Mycorrhizae are the most widespread type of association established between plants and soil microorganisms. The roots of 90% of land plants associate with many soil fungi to form complex systems, whose structure and function depend on the specific combination of the eukaryotic partners. During the formation of mycorrhizae, numerous events of specificity and recognition occur at different levels: species, plant organ, root tissue and cell type.

Specificity and recognition in mycorrhizae

Among the different 260 000 plant species, 200 000–240 000 have been estimated to have the potential to form mycorrhizal associations (Law & Lewis, 1983). On the other hand, many fungal representatives of the Zygomycotina, Ascomycotina, Basidiomycotina and Deuteromycotina depend on the mycorrhizal association for the completion of their life cycle (Harley, 1989). The single word ‘mycorrhiza’ encompasses a complexity of forms of interaction, since at least six main types of mycorrhiza can be recognised (Table 1). Extensive observations indicate that some specificity exists in the interaction, as a range of compatible and incompatible hosts can be defined. According to Smith & Douglas (1987) specificity of a symbiosis refers ‘to the degree of taxonomic difference between acceptable partners and may vary from very low to high or very high’. From Table 1 it is clear that mycorrhizae have a low degree of specificity compared to other associations, as a single fungus may associate with plants of more than one class and also a single host plant may associate with different fungal endophytes.