Australia: The Land Where Time Began

A biography of the Australian continent 

These are the most common, widespread of the mycorrhizal types, being found associated with about 80 % of plant species. Some examples of their host trees are monkey puzzles, bananas, maple oaks, as well as many shrubs and the majority of herbaceous plants, such as ferns. They occur very widely, being found in tropical rainforests, deserts and alpine meadows. A common feature of many of the soils they are associated with is a degree of phosphorus deficiency, their role in the symbiosis being to gather enough phosphorus to supply the plant host. It is believed this type of mycorrhiza may be the type that first established a symbiotic role with the earliest vascular plants, as indicated by the very wide range of plant hosts.

This type of mycorrhizal association is formed by zygomycetes. The fungal hyphae penetrate the epidermal root layer, forming a highly branched structure, arbuscules,  in the cortex cells, but don't penetrate deeper into the vascular tissues. Sugar and nutrients can then be exchanged between the fungus and the host plant across the major interface of the arbuscules and hyphae. The plasma membrane, the plasmalemma, is not penetrated when the hyphae penetrate the cell walls, the the plasma membrane expanding to surround the hyphae and arbusculae. The result is that the cell membranes of the plant cell and fungus are separated only by apoplasm through which the nutrients and sugar can pass between the partners, the flow being controlled by the both partners. The surface to volume ratio of the parts of the fungus inside the plant tissue is increased by more than fivefold by the branched structure of the arbuscules. The external hyphae form a mycelium around the infected root that can extend more than 12 cm from the root surface, which increases greatly the volume of soil that nutrients can be extracted from.

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003

• Arbuscular Mycorrhizas
• Ectomycorrhizas
• Ericoid Mycorrhizas
• Orchid Mycorrhizas