Visualization of Infection of an Endophytic Actinomycete Streptomyces galbus in Leaves of Tissue-cultured Rhododendron

Infection of a Streptomyces galbus strain R-5 in leaves of tissue-cultured seedlings of rhododendron was examined using transmission electron microscopy and a histochemical method. Mycelia invaded the host through stomatal openings but not through other sites. Mycelia colonizing leaf surfaces and those growing through stomatal openings were embedded in an electron-dense, thick, amorphous material, while single hyphae were not. Within host tissues, hyphae were observed individually or in colonies in intercellular spaces but not in epidermal or mesophyll cells. The mycelia colonizing in intercellular spaces were also embedded in a similar amorphous material. Wall appositions formed within the epidermal and mesophyll cells, the outer surfaces of which were attached to the intercellular mycelia, suggesting that those cells had recognized the presence of mycelia of R-5 and responded to it. On the leaf surfaces, the wax that covered the undulating cuticular layer was degraded beneath the growing mycelia. This observation was supported by the presence of indigo blue crystals that formed as hydrolytic products of mycelial nonspecific esterase along the mycelia on fresh leaves.