What do root pathogens see in mycorrhizas?

The obligately biotrophic relationship of the arbuscular mycorrhizal (AM) fungus in the root cortex and the regulation of colonization by carbon supply are strongly suggestive that mycorrhizas interact directly with root pathogens that have similar trophic requirements. Aphanomyces euteiches is a biotrophic pathogen that attacks root cortical tissues of pea and other legumes. The potential exists for resource competition between the symbiotic fungus and pathogen, leading to the reduction of each other’s colonization and reproduction when they coinhabit roots. Larsen Ratnayake et al., 1978). With attainment of P sufficiency in tissues, mycorrhizal mediated decrease in sugar availability from roots is coincident with reduction in pathogen colonization and damage (Graham Graham open triangles – mycorrhizal, high P, %P > 0.11). Curve fit: r2 = 0.835. Data: J. H. Graham, unpublished. The inability of external hyphae to assimilate sugars diminishes the role of extramatrical hyphae as sites for C interactions. Only a small fraction (< 2%) of the total below-ground labelled 14C in mycorrhizal citrus roots is released into the rhizosphere and an even smaller fraction (< 0.2%) is attributable to release into the mycorrhizosphere (Eissenstat et al., 1993). Populations and metabolic activity of Pseudomonas fluorescens are reduced in the hyphosphere of G. intraradices compared to nonrhizosphere soil (Ravnskov et al., 1999). Thus, microorganisms compete intensely for C in the hyphosphere as this arena is perhaps even more growth-limiting than rhizosphere soil. Field evidence remains unconvincing that AM fungi substantially improve P relations and growth of temperate crops in agroecosystems (McGonigle & Miller, 1996). Even in natural systems, plants with fibrous root architecture show only marginal reduction in P acquisition when their mycorrhizas are impaired by fungicides (West & Fitter Watkinson, 1993). This directs attention to other functional attributes of mycorrhizas that might confer competitive advantage for plants to maintain the symbiosis in the absence of substantial P benefit. In a temperate grassland ecosystem, direct interaction of AM fungi, not P uptake, reduces the deleterious effects of the root pathogenic fungi and thereby may increase plant fecundity (Newsham et al., 1994). Newsham et al. (1995) propose that root protection by AM fungi may be as important as nutritional benefits in natural systems, though they do not specify a mechanism for the interaction of mycorrhizas and root-infecting fungi. Their conclusion is derived from study of AM function in only a few plant species under natural conditions, thus, the broader significance of direct mycorrhiza–pathogen interactions requires much greater support from field experimentation. Refined understanding of the exchanges of P, C and other nutrients in arbuscular mycorrhizas and the mycorrhizosphere permits us better to answer the question ‘what do root pathogens see in mycorrhizas?’Larsen and Bodker (1994) employ neutral lipid fatty acids for more precise quantification of the interaction of a root pathogen and an AM fungus than here-to-fore possible. Moreover, these fatty acids give estimates of the ‘energy status’ of the interacting fungi, as the signature acids comprise more than 50% of the storage lipid in each fungus. The concomitant reduction in each fungus’s biomass and energy status provides preliminary evidence for competition for resources between the obligate symbiont and the biotrophic pathogen. The specificity of signature fatty acids for evaluation of direct interactions of selected microbes and plant species should greatly facilitate studies, particularly under field conditions.