Twenty-two strains were tested for nodulation to Acacia species and other genera. All tested strains could nodulate and showed nitrogen-fixing activity on Acacia mangium. No root hairs had been observed on the surface of Acacia young roots. However, a few short, deformed root hairs had emerged from the root when the seedlings were inoculated with the symbiotic bacteria. The infection thread could be recognized in the induced root hair around 2 weeks after inoculation. The nodule morphology of Acacia was typical determinate type. The existence of bacteria in the nodule cell was exhibited in two groups. That is, the symbiosome is formed by several bacteroid cells enwrapped in a peribacteroid membrane (PBM), called plural type, and by single bacteroid cell in a PBM, called singular type.
The phylogenetic analysis of 16S rRNA gene revealed that the Acacia strains fell into three genera, Rhizobium, Bradyrhizobium and Ochrobacterum. This result showed that the Acacia strains from Southeast Asia were very diverse. Strain DASA35030, from Thailand, showed 93% similarity to Ochrobacterum intermedium. This strain showed rod shape and grew faster (generation time 60 min) than other Rhizobium and Bradyrhizobium strains. The optimum temperature for growth of strain DASA35030 was 37°C though other strains were 30°C. Furthermore, pH growth range of strain DASA35030 was very wide pH 5 to pH 9.5 comparing with other strains. However, most Acacia strains preferred the alkaline condition rather than the acidic condition.
Lebunh et al. (2000) isolated Ochrobacterum strains from rhizoplane of wheat. Our report might be the first one that Ochrobacterum strain was isolated from root nodule of leguminous plant. The most plausible interpretation of this fact could be an example that it is a horizontal transfer of symbiotic gene set of Rhizobium into Ochrobacterum.
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