Sachiko Hasegawa

An endophytic actinomycete, Streptomyces sp. AOK-30, isolated from mountain laurel and its antifungal activity

To survey endophytic actinomycetes as potential biocontrol agents against fungal diseases of mountain laurel, young plants of mountain laurel (Kalmia latifolia L.) were used as an isolation source. From a total of 73 actinomycetes isolates obtained from leaves, stems and roots of test plants, Streptomyces sp. AOK-30 was selected, because i) it had a broad and intense antimicrobial spectrum against various yeasts and fungal pathogens of Ericaceae, ii) it grew on the multiplication and rooting media for tissue culture for this plant, and iii) the tissue-cultured seedlings that had been treated with this isolate became resistant to Pestalotia disease without any adverse growth effects such as malformation, dwarfing, discolorization and defoliation. This isolate was identified as Streptomyces sp., based on cultural, physiological and morphological characteristics. The present results indicate that isolate AOK-30 is a potential biocontrol agent against diseases of mountain laurel.

Disease resistance induced by nonantagonistic endophytic Streptomyces spp. on tissue-cultured seedlings of rhododendron

Of 82 strains of endophytic actinomycetes isolated from rhododendron plants, 12 were not antagonistic against Pestalotiopsis sydowiana, which is the causal agent of Pestalotia disease. Of these 12, MBR-37 and MBR-38 (identified as Streptomyces spp.) grew on IMA-2 medium. Tissue-cultured seedlings of rhododendron treated with these nonantagonistic strains showed less wilting and/or smaller lesions to P. sydowiana, although the degree of resistance was a little lower than that conferred by antagonistic Streptomyces galbus strain R-5. These seedlings accumulated the anthocyanin(s), suggesting that resistance induced by these strains could depend on activated defense responses associated with the phenylpropanoid pathway rather than with antibiosis.

Genes expressed in tissue-cultured seedlings of mountain laurel (Kalmia latifolia L.) with colonizing Streptomyces padanus AOK30

An endophytic actinomycete, Streptomyces padanus AOK30, is capable of protecting mountain laurel against infection by Pestalotiopsis sydowiana, a causal agent of Pestalotia disease, when applied to the seedling of the plant. In this study, suppression subtractive hybridization was used to identify genes differentially expressed in seedlings of mountain laurel after application of S. padanus AOK30. Subsequent dot hybridization with independent RNA from S. padanus-colonized and control plants identified nonredundant 180 cDNAs involving 71 and 109 clones, which were up- and downregulated after inoculation with the bacteria, respectively. Comparison of the sequences with databases revealed that a number of transcripts encoding proteins or enzymes that function directly in defense or stress response and regulatory proteins were regulated differentially in the seedlings with colonizing S. padanus AOK30. Semi-quantitative RT-PCR analysis for the selected genes demonstrated that inoculation of mountain laurel seedlings with S. padanus AOK30 increased expression of defense-related genes as well as distinct classes of glutathione S-transferase, although endochitinases were exclusively suppressed. These results clearly indicate that the S. padanus-colonizing seedlings likely initiate or prime plant defense responses toward pathogen infection. Selected genes were also differentially expressed in S. padanus-colonized seedlings, compared to those solely challenged with the fungal pathogen P. sydowiana. This approach will assist in efforts not only to understand the molecular basis of the enhanced tolerance and/or enhanced disease resistance of mountain laurel, but also to define a core set of genes during colonization or association with S. padanus AOK30.