Effect of the Inoculation Method on the Potential Plant Growth-Promoting Activity of a Microbial Synthetic Consortium

Plant microbiomes represent a reservoir of adaptive traits that can enhance plant resilience and productivity. In this study, we investigated the potential of a synthetic microbial consortium (SynCom), composed of five nitrogen-fixing Bacillaceae derived from the phyllosphere of Pistacia lentiscus L., to promote the growth of Lactuca sativa L. under nutrient-limiting availability. The short-term effects of the SynCom were evaluated in a hydroponic system, and four inoculation treatments were compared. The root and leaf inoculation (RL) significantly increased total plant biomass, driven by higher above-ground (+33%) and below-ground (+31%) biomass. The number of leaves per plant and leaf mass per area were also significantly enhanced in RL (+12% and +34%, respectively). While root-only inoculation (R) did not alter plant biomass, structure, or leaf traits, foliar spraying (L) significantly decreased the total leaf area and increased root biomass and the root-to-shoot ratio compared to non-inoculated plants, suggesting a direct influence of microbial metabolites on root growth and nutrient uptake. Compared to the individual R and L treatments, the synergistic effect observed in RL highlights the complex interplay between plant–microbe interactions in the rhizosphere and phyllosphere and the importance of ensuring adequate nutrient availability to nitrogen-fixing bacteria to achieve their growth-promoting potential fully. These findings suggest the potential for utilizing SynComs as bioinoculants to promote plant growth, emphasizing the need to optimize application strategies, considering both the SynCom composition and the host plant’s nutritional status to ensure efficacy.