Herbicidal Characteristics and Structural Identification of the Potential Active Compounds from <i>Streptomyces</i> sp. KRA17-580

Abstract

Weeds are notorious plant species exhibiting a harmful impact on crops. Biological weed control is an efficient and environmentally friendly technique, usually constitutes naturally derived compounds, including bioherbicidal metabolites produced by <i>Streptomyces</i> sp. The isolation and structural identification of phytotoxic compounds from <i>Streptomyces</i> have recently been proposed as an effective way to the discovery of novel bioherbicides. In the screening of bioherbicidal agents, isolated <i>Streptomyces</i> strain KRA17-580 demonstrated significant phytotoxic activity against <i>Digitaria ciliaris</i>. Phylogenetic analysis of the 16S rRNA sequence indicated that isolated KRA17-580 is similar to <i>Streptomyces olivochromogenes</i>. The bacterial culture conditions were optimized for temperature, agitation, and initial pH. <i>Streptomyces</i> strain KRA17-580 showed intense phytotoxic activity and high cell mass at an initial pH of 5.5-7.0, more than 150 rpm, and 25-30 °C. The herbicidal compounds isolated from the culture filtrate of strain KRA17-580 were purified by solvent partition, C₁₈, Sephadex LH20 column chromatography, and high-performance liquid chromatography. By 1D-NMR, 2D-NMR, and electrospray ionization mass spectrometry analysis, the 580-H1 and 580-H2 compounds were identified as a cinnoline-4-carboxamide (<i>M</i>_W, 173.0490; C₉H₇N₃O₂) and cinnoline-4-carboxylic acid (<i>M</i>_W, 174.0503; C₉H₆N₂O₂), respectively. Only these two herbicidal compounds showed strong phytotoxic activity against <i>D. ciliaris</i> in foliar applications. However, compound 580-H2 was more phytotoxic than 580-H1 and the toxicity was dose-dependent. The herbicidal metabolite KRA17-580 produced by <i>Streptomyces</i> sp. is a new bioherbicidal candidate that may provide a new lead molecule for more efficient phytotoxic compounds.