Drought Tolerance of Tissue-cultured Seedlings of Mountain Laurel (Kalmia latifolia L.) Induced by an Endophytic Actinomycete II. Acceleration of Callose Accumulation and Lignification

Colonization of host plants by microbial symbionts and avirulent parasites often elicits physiological modifications such as localized and systemic disease resistance1–4) and drought or dehydration tolerance5–7). Among such microbes, soil-inhabiting actinomycetes have attracted much attention as potential biocontrol agents in agriculture because of their ability to produce a variety of antibiotics. Recently, however, the usefulness of endophytic actinomycetes for the same purpose was identified 2,8–13) after Matsukuma et al.14) and Okazaki et al.15) isolated a variety of actinomycetes inhabiting a wide range of plants as either symbionts or parasites. Igarashi et al.16–18) discovered novel antibiotics and growth regulators of plants and animals from several species of actinomycetes of plant origin, showing the physiological potential of endophytic actinomycetes in aiding development of novel technology to enhance human life. Based on these earlier reports, Shimizu et al.8–10) developed unique biocontrol methods for endophytic actinomycetes. They successfully produced disease-resistant rhododendron seedlings by isolating Streptomyces galbus strain R-5, which has broad antimicrobial activities, from field-grown rhododendron and added it to flasks in which tissue-cultured seedlings were growing. Subsequently, Nishimura et al.19,20) isolated an antifungal strain of S. padanus, AOK-30, from potted mountain laurel (Kalmia latifolia L.) and induced disease resistance in tissue-cultured seedlings. We further found that colonization with AOK-30 rendered tissue-cultured seedlings drought tolerant and enhanced osmotic pressure in seedling cells21). We also prepared protoplasts from AOK-30-treated and -untreated seedlings and found that those from untreated seedlings were released readily with cellulase, but an additional xylanase was required to prepare those from treated seedlings. These findings led us to postulate that AOK-30 colonization induces structural cell wall modifications in tissue-cultured seedlings of mountain laurel and that these modifications might be associated with AOK-30-induced drought tolerance.