principally found in cool north temperate and arctic environments, but do occur as far south as Mexico and the West Indies (Heywood 1993).
Arctostaphylos and Pyrola species are widely distributed in several large forest ecosystems across North America including mixedwood, boreal and sand dune habitats. Some arbutoid species (e.g. A. uva-ursi) provide important ground cover and appreciable biomass in many northern regions.
The fact that some (although probably not all) arbutoid hosts associate with mycorrhizal fungi that may also form ectomycorrhizal symbioses with other tree species, suggests a strong potential for the development of fungal linkages and nutrient exchange; in addition, arbutoid species may also act as repository (refuge) for mycorrhizal fungi, following tree harvest.
Exceptions to this “broad” host specificity has been suggested for some fungi, such as certain Leccinum species, which may only associate with Arbutus or Arctostaphylos (Thiers 1975; Acsai and Largent 1983a,b; Molina et al. 1992). Horton et al. (1999) suggest that fungi symbiotic with Arctostaphylos glandulosa facilitate the survival and growth of Douglas-fir seedlings growing in the central coast area of California. Arbutoid Mycorrhiza
When grown in A. glandulosa patches, 1-year-old Douglas-fir seedlings shared 17 species of fungi with Arctostaphylos, allowing for possible fungal linkages and facilitating seedling establishment.
A field study by Hagerman et al. (2001) documented that bearberry (Arctostaphylos uva-ursi) growing on sites 3 years after harvest in southern British Columbia had 10 morphotypes (out of 17) in common with Douglas-fir seedlings on the same sites.
With this important mycorrhizal group, we are just beginning to explore the importance and magnitude of fungal linkages, as well as refugia for mycorrhizal fungi, and their impact on maintaining species diversity, especially following ecosystem disturbance. Arbutoid Mycorrhiza