A

FIGURE 3.10. Conceptual diagram of a root-rhizosphere-soil system (from Trofymow and Coleman, 1982).

Bulk soil

Transect B

FIGURE 3.10. Conceptual diagram of a root-rhizosphere-soil system (from Trofymow and Coleman, 1982).

The field of mycorrhizosphere (Garbaye, 1991; Andrade et al., 1998) research has taken a quantum leap forward with elegant microscopic methods, in conjunction with molecular tools to pinpoint organisms that are co-associates. Artursson and Jansson (2003) used bromodeoxyuri-dine (BrdU), as a thymidine analog, to identify active bacteria associated with AM hyphae. After adding BrdU to the soil and incubating for 2 days, DNA was extracted, and the newly synthesized DNA was isolated by immunocapture of the BrdU-containing DNA. The active bacteria in the community were identified by 16S rRNA gene PCR amplification and DNA sequence analysis. Based on gene sequence information, a selective medium was used to isolate the corresponding active bacteria. Bacillus cereus strain VA1, one of the bacteria identified by the BrdU method, was isolated from the soil and tagged with green fluorescent protein. By using confocal microscopy, this was shown to clearly attach to AM hyphae. This study by Artursson and Jansson (2003) is a pioneering attempt, using molecular and traditional approaches, to isolate, identify, and visualize (Fig. 3.11) a specific bacterium that is active in fallow soil and associates with AM hyphae.

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