Germination-Arrest Factor (GAF): Biological properties of a novel, naturally-occurring herbicide produced by selected isolates of rhizosphere bacteria

Five strains of deleterious rhizosphere bacteria (DRB), identified as Pseudomonas fluorescens, were shown to produce and secrete a naturally-occurring herbicide that targets the seeds of certain graminaceous plants, including annual bluegrass (ABG; Poa annua L.), considered a weed in many agronomic systems. The herbicide arrests germination of the seeds in a developmentally-specific manner, typically irreversibly blocking the germination process immediately after the emergence of the plumule and coleorhiza. Because of its unique mode of action, this herbicide has been termed a Germination-Arrest Factor (GAF). Bacterial-culture filtrates containing GAF arrest seed germination of a large number of graminaceous species, including grassy weeds and grass species grown for seed and food. The germination of corn seeds did not appear to be affected by the herbicide. The seeds of dicot species appear to be less sensitive to GAF than seeds of graminaceous species. The biological effects of GAF on graminaceous species is primarily limited to germination, although transient slowing of ABG seedling growth was observed after post-germination exposure to GAF. Exposure of non-germinated ABG seeds to GAF for periods as short as 24 h was sufficient to irreversibly arrest germination. A quantitative bioassay for GAF was developed based on the sensitivity of ABG seed to this compound.