Structural diversity in (vesicular)–arbuscular mycorrhizal symbioses

Abstract

summary This review describes diversity in the structure of (vesicular)‐arbuscutar (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‐type, defined on the basis of an extensive intercellular phase of hyphai growth in the root cortex and development of terminal arbuscules on intracellular hyphai branches; (2) the Paris‐type, defined by the absence of the intercellular phase and presence of extensive intracellular hyphai coils. Arbuscules are intercalary structures on the coils. However, there have been many reports that in Paris‐types arbuscules are relatively 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 and many wild angiosperms. The cultivated herbs that are the subject of much experimental work are mostly Arum‐types. Although evidence is still limited, there are differences at the family level. In 41 angiosperm families there are records of only Poris‐type VA mycorrhizas and in 30 families records of only Arum‐types. Another 21 families have examples of both classes, or intermediates between them. Accordingly, we consider whether the original division into two classes is still useful. We conclude that it is when considering the physiology of the symbiosis and especially the issue of whether different fungus/host interfaces have specialized roles in transfer of inorganic nutrients and organic carbon between the partners, if there is no such specialization between hyphai coils and arbuscules, then the latter might not be necessary1 for the function of Paris‐types. This would account for reports of the infrequency or absence of arbuscules in this class. The control exerted on structures by the genomes of host and fungus, and possible reasons (anatomical and physiological) for the existence of the VA mycorrhizal structures, are discussed. The presence or absence of extensive intercellular spaces and differences in the wall structure of cortical cells might be particularly important in determining which type of VA mycorrhiza is formed.

Arbuscular mycorrhizal fungi associated with the Meliaceae on Hainan island, China

“…One possible explanation is that Meliaceae may form typical Paris-type or intermediate-type mycorrhizae. This finding supports the results of a previous work that was restricted to only one genus of Meliaceae (summarized in Smith and Smith 1997) and extends the number of genera in the Meliaceae examined to 15. There were significant differences in the species richness and spore density of the AM fungi in the rhizopheres of the plants on Hainan island.…”

Arbuscular mycorrhizal colonization of vascular plants from the Yungas forests, Argentina

“…The occurrence of AM fungi in these families agrees with the reports of Harley and Harley [40], Newman and Reddell [63], Godoy et al [37], and Zhao [86]. The presence of the Paristype predominates in ferns [76]. Only in one fern species we have observed arbuscules (Tab.…”

“…A root was considered AM colonized when the following structures were observed: arbuscules, vesicles, hyphal coils, intraradical aseptate hyphae, and appressoria. We also determined the AM morphological type of the species studied, to find out whether they were the Arum-or the Paris-type [76]. The quantification of the AM root colonization was estimated visually and characterized using five classes of mycorrhizal root colonization: Class I, 1-5%, Class II, 6-25%, Class III, 26-50%, Class IV, 51-75%, and Class V, 76-100% [70].…”

Root tissue structure is linked to ecological strategies of grasses

“…However, an association between interspecific variation in these functions and in RGR is unknown. Cortical tissue is also important as an interface between arbuscular mycorrhizal fungi and plant (Smith & Smith, 1997), but general differences between fast-growing and slow-growing species with respect to relative importance of mycorrhizal symbiosis have so far not been shown (Koide, 1991 ;Smith & Smith, 1996). The function of the relatively larger cortex in fast-growing species might simply be to increase the root surface area and absorption rate per root length.…”