Update on Mycorrhizal Symbiosis Arbuscular Mycorrhizal Fungal Colonization. Factors Involved in Host Recognition

The arbuscular mycorrhizal (AM) symbiosis is the association between fungi of the order Glomales (Zygomycetes) and the roots of terrestrial plants (Harley and Smith, 1983). Conservative estimates suggest that this ancient symbiosis, dating back to the early Devonian age (398 million years ago), affects approximately 90% of the Earth’s land plant species (Remy et al., 1994). This symbiosis is increasingly being recognized as an important and integral part of natural ecosystems throughout the world. The AM fungus-plant association is a mutually beneficial event: The plant supplies the fungus with carbon (from its fixed photosynthates) while the fungus assists the plant in its uptake of phosphate and other mineral nutrients from the soil (Smith and Gianinazzi-Pearson, 1988; Smith and Read, 1997). This bidirectional exchange of nutrients takes place through extensively branched haustoria, termed arbuscules. In addition to increased nutrition, mycorrhizal plants also show increased resistance to root pathogens and tolerance to drought stress, and their hormonal balance is altered (Smith and GianinazziPearson, 1988; Hwang et al., 1992). A major challenge for the mycorrhizologist is to understand the extremely harmonious AM fungushost signaling mechanisms and the colonization process. This harmonious symbiotic relationship is reflected in the obligate biotrophic nature of the fungi, which cannot be cultured in the absence of a host (Williams, 1992). The most accepted reason for the obligate biotrophy is that the fungus, during the long evolution of its symbiotic relationship with the host plant, lost some of its carbon-fixing capabilities or the genetic machinery that supports them, and became completely dependent on the host plant for fixed carbon supply. The empirical evidence for this hypothesis is still lacking, but several indirect approaches to the study of this relationship have been developed. In all of the current methods of cultivating AM fungi, the presence of the host plant is indispensable. Many variants of these methods have been developed, including the classical soil-based system, aeroponic and hydroponic systems, and the recent in vitro root organ culture system. The root organ culture system is the most attractive cultivation methodology for research; it uses root-inducing transferDNA-transformed roots of the host plant to develop the symbiosis on a specific medium in vitro (Bècard and Fortin, 1988). These techniques, though challenging, have proven useful in adding to our understanding of various aspects of the AM fungal-host symbiosis (Douds, 1997). The observation that approximately 150 species of AM fungi (Morton and Bentivenga, 1994) colonize an estimated 225,000 species of plants (Law and Lewis, 1983) has led to the conclusion that AM fungi have wide host ranges. This situation indicates a high degree of adaptability and integration of the symbiotic process across a wide range of plant species (Smith and Read, 1997); but does it mean that the fungi have no preferences among plants? Do all host plants emit signals that indicate their availability for colonization by all AM fungi? The very fact that plants respond to colonization by other soil biota, e.g. by initiating diverse biochemical and physiological changes, but do not do so when “invaded” by AM fungi supports the hypothesis that a specific signal(s) emitted by the AM fungi trigger(s) a cascade of events that culminate in colonization without eliciting any adverse defense reaction from the host. Here, we discuss the question of host specificity in this unique category of symbiotic interactions and update the reader on the existing evidence for mutual recognition mechanisms between the host and the fungus. Emphasis will be placed on how the host responds to colonization by the AM fungus during the early stages of the interaction and on the basic mechanism of recognition by the host. Current exciting developments in the field have set the stage for revealing the roles played by the factors involved in recognition and colonization of the host plant.