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

Abstract

There has been a large body of literature describing potential uses of plant-associated bacteria as agents stimulating plant growth and managing soil and plant health 1). Some of these bacteria colonize the root surfaces and the closely adhering soil surface, rhizosphere, but some others enter root interior, followed by establishing endophyt-ic populations. Regardless of the survival sites, almost all microbes produce and secrete secondary metabolites such as lytic enzymes 2,3) , plant hormones, toxins, antibiotics, siderophores and other biologically active materials, as well-documented in reviews by Compant et al. 1) , Takahashi and-Omura 4) and Kohmoto and Otani 5). As is expected, these molecules very often interfere with growth of other organisms. Since Matsukuma et al. 6) and Okazaki et al. 7) discovered that a variety of actinomycetes inhabit a wide range of plants as either symbionts or parasites, endophytic actin-omycetes have been studied extensively as potential sources of novel antibiotics and physiological activa-tors 8-10). Shimizu et al. 11-13) developed a unique biocontrol method using an endophytic actinomycete: they isolated Streptomyces galbus strain R-5 that shows a broad spectrum of antimicrobial activity, from field-grown rhododendron and added it to flasks in which tissue-cultured seedlings of rhododendron were growing to successfully render them disease-resistant. Similarly, Nishimura and colleagues 14,15) succeeded to produce disease-resistant tissue cultured seedlings of mountain laurel by applying an endophytic strain AOK-30 of S. padanus to the flasks with growing mountain laurel seedlings. Furthermore, Hasegawa et al. 16,17) showed increase of drought tolerance in the AOK-30-treated seedlings by enhancement of osmot-ic pressure in leaf cells, acceleration of callose and lignin accumulation in cell walls. Igarashi et al. 18-20) discovered novel antibiotics and growth regulators of plants and animals from several species of actinomycetes of plant origin. These papers show that secondary bioactive metabo-lites of endophytic actinomycetes apparently affect physiological properties of host plants. In this paper, we report isolation and selection of a strain of endophytic actinomycete that is able to stimulate rooting of plants. MATERIALS AND METHODS Isolation of actinomycetes from field-grown rhododendron Endophytic actinomycetes were isolated from young plants of rhododendron that were harvested from the Akatsuka Garden Co. Ltd. (Tsu-City, Mie Prefecture, Japan), by following the method of Shimizu et al. 11). Briefly, small pieces of leaves, stems and roots were rinsed in 0.1% Tween 20 for a few seconds and then in 1% sodium hypochlorite for 5 min and washed in sterilized distilled water for a few minutes. They were further surface-sterilized in 70% ethanol for 1 min and air-dried in a laminar flow chamber. Each piece was placed on IMA-2 agar medium 11). Actinomycetous filaments growing from the respective samples were transferred onto freshly pre-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.