Choice of Host Root

Various plants have been used to establish monoxenic cultures with AM fungi (see Chap. 7, Table 1). The first host roots used in monoxenic cultures were Lycopersicon esculentum Mill. (tomato) and Trifolium pratense L. (red clover) associated with Glomus mosseae Nicolson & Gerd. (Mosse and Hepper 1975), and Fragaria x ananassa Duchesne. (strawberry) and Alium cepa L. (onion) associated with several Glomus species (Strullu and Romand 1986; Table 1).

A natural genetic transformation of roots with the soil bacterium Agro-bacterium rhizogenes Conn. was achieved decades ago (Riker et al. 1930; Ark and Thompson 1961), but only applied in mycorrhizal research since the mid-1980s (Mugnier and Mosse 1987). Since the development of the carrot hairy root line (Daucus carota L.), established by Becard and Fortin (1988), this root has become the most widespread host for monoxenic cultivation of AM fungi. Transformed roots of strawberry (Nuutila et al. 1995) and tomato (Simoneau et al. 1994; Khaliq and Bagyaraj 2000) were also developed with fewer studies published. Recently, transformed roots of Medicago truncatula Gaern. have been obtained and associated with AM fungi (Boisson-Denier et al. 2001). It is probable that this association will receive increasing attention in the coming years, thanks to its usefulness as a legume model plant in molecular biology (Cook 1999; May and Dixon 2004). Transformed roots have several advantages over non-transformed roots for monoxenic cultivation. Their hormonal balance is modified, allowing profuse proliferation on synthetic media. It is generally accepted that this modification induces the production of growth hormones in the roots, thereby eliminating the necessity of incorporating of plant hormones into the culture medium. Stability of the transformation over time is dependent on the host cultivar and bacterial strain combination (Labour et al. 2003), but hairy roots used for further experiments have always tested positive.

In association with AM fungi, Ri T-DNA transformed roots show greater AM intraradical colonization and sustain higher extraradical hyphal development than non-transformed roots, which is an advantage for fungal production (Fortin et al. 2002). However, the pattern of colonization, distribution of vesicles, mycorrhizal spread and sporulation mechanisms - all the important growth stages of the fungus - can vary within different cultivars of the same host (Tiwari and Adholeya 2003). Furthermore, transformation of the same host cultivar with a different bacterial strain results in a different mycorrhizal susceptibility (Labour et al. 2003). More research is therefore needed to get a better insight into the physiological host preference/specificity of these fungi.

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