Plant growth-promoting bacteria (PGPB) inhabit the rhizosphere of plants and are capable of enhancing plant growth through a number of mechanisms. A strain of Pseudomonas fluorescens DUS1-27 was identified as a potential PGPB candidate based on its ability to increase the growth of Brassica napus L. (canola) over that of uninoculated control plants in a soil-based system. The same P. fluorescens isolate was found to reduce plant growth in a hydroponic growth system, with plants showing the symptoms of a microbe-associated molecular pattern (MAMP) response to the bacteria. The amperometric quantification of H₂O₂, fluorescence-based total peroxidase assays, and quantification of catalase gene expression levels using qRT-PCR revealed that oxidative stress reduced plant growth in the hydroponic system. The addition of the cyanobacterium Nostoc punctiforme (known to have high catalase activity levels) in the hydroponic system as a co-inoculant reduced oxidative stress (49.7% decrease in H₂O₂ concentrations) triggered by the addition of P. fluorescens DUS1-27, thereby enabling plants to grow larger than uninoculated control plants. These results show the advantage of inoculating with multiple bacteria to promote plant growth and, for the first time, demonstrate that N. punctiforme beneficially assists plants under oxidative stress through its catalase activity in planta.