Phytotoxicity and indole acetic production by rhizobacterial strains

Deleterious rhizobacteria (DRB) constitute an emerging category of bioherbicides with the capacity to inhibit weed germination through phytotoxic metabolites. This study aimed to isolate and screen rhizobacteria for deleterious potentials. Bacterial isolation was carried out using the standard pour plating method. Five agricultural crops (cucumber, soybean, sorghum, cowpea and maize) were assayed for phytotoxic screening against the DRB. In addition, antimicrobial potential of the DRB against test pathogens and indole acetic acid (IAA) were also investigated. Polymerase chain reaction and gene sequencing of the isolates revealed three strains of Pseudomonas aeruginosa and one each of P. fluorescens and E. hormaechei. The antimicrobial outcomes of the test DRB revealed broad-spectrum inhibition against the test pathogens. In terms of culture age, younger cultures (≤48 h) mostly stimulated or matched control germination and vigor indices with toxicity increasing progressively with age beyond 120 h for most of the seeds. In the case of steeping duration, short steeping durations (1–2 h) typically maintained germination and vigor near control levels. Progressive extension of steeping time increased inhibition across all isolates. At different inoculum concentrations, higher inoculum concentrations produced stronger inhibitory effects than diluted suspensions. Full-strength inoculum (100%) suppressed vigor drastically, whereas 20–40% dilutions produced diminished or reversible effects. Also, all isolates produced measurable IAA in tryptophan-supplemented media with output increasing with incubation age, correlating with suppression of vigor at later time points. The indigenous origin of the DRB evaluated in this study enhances environmental compatibility and reduces ecological disturbance risks.