Identification of 6-prenylindole as an antifungal metabolite of Streptomyces sp. TP-A0595 and synthesis and bioactivity of 6-substituted indoles.

The biological control is an alternative measure to agrochemicals to control fungal pathogens in agricultural production. We recently reported the utilization of an actinomycete to the biological control in horticultural plant. Tissue-cultured seedlings of horticultural rhododendron suffer from significant damage caused by Pestalotiopsis sydowiana, a major pathogen of rhododendron, during the acclimatization. The inoculation of an endophytic Streptomyces galbus R-5 isolated from a field-grown rhododendron to the seedlings made them resistant to the pathogen1-3) Although the major antifungal metabolites of strain R-5, actinomycin X2 and fungichromin, were speculated to be responsible for the resistance, their involvement in plant protection was not proved2). In spite of this observation, inoculation of such strains that produce secondary metabolites with antifungal activity is expected as a new strategy to control disease resistance in plants. Alternaria brassicicola is a casual pathogen of Brassica species and causes damage and loss of quality to Brassica crops worldwide4). In our previous screening of the microbial secondary metabolites which suppress the disease symptom caused by A. brassicicola in the seedlings of Chinese cabbage, two positive strains producing metabolites antagonistic and non-antagonistic to the in vitro