Metabolites from Pseudomonas fluorescens strain D7 inhibit downy brome (Bromus tectorum) seedling growth

Phytotoxins produced by the naturally-occurring rhizobacterium, Pseudomonas fluorescens strain D7 (strain D7), in shake cultures from several media sources adversely affected downy brome at several growth stages and affected seeds, whole plants, and isolated cells. These uncharacterized phytotoxins inhibited root and shoot growth of downy brome by 80% in a seed germination agar assay at 2 and 400 ng total solids/ml, respectively. In a hydroponics assay, downy brome and winter wheat dry weights were reduced 50% by cell-free broth concentrations of 15 and 26% of the initial broth concentration, respectively, indicating an almost 2× selectivity between winter wheat and downy brome. Plant water use was reduced similarly. Plasma membranes of isolated protoplasts of downy brome were damaged within 2 h by diluted cell-free broth. Cold-precipitated phytotoxins of strain D7 produced in semi-synthetic and synthetic media reduced dry weight of downy brome seedlings by 100 and 89%, respectively, and killed plants rapidly.