The arbuscular mycorrhizal symbiosis: a molecular review of the fungal dimension

Mycorrhizal associations vary widely in structure and function, but the most common interaction is the arbuscular mycorrhizal (AM) symbiosis. This interaction is formed between the roots of over 80% of all terrestrial plant species and Zygomycete fungi from the Order Glomales. These fungi are termed AM fungi and are obligate symbionts which form endomycorrhizal symbioses. This symbiosis confers benefits directly to the host plant's growth and development through the acquisition of P and other mineral nutrients from the soil by the fungus. In addition, they may also enhance the plant's resistance to biotic and abiotic stresses. These beneficial effects of the AM symbiosis occur as a result of a complex molecular dialogue between the two symbiotic partners. Identifying the molecules involved in the dialogue is a prerequisite for a greater understanding of the symbiosis. Ongoing research attempts to understand the underlying dialogue and concomitant molecular changes occurring in the plant and the fungus during the establishment of a functioning AM symbiosis. This paper focuses on the molecular approaches being used to study AM fungal genes being expressed in the symbiotic and asymbiotic stages of its lifecycle. In addition, the importance of studying these fungi, in relation to understanding plant processes, is discussed briefly.