In vitro inhibition of acetolactate synthase by alkaloid‐rich extract from guinea grass ( <i>Megathyrsus maximus</i> ) for herbicidal potential

Abstract Acetolactate synthase (ALS) is a well‐established target enzyme for weed control, but widespread resistance to synthetic ALS inhibitors has necessitated the search for alternative herbicidal compounds. Plant‐derived alkaloids offer a diverse chemical framework that may exhibit unique interactions with ALS, potentially overcoming resistance mechanisms. In this study, an alkaloid‐rich extract from the leaves of Megathyrsus maximus (36.9741 ± 0.0056 mg CE/g) was evaluated for its phytotoxicity and in vitro ALS inhibition against three representative weed species: Echinochloa crus‐galli (grass), Cyperus iria (sedge), and Amaranthus spinosus (broadleaf). A lettuce seed germination assay demonstrated a dose‐dependent inhibition, with an IC50 of 609 ppm. The extract significantly reduced ALS activity (10 ppb), with mean relative activity of 41.79% for C. iria , 50.85% for E. crus‐galli , and 70.86% for A. spinosus . Thin‐layer chromatography led to the isolation of an alkaloid compound, confirmed by chemical tests and Fourier transform infrared (FTIR) spectroscopy. While the exact mode of action remains to be fully elucidated, these findings suggest that M. maximus alkaloids could interact with ALS differently than synthetic herbicides, highlighting their potential as a novel bioherbicidal approach. Further studies on their binding mechanism and field efficacy could help determine their role in sustainable weed management strategies.