Microbiological assays
At the R1 growth stage (42 and 38 DAS for GR1 and GR2, respectively), shoots were clipped close to the ground, roots were carefully removed from soil and soil tightly adhering to the root surface (rhizoplane) was rigorously removed using a camel-hair brush and retained as ‘rhizosphere soil.’ A subsample of the cleaned roots was washed under running water and gently blotted on paper towels in preparation for microbiological assays. Roots were surface sterilized in 1Æ25% sodium hypochlorite for 2 min followed by rinsing three times in sterile water and blotted dry on sterile paper towels. Fusarium colonization of the surface-sterilized soybean root segments (8, 2-cm segments per agar plate) was assessed by the root-plating procedure of Le´vesque et al. (1993) on Fusarium-selective agar medium (Nash and Snyder 1962). After 5-day incubation at 25C, the number of fungal colonies developing on root segments was recorded.
A Fusarium colony that developed on a root segment was counted as a single colony-forming unit (CFU) of rhizoplane Fusarium. Total numbers of Fusarium colonies per plate were converted to Fusarium CFU per 100 cm of root. Fusarium colonies were randomly selected, subcultured on potato dextrose agar and tentatively identified using descriptions of cultural and microscopic morphologies (Nelson et al. 1983). Identification of isolates was confirmed by the USDA-ARS Microbial Genomics Unit, Peoria, Illinois, by molecular analysis using partial translation elongation factor sequences (Skovgaard et al. 2001).
A 1-g portion of rhizosphere soil from each sample was suspended in 0Æ01 mol l)1 MgSO4 buffer, and appropriate 10-fold dilutions were plated on S1 agar medium selective for fluorescent pseudomonads (Gould et al.1985) and on Gerretsen’s agar medium for detecting Mn-oxidizing and Mn-reducing micro-organisms (Huber and Graham 1992). After incubation at 25C (48 h for pseudomonads; 7 day for Mn-transforming bacteria), colonies developing on agar medium were recorded as colony-forming units in the rhizosphere. Mn-oxidizing colonies are brown to black on Gerretsen’s medium, and Mn-reducing bacteria are white to opaque within a cleared halo. Mn-transforming bacterial components were further expressed as ratios of Mn reducers to Mn oxidizers to detect potential effects of microbial activity on plant-available Mn (Rengel 1997).
Rhizosphere soils were screened for ı´ndoleacetic acid–producing (IAA) bacteriaby plating on S1 agar medium supplemented with 5 mmol l) 1 l-tryptophan. A nitrocellulose membrane(90 mm diameter) was placed directly on the agar surfaceafter plating prior to incubation (Bric et al. 1991). After 48 h, the nitrocellulose membrane was removed from each plate and placed on filter paper (Whatman 42) saturated with Salkowski reagent (2% 0 Æ 5 mol l)1 FeCl3 in 35% HClO4) in a clean petri plate for up to 4 h (Gordon and Weber 1951). Colonies that developed a pink colourwere scored positive for IAA production and were considered ‘IAA-producing rhizobacteria.’ Representative bacterial colonies from the pseudomonad, Mn transformer and IAA-producing assays were selected and subcultured on S1 and tryptic soy agars to obtain pure, single-colonyisolates. Isolates were characterized for colony morphology and fluorescent pigment production; gram stain and oxidase reactions; and classified taxonomically by cellular fatty acid profiles using gas chromatography-fatty acidmethyl ester analysis (Kennedy 1994).