Endophytic Actinomycetes : Attractive Biocontrol Agents(Presidential Address)

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In modern agriculture, application of agrochemicals is still an invaluable and effective method to control plant diseases. However, since use of agrochemicals has been fallen into disfavor because of environmental pollution and detrimental effects on a variety of nontarget organisms, potential use of microbe-based biocontrol agents as replacements or supplements for agrochemicals has increased in agricultural importance. The success of biocontrol largely depends on basic knowledge about the selected antagonist. Generally, biocontrol candidates are selected using an in vitro test of their antagonistic activity against target pathogens and an environment-controlled pot test. However, subsequent in vivo experiments hold the key to success in biocontrol; the selected agent may lose its antagonism when environmental conditions change, and the respective microbes may respond differently to these changes^7). Plant-associated microbes sometimes give physiological and environmental advantages to their host plants. For example, Narisawa et al.^5) reported that Chinese cabbage seedlings grown from seeds treated with endophytic Heteroconium chaetospira became resistant to Plasmodiophora brassicae, and their growth was significantly promoted. Furthermore, Varma et al.^12) found that when calli of tobacco and seeds of maize, tobacco and parsley were treated with Piriformospora indica, growth of shoots and roots of these plants was greatly promoted. Van Wees et al.^11) demonstrated that selected strains of nonpathogenic rhizobacteria from the genus Pseudomonas induced broad-spectrum systemic resistance. In all these reports, endophytic microbes affected the physiology of their host plants. The endophytic presence of Streptomyces spp. may have important roles in plant development and health because they can affect plant growth either by nutrient assimilation or by secondary metabolite production. Matsukuma et al.^4) and Okazaki et al.^6) reported that a variety of actinomycetes inhabit a wide range of plants as either symbionts or parasites. They also reported that several new or rare species of actinomycetes were discovered from plants and suggested that their secondary metabolites might be promising sources of novel antibiotics and growth regulators of other organisms. Actinomycetes, especially Streptomyces spp., isolated from the rhizosphere have proven to be excellent biocontrol agents of soilborne plant pathogens^13). Such an effective activity is largely dependent on secondary metabolites produced by these organisms. These earlier reports led us to assume that if a useful endophytic actinomycete isolated from a field-grown plant can successfully colonize tissue-cultured seedlings of the plant, the seedlings could become resistant to various plant pathogens. Because tissue-culture flasks are usually axenic, such a novel technique should allow this actinomycete to colonize its host plant without competition and/or antagonism by any other microbes in soil. Here, I would like to address the recent progress of our research on a novel biocontrol technique using endophytic actinomycetes^1),2),10). In this study we used tissue-cultured seedlings of rhododendron that were supplied from Akatsuka Orchid Co. in Tsu city when necessary.
日本植物病理学会の論文
2002-08-25

Endophytic Actinomycetes : Attractive Biocontrol Agents(Presidential Address)

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