Mutualistic effectors : architects of symbiosis

All living things are heavily influenced by their environment and the flora and fauna living within, on, and around them. It is thought that since the time plants first colonized earth, fungal symbiosis was valuable, if not essential, for plant survival (Baylis, 1972; Pirozynski and Mallock, 1975). Reasoning for this claim is both theoretical and based on the fossil record (Kidston and Lang, 1921; Gehrig et al., 1996; Redecker et al., 2000; Brundrett, 2002; Read and Perez-Moreno, 2003). If fungal-host symbiosis occurred in a similar fashion to present-day mutualistic examples, it is assumed that the interaction between bryophyte-like plants and filamentous fungi allowed the plant to better anchor to the substrate as well as gain nutrients and water from its fungal partner. These early fungal symbionts were likely to be related to the present day Glomus genus"”obligate biotrophs that invade plant cells to form a multilobed hyphal structure called an arbuscule (Fig. 12.1). Across the surface area of the arbuscule, a constant flux of nutrients (e.g., nitrogen, phosphorus, sulfur) from the fungus is exchanged for plant-based sugars.