Two New Plant Growth Inhibitors Isolated from Colletotrichum acutatum/Alternaria destruens CIY4 Coculture Fermentation

Herbicides derived from plant pathogenic fungi or their metabolites are promising candidates for new herbicide development, offering abundant resources, low toxicity, and environmental friendliness. Two new indole derivatives, colletotriauxin G (1) and H (2), along with three known analogs colletotryptin A (3), colletotryptin B (4), and tryptophol (5), were isolated from coculture fermentation of Colletotrichum acutatum J. H. Simmonds and endophytic fungus Alternaria destruens CIY4 from Cichorium intybus. Their structures were elucidated using comprehensive spectroscopic data, computational chemistry, and X-ray crystal diffraction. Compounds 1, 2, and 5 demonstrated obvious inhibitory effects on the growth of Echinochloa crus-galli seedlings. Specifically, compounds 1 and 2 significantly inhibit the growth of E. crus-galli primary roots at a concentration of 50 μg/mL, and the activity is comparable to the positive control glyphosate. In addition, they elevated the content of H₂O₂ in E. crus-galli and inhibited the activities of the antioxidant enzymes CAT, POD, and SOD. Confocal laser scanning microscopy revealed significant damage to the root border cell membrane. Compound 2 exhibited appreciable broad-spectrum herbicidal activity along with partial crop selectivity in the pot experiments. The findings indicate that colletotriauxin G and H hold promising potential as candidates for the development of new agricultural herbicides.