Quantity and distribution of arbuscular mycorrhizal fungal storage organs within dead roots

Abstract

The formation of storage organs, such as spores and vesicles, is a central part of the life cycle of an arbuscular mycorrhizal fungus (AMF), but the conditions under which this occurs in AMF are not well understood. Here, quantity and distribution of storage organs formed by the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae within dead (excised) roots were characterised. ‘Trap roots’ (TR), separated from the growth substrate by a 30-μm mesh, supported hyphal growth and formation of storage organs of the AMF. Hyphae developed both inside and on the outside of the TR and also within air gaps of surrounding nylon mesh compartments, but formation of vesicles and spores was confined to the interior and to the surface of the TR. Up to 20 % of the TR length harboured newly formed storage organs, resulting in a number of about 60 per mg TR dry weight. The portion of TR length containing storage organs was greater in coarse (diameter >300 μm) than in thin (<150 μm) TR, irrespective of whether the TR were sourced from an AMF host or non-host plant. We conclude that the AMF’s extraradical mycelium produces its storage organs within dead roots in preference to air space in the substrate. Dead roots may indirectly supply nutrients to AMF (once they have been mineralised) or represent a protected space for the fungal structures to develop. The experimental technique described here allows for the preparation of AMF spores and vesicles of F. mosseae free of any mineral substrate.

Resolving the mycorrhizal status of important northern hemisphere trees

“…Using arbuscules to define AM is required to distinguish mycorrhizal from endophytic activity (represented by vesicles and hyphae without arbuscules in roots) which is common in NM plants (Demars and Boerner 1996;Cosme et al 2018) and EcM plants (Cázares and Trappe 1993;Wagg et al 2008). AM fungi also commonly produce hyphae and vesicles in dead roots, decomposing leaves, old seeds, etc., but it is not clear if this is a form of necrotrophic interaction or if these fungi often seek shelter in soil organic materials (Aristizábal et al 2004;Müller et al 2017).…”

Misdiagnosis of mycorrhizas and inappropriate recycling of data can lead to false conclusions

“…These observations and many since then support the idea that AM fungi form endophytic associations in roots of nonhost that cannot be classified as AM (see Brundrett, , ). Further evidence is provided by the frequent observations that (1) fungal activity is greater when nonhosts are grown with a companion plant that has AM; (2) colonization by AM fungi is most common in old roots, presumably due to declining resistance to fungal colonization, and that AM fungi grow into and sporulate in dead roots (Brundrett, ; Müller et al ., ). The increased frequency of AM fungi hyphae in roots of NM plants in soils containing high heavy metal concentrations (Orlowska et al ., ; Vogel‐Mikuš et al ., ) suggests that plant stress may increase proliferation of hyphae of nonmutualistic fungi in NM roots.…”

Carbon for nutrient exchange between arbuscular mycorrhizal fungi and wheat varies according to cultivar and changes in atmospheric carbon dioxide concentration

“…Intracellular plant-fungal interfaces are formed, and degenerate, throughout the lifetime of the symbiosis. As such, the abundance of these structures, particularly those believed to serve fungal storage organs, may be used to infer relative plant carbon investment (Müller, Ngwene, Peiter, & George, 2017) over a longer period of time than the instantaneous measurements made through the isotope tracing approach used here. The frequency of vesicles, as fungal lipid stores, may be indicative of AMF carbon acquisition (Smith, Grace, & Smith, 2009).…”