Acidophilic actinomycetes from rhizosphere soil: diversity and properties beneficial to plants.

Three hundred and fifty-one isolates of actinomycetes were recovered from 21 rhizospheric soil samples using acidified media of pH 5.5. They were evaluated for their antifungal, siderophore production and phosphate solubilization activities. The total count of actinomycetes growing on acidified starch casein agar and Gause no. 1 agar were below 2.48 × 10(4) CFU g(-1) soil. Two hundred and twelve isolates were assigned to acidophiles and the remaining 139 isolates were neutrophiles. Of these actinomycetes, 57.8, 32.5 and 50.4%, showed antagonistic activity against three rice pathogenic fungi; Fusarium moniliforme, Helminthosporium oryzae and Rhizoctonia solani, respectively. More than half of the isolates (68.1%) inhibited at least one tested pathogenic fungus, whereas 25.9% exhibited antifungal activities against all tested fungi. Three hundred and thirty-eight isolates (96.3%) produced siderophore and 266 isolates (75.8%) solubilized phosphate. A greater proportion of the acidophilic actinomycetes exhibited antifungal, siderophore production and phosphate solubilization activity compared with the neutrophiles. Three hundred and twenty-five isolates (92.6%) were classified as streptomycetes based on their morphological characteristics and the presence of the LL-isomeric form of diaminopimelic acid in whole-cell hydrolysates. The 16S ribosomal RNA (rRNA) gene analysis of representative non-streptomycete strains showed that the isolates belonged to seven genera, that is, Allokutzneria, Amycolatopsis, Mycobacterium, Nocardia, Nonomuraea, Saccharopolyspora and Verrucosispora. The potential antifungal acidophilic isolates, R9-4, R14-1, R14-5 and R20-5, showed close similarity to Streptomyces misionensis NBRC 13063(T) (AB184285) in terms of morphological characteristics and 16S rRNA gene sequences.