(2004). Microphotographs of the outer surface of white sweetclover (Melilotus alba) roots show that both DsRed-labeled strain Rm1021 and the green fluorescent protein (GFP)-labeled
strain GMI6032 of S. meliloti attached to the root surface, forming aggregates and infection threads that contained only DsRed-labeled cells (Fig. 2a). Infection threads and small aggregates that contained either GFP- or DsRed-expressing rhizobia were observed (Fig. 2b). Where the two strains overlap, fluorescence is yellow. In infection threads containing both, GFP- and DsRed-expressing rhizobia were not randomly intermixed (Fig. 2c). Surface components are involved in the early stages of nodulation elicited by rhizobia, and are critical for biofilm formation. The change from a planktonic to a biofilm lifestyle in S. meliloti is mediated by numerous environmental signals selleck compound (Rinaudi et al., 2006). Biofilms are the most common life strategy for bacteria in natural environments, including the rhizosphere, as typified by S. meliloti. Mycelial colonization and biofilm formation by bradyrhizobia with common soil fungi have been reported (Seneviratne & Jayasinghearachchi, 2003). Such biofilms showed nitrogenase activity (Jayasinghearachchi & Sereviratne, 2004a, b; Sereviratne & Jayasinghearachchi, 2005) and enhanced availability of nitrogen and phosphate when inoculated
to soil (Sereviratne & Jayasinghearachchi, 2005). Heavy mycelial colonization by Bradyrhizobium elkanii SEMIA 5019 was observed in Pleurotus ostreatus-bradyrhizobial biofilms 16 days postincubation (Jayasinghearachchi Apoptosis inhibitor & Sereviratne, 2004a). Nitrogenase activity was detected in the biofilm, but not in the fungus or Bradyrhizobium PD184352 (CI-1040) alone. This study proved that symbiotic bacteria within biofilms can
fix nitrogen, and that the fungi are not responsible for nitrogenase activity, as was claimed previously. Similar findings were reported in a B. elkanii SEMIA 5019/Penicillium spp. system. Shoot, root, and nodule weights of soybean plants treated with a biofilm inoculum were significantly higher than those of control plants under greenhouse conditions (Jayasinghearachchi & Sereviratne, 2004b). Biofilm-inoculated plants also showed significantly higher shoot and root nitrogen accumulation. Therefore, use of nitrogen-fixing biofilms as inoculants may promote soil nitrogen fertility and plant growth. Mycelial growth in the rhizosphere may facilitate the movement of rhizobia, which normally show reduced vertical mobility (McDermott & Graham, 1989), because plants inoculated with a bradyrhizobial-fungal biofilm displayed better nodule distribution than conventionally inoculated plants (Jayasinghearachchi & Sereviratne, 2004b). Application of the B. elkanii SEMIA 5019–Penicillium spp. mix enhanced phosphate mineralization, in addition to increasing nitrogen availability in soil.