Potential use of Rhizobium spp. to improve fitness of non-nitrogen-fixing plants

Plant-growth-promoting bacteria (PGPR) stimulate plant growth by producing and/or inducing the plant to release secondary metabolites facilitating the uptake of nutrients and/or inhibiting plant pathogenic organisms in the rhizosphere. Recently, legume bacteria (Rhizobium spp.) have been reported to act as PGPR. In this study, we investigated the potential use of legume bacteria as PGPR in nonlegumes by inoculating eight different rhizobial strains onto mixtures of six botanically different non-nitrogen-fixing plant species. Each seed mixture was inoculated before sowing with the same concentration of the respective rhizobial strain. The results showed that inoculation with certain rhizobial strains increased plant biomass. In a follow-up experiment, we investigated the importance of cell density of the rhizobial inoculant by inoculating seeds of one species (Linum usitatissimum) with only one rhizobial strain, but at different concentrations. A concentration of level 10⁴ c.f.u. (colony-forming units) mL⁻¹ proved to be the best for successful growth. A supplementary in vitro study investigating potential mechanisms behind the plant stimulatory effect of rhizobia found that some rhizobial strains have a capability to dissolve the fungal mycelium at the initial stage. These results demonstrate that certain rhizobial strains have interesting features that deserve further attention when evaluating and developing biological plant-protection systems and/or plant-stimulating agents.

Keywords

• Fitness,
• non-nitrogen-fixing plants,
• Rhizobium