More than 100 isolates of endophytic actinomycetes were obtained from field-grown rhododendron. By root-growth tests using cucumber germlings, one isolate, MBR-52, was selected as a final candidate to accelerate rooting of tissue-cultured rhododendron. Re-isolation test proved that when tissue-cultured seedlings of rhododendron were treated with MBR-52 in flasks, this isolate colonized in the seedlings and survived there even after their transplanting in soil. Another transplanting test revealed that emergence of adventitious roots was prominently accelerated in the MBR-52-treated tissue-cultured seedlings, suggesting that MBR-52 might release some type of rooting-promoting plant hormone(s). In addition to nucleotide sequence analysis of 16S rDNA, cultural, physiological, chemical and morphological characterizations showed that MBR-52 belongs to a strain of Streptomyces sp. MBR-52 certainly gives a great practical advantage to shorten the acclimatization period of tissue-cultured seedlings in warm and humid environment which very frequently increases the disease-infection risk of the seedlings.

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Five strains of actinomycetes were isolated from soils in Thailand. PNK1–3, PNK1–5, TT2–9 and KN-6 were belonged to Streptomyces and FLM-2 was Kitasatospora based on their phenotypic and chemotaxonomic characteristics. Phylogenetic analysis using 16S rDNA sequences revealed that PNK1–3 and PNK1–5 were identified as S. hygroscopicus (99.7%), TT2–9 was closed to S. nogalater DSM 40546^T (98.0%), KN-6 was S. purpurascens (99.8%); and FLM-2 showed highest similarity with K. kifunensis LMG 19957^T (97.6%). The selected strain, PNK1–3 produced geldanamycin that showed inhibition on the growth of Gram-positive bacteria and strong cytotoxicity against KB and BC cells lines. Its taxonomic characterization has been described.

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I have studied actinomycetes as source of new bioactive substances for approximately 30 years. The ability of actinomycetes to adapt to different environmental conditions and their capacity to produce various antibiotics has always made a strong impression on me.

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