An Endophytic Actinomycete, Streptomyces sp. MBR-52, That Accelerates Emergence and Elongation of Plant Adventitious Roots

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.