Bioactive secondary metabolites from two sponge-derived actinomycetes

Abstract

Actinomycetes residing in marine habitat are an excellent treasure house of structurally novel and bioactive natural products with potent medicinal values. To exploit chemically diverse and bioactive natural products from actinomycetes living in underexplored ecological niche, we isolate a large number of marine actinomycetes from marine sponges, corals, algae, mollusks, sea grasses, and sediments collected from the South China Sea. In order to assess the values of these special marine actinomycetes in the green development of tropical agriculture, we screen the activity of marine actinomycetes against phytopathogenic fungi and bacteria. As a result, we identify two strains of sponge-derived actinobacteria <italic>Streptomyces</italic> sp. HMH1 and HML1 possessing antagonism effect against tested phytopathogenic microbes. The secondary metabolites produced by these two actinomycete strains and their anti-phytopathogenic fungi activity are investigated. The secondary metabolites from the fermentation extract of <italic>Streptomyces</italic> sp. HMH1 and HML1 are isolated and purified using various chromatography methods including silica gel column, reverse gel column and Sephadex LH-20 column chromatography. The structures of the isolated compounds are characterized by NMR, HR-ESI-MS spectroscopic data analyses and comparison with previously reported data. Three major compounds belonging to the family of actinomycin are purified from the solid fermentation extract of the sponge-derived actinobacterium <italic>Streptomyces</italic> sp. HMH1. They actinomycin D, actinomycin X₂, and actinomycin X_oβ. A compound identified as K-252d belonging to the family of indolocarbazole alkaloid is isolated from the solid fermentation extract of another sponge-derived actinobacterium <italic>Streptomyces</italic> sp. HML1. The anti-phytopathogenic fungi and bacteria activities of these compounds are evaluated against ten species of plant pathogenic fungi including <italic>Fusarium oxysporum</italic> FOC4, <italic>Lasiodiplodia theobromae, Phomopsis caricae-papayae</italic> Fetrak&amp;Cif., <italic>Corynespora cassiicola, Phytophthora capsica, Pythium aphanidermatum</italic> (Edson) Fitzp, <italic>Corynespora</italic> sp., <italic>Fusarium graminearum, Verticillium dahlia, Pestalotiopsis microspora</italic>, and two species of plant pathogenic bacteria including <italic>Xanthomonas oryzae</italic> pv. <italic>oryzae</italic> and <italic>Ralstonia solanacearum</italic>. The bioactivity test results of the compound against agricultural pathogens show that actinomycin D possesses inhibitory activity against the growth of these tested phytopathogenic fungi and bacteria. among which, actinomycin D has the most potent inhibitory activity against the plant pathogenic fungi <italic>Lasiodiplodia theobromae, Phomopsis caricae-papayae</italic> Fetrak&amp;Cif., <italic>Fusarium graminearum</italic> and <italic>Phytophthora capsica</italic>, with inhibitory rate of (60.09 ± 0.66)%, (48.82 ± 0.66)%, (46.47 ± 1.14)% and (44.13 ± 0.66)%, respectively. In addition, actinomycin D displays strong antibacterial activity on the plant pathogenic bacteria <italic>Xanthomonas oryzae</italic> pv. <italic>oryzae</italic> and <italic>Ralstonia solanacearum</italic> with bacteriostatic diameter at (36.00 ± 0.82) mm and (15.00 ± 0.47) mm, respectively. These results provide new compounds and bacteria resources for the development of high-efficiency biocontrol agents for agricultural disease control.