TANSLEY-REVIEW-NO-96 STRUCTURAL DIVERSITY IN (VESICULAR)-ARBUSCULAR MYCORRHIZAL SYMBIOSES

This review describes diversity in the structure of (vesicular)-arbusc ular (VA) mycorrhizas, i.e. endomycorrhizas formed by Glomalean fungi. In particular, we consider the extent in the plant kingdom of the two classes first described by Gallaud (1905). These are: (1) the Arum-ty pe, defined on the basis of an extensive intercellular phase of hyphal growth in the root cortex and development of terminal arbuscules on i ntracellular hyphal branches; (2) the Paris-type, defined by the absen ce of the intercellular phase and presence of extensive intracellular hyphal coils. Arbuscules are intercalary structures on the coils. Howe ver, there have been many reports that in Paris-types arbuscules are r elatively few in numbers, small, or absent altogether. A survey of the literature has revealed that Paris-types occur more frequently in the plant kingdom than Arum-types and predominate in ferns, gymnosperms a nd many wild angiosperms. The cultivated herbs that are the subject of much experimental work are mostly Arum-types. Although evidence is st ill limited, there are differences at the family level. In 41 angiospe rm families there are records of only Paris-type VA mycorrhizas and in 30 families records of only Arum-types. Another 21 families have exam ples of both classes, or intermediates between them. Accordingly, we c onsider whether the original division into two classes is still useful . We conclude that it is when considering the physiology of the symbio sis and especially the issue of whether different fungus/host interfac es have specialized roles in transfer of inorganic nutrients and organ ic carbon between the partners. If there is no such specialization bet ween hyphal coils and arbuscules, then the latter might not be necessa ry for the function of Paris-types. This would account for reports of the infrequency or absence of arbuscules in this class. The control ex erted on structures by the genomes of host and fungus, and possible re asons (anatomical and physiological) for the existence of the VA mycor rhizal structures, are discussed. The presence or absence of extensive intercellular spaces and differences in the wall structure of cortica l cells might be particularly important in determining which type of V A mycorrhiza is formed.