Plants of the Ericales live in inhospitable environments. In Australia, members of the Ericaceae are found in heath and open woodland vegetation. The plants are characterised by sclerophylly, with thick walled cells and heavily lignified tissues. The plants are found in acid to extremely acid soils. Those in calcareous sands in WA will have their root systems in pockets of acidic organic soil, within the alkaline matrix. Mineral nutrients especially nitrogen and phosphorus are particularly unavailable. The mycorrhizal associations of these plants play a crucial role in plant access to both N and P.
Growth of the host plant is highly seasonal. The fine hair roots in which mycorrhizas are found emerge as the wet season begins. They die back as the soil dries. Thus the pattern of root growth determines the process of colonisation, as mycorrhizal colonies form in these hair roots.
The hair root has a limited construction. It consists of a stele with single sieve element with companion cell, with one or two tracheids. Surrounding the stele are two layers of cells: an inner endodermis and an outer hypodermis. Only the outer hypodermal cells become colonised.
The precise source of inoculum for initiating colonisation is unclear. Spores of many epacrid mycorrhizal fungi have the potential to survive steam treatment of soil, a characteristic typical of the ascospores of many Ascomycota. This indicates that soils may contain fungal propagules in excess of that necessary for initiation of mycorrhizas, perhaps because the fungi function in other, as yet unknown, areas.
A second potential source of propagules has been discovered in the epacrid host Woolsia pungens and other species of the drought-prone southern Australia. Some colonised cells develop thick walls. The cells appear to be shed as the soil dries and the hair root dies. These cells contain viable hyphae. The thick walls probably protect the enclosed hyphae during the dry period, enabling re-emergence of the fungus as the hair roots are formed at the beginning of the next growth season.
Colonies are initiated by hyphae invaginating the host wall from a securely attached appressorium. Specific recognition between the symbionts is necessary for colonies to be initiated. However, the mechanisms are not known at this stage. Coils are formed in the outer hypodermal cells of the hair root. Each cell is usually colonised by a single fungus. Adjacent cells may be colonised by different fungi. Mycelia form over the root surface and ramify into the surrounding soil.
The fungi appear to possess a wide array of saprotrophic characteristics. Of particular importance to the host is the capacity of the endophytes to acquire nitrogen and phosphorus from complex plant polymers in extremely acidic conditions. The fungi access protein held on protein-phenol complexes. In these acidic soils, the most important source of phosphate is in organic form, probably as phytates complexed with iron and aluminium. The products of digestion are transported to the host plant.
In acidic soils, iron, manganese and aluminium are present in highly available forms. The fungus appears to regulate plant access to these ions, either through siderophores (Fe), or tolerance of high concentrations (Al, Mn). Ericoid species are also common colonisers of metal tailings. The fungi associated with these plants tolererate the metal. In most cases, the mechanism underlying tolerance has not been clarified. In principle, the fungus may either sequester the metal on the wall, or place the toxin in vacuoles within the hyphae through the activity of metalothiones or other polypeptides.
Ericoid plants tolerate extremely difficult, commonly acidic environments. Their capacity to function in these environments is in large part attributable to their fungal symbiont. The fungus accesses recalcitrant sources of minerals and provides protection from the consequences of the acid soils. We still lack a clear appreciation of the diversity of fungi associated with members of the Ericales and their different functional attributes. Thus the variety of functions, and variation within each function, provided by the fungal partner and the general characteristics of the broader group are still unclear. Given the evolution of a putative protective function by the plant for its mycorrhizal fungus, it is clear that the association is important for both partners.
Smith SE & Read DJ (2008) Mycorrhizal Symbiosis. Academic press, London.