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Arbuscular Mycorrhizal Fungal Colonization. Factors Involved in Host Recognition

2001; Oxford University Press; Volume: 127; Issue: 4 Linguagem: Inglês

10.1104/pp.010783

1532-2548

Vijay Gadkar, Rakafet David-Schwartz, Talya Kunik, Yoram Kapulnik,

Plant and Fungal Species Descriptions

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 Gianinazzi-Pearson, 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