Potential Allelopathic Indole Diketopiperazines Produced by the Plant Endophytic Aspergillus fumigatus Using the One Strain–Many Compounds Method

On the basis of the OSMAC (one strain–many compounds) strategy, 14 indole diketopiperazine (DKP) alkaloids, including spirotryprostatins (1–3), tryprostatins (4–6), and cyclotryprostatins (7–14), were isolated from the endophyte Aspergillus fumigatus associated with Melia azedarach L. Their structures were identified by nuclear magnetic resonance and electrospray ionization mass spectrometry data. All the indole DKPs were evaluated for plant growth regulation using the lettuce (Lactuca sativa) seedling growth bioassay, which showed the plant growth influence of the seedling. Among these compounds tested, a tryprostatin-type compound, brevianamide F (6), was identified as a new type of natural potential plant growth inhibitor with a response index (RI) higher than that of the positive control glyphosate, a broad-spectrum systemic herbicide. 6 can also inhibit turnip (Raphanus sativus) shoot and root elongation with RIs of −0.76 and −0.70, respectively, at 120 ppm, and it strongly inhibits amaranth (Amaranthus mangostanus) seedling growth with a high RI of −0.9 at 40 ppm. The structure–allelopathic activity relationship analysis of these isolated alkaloids indicates that tryprostatin-type alkaloids without the C5 prenyl and methoxy group give the most potent inhibition of seedling growth. Brevianamide F (6) could be used to develop a natural eco-friendly herbicide.