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.

Allokutzneria oryzae sp. nov., isolated from rhizospheric soil of Oryza sativa L.

The taxonomic status of a rhizospheric soil actinomycete, designated R8-39(T), was established using a polyphasic approach. The organism had phenotypic and morphological characteristics consistent with its classification in the genus Allokutzneria. Phylogenetic analysis based on an almost complete 16S rRNA gene sequence showed that the strain formed a monophyletic clade with the type strains of members of the genus Allokutzneria. Strain R8-39(T) displayed the highest levels of 16S rRNA gene sequence similarity to Allokutzneria albata DSM 44149(T) (98.8%) and Allokutzneria multivorans YIM 120521(T) (98.3%). However, the DNA-DNA hybridization values between strain R8-39(T) and A. albata and A. multivorans were clearly below the 70% threshold. The organism was found to have chemical characteristics consistent with its classification in the genus Allokutzneria. Whole-cell hydrolysates contained meso-diaminopimelic acid, arabinose, galactose, glucose, mannose, rhamnose and ribose. The main menaquinone was MK-9(H4). No mycolic acid was detected. The G+C content of the genomic DNA was 71.8 mol%. In addition, strain R8-39(T) had a phenotypic profile that readily distinguished it from recognized representatives of the genus Allokutzneria. It is evident from the combined genotypic and phenotypic properties that strain R8-39(T) represents a novel species of the genus Allokutzneria. The proposed name for this species is Allokutzneria oryzae sp. nov.; the type strain is R8-39(T) ( = BCC 60399(T) = NBRC 109649(T)).