Arbuscular mycorrhizas (AM) are probably the most widespread plant symbioses. They are formed by c. 80% of land plant species and fungal species belonging to the order Glomales (Zygomycotina) (Smith & Read, 1997). The symbiosis is of ancient origin, as shown by fossils in the Devonian Rhynie Chert (450 million yr BP) (Nicolson, 1975; Remy et al., 1994), the Ordovician of Wisconsin (460 million yr BP) (Redecker et al., 2000) and by molecular phylogeny of the fungi (Simon et al., 1993). Compared with the interactions between plants and other microorganisms, AM symbioses have extremely low specificity and high compatibility between plant and fungal symbionts, which may reflect both their mutualistic biotrophic nature and the very long period of coevolution. Studies on the mechanisms controlling the development of AM associations by both plant and fungal symbionts are significant for understanding the interactions between plants and microbes in general.
The development of AM colonization involves several well-defined stages. The phenotypic characteristics of each stage have been summarized by Smith (1995). The interactions between plant and fungus start before they come into physical contact. Hyphal growth and branching are induced by the roots of host plants, followed by the formation of appressoria which are thought to be the key event in recognition (Staples & Macko, 1980), leading to the hyphal penetration of the roots (Giovannetti et al., 1994; Smith & Read, 1997). By contrast, in nonhost plants, the signals inducing hyphal growth and branching, and recognition at the root surface, are apparently switched off (Giovannetti & Sbrana, 1998). Following the penetration of host root epidermal cells, hyphae grow inter- or intracellularly in the root cortex and intracellular hyphae form arbuscules. Exchange of P and carbohydrate between plant cells and fungal hyphae occurs across these plant–fungus interfaces (Smith & Read, 1997).
Plant mutants in which the formation of AM structures is impaired have been identified in both legume and nonlegume plants and indicate the existence of plant genes controlling AM development. Among these mutants, three main phenotypes have been characterized in legumes. Those referred to as myc−1 are most frequently found among nod−fix− mutants. In this phenotype, fungal growth is blocked on the root surface following formation of appressoria (Duc et al., 1989; Bradbury et al., 1991; Gianinazzi-Pearson et al., 1991; Bradbury et al., 1993; Balaji et al., 1994; Sagan et al., 1995; Shirtliffe & Vessey, 1996; Senoo et al., 2000). Mutants with fungal development arrested in the epidermal cells have been identified in Lotus japonicus (Wegel et al., 1998; Senoo et al., 2000). Another phenotype, designated as myc−2, has been identified among Nod+fix− mutants in Pisum sativum (Gianinazzi-Pearson et al., 1991), Medicago truncatula (Sagan et al., 1995) and M. sativa (Bradbury et al., 1993). In this phenotype, fungi can penetrate the roots and colonize cortical cells, but arbuscular development is reduced to a few stumpy branches.
These different phenotypes of mycorrhiza mutants were thought to be entirely controlled by the altered genes in the plants (Gianinazzi-Pearson et al., 1991; Wegel et al., 1998). The influence of the identity of AM fungi on the phenotypes has not been reported. Gianinazzi-Pearson et al. (1996) suggested that no isolate of microsymbiont (either mycorrhizal fungus or Rhizobium) had been found to infect myc−1 pea mutants. However, in most cases only one or two fungal species have been used to assess the phenotypes of each mutant. One exception is the M. sativa which were challenged by G. versiforme, G. intraradices, G. monosporum, G. fasciculatum and Gi. margarita. The phenotypes differed only in the number and the morphology of appressoria, and not in the extent to which root tissues could be penetrated by the fungi (Bradbury et al., 1991; Bradbury et al., 1993).
A mycorrhiza-defective mutant (rmc) has been identified in a nonlegume plant species, Lycopersicon esculentum (Barker et al., 1998). The phenotype was initially tested against G. mosseae (also used to test the Pisum mutants) and found to be similar to myc−1. Some differences in the development of G. mosseae, G. intraradices and Gi. margarita were observed (Barker et al., 1998), which suggested that the interactions with rmc might vary with fungal genotype. In order to confirm the phenotype of this mutant and to extend the study to include a wider range of AM fungal genotypes, we challenged the mutant rmc with eight species of AM fungi. These were G. intraradices, G. mosseae, G. coronatum, G. versiforme, G. etunicatum, G. fasciculatum, Gi. margarita and S. calospora. Except for G. coronatum and S. calospora, the species were chosen from among those used to characterize phenotypic features of other mycorrhiza-defective mutant plants. The results showed differential colonization patterns of species of AMF on rmc, highlighting diversity in the interactions, influenced by the genotypes of both plant and fungal partners.