## 4 Discussion
Exclusive foliar SynCom application decreased total leaf area and stimulated root biomass accumulation without affecting leaf biomass or leaf mass per area (Figure 3b; Table 2). This plant response increased the root-to-shoot ratio compared to the non-inoculated and RL-inoculated plants (Figure 3d), suggesting that foliar-applied PGPR triggers a signaling cascade that promotes root development. The differential stimulation of leaf and root biomass production was also observed by [PERSON] et al. [71] in wheat, where foliar spray inoculation of the diazotroph _Azospirillum brasilense_ resulted in increased root systems and N accumulation in the shoots. The authors attributed these effects to microbially derived hormonal signaling, primarily involving indole acetic acid, jasmonic acid, and salicylic acid. Notably, jasmonic and salicylic acids induced an up-regulation of oxidative stress and plant defense genes in leaves while concurrently down-regulating these genes in roots [53,71]. We can hypothesize that similar mechanisms took place in our experiment, which can be coupled to the low nutrient concentration provided to plants. Together, these results highlight insufficient nutrient availability to support above-ground growth, considering the costs of plant responses to the oxidative stress induced by inoculation. Thus, no increase in leaf biomass was observed despite the increased root biomass conferred by leaf PGPR inoculation. Interestingly, root inoculation significantly alters above- and below-ground biomass only if combined with applying the SynCom at the leaf level, indicating a complex interplay between PGPR inoculation, root development, and leaf growth (Figure 3). In conclusion, to optimize the benefits of PGPR in sustainable agriculture, application strategies should be tailored to the composition of the microbial consortium and the host plant's nutritional status. When nutrient availability is high, foliar application of N-fixing diazotrophs that stimulate root growth may be sufficient to determine an increase in aboveground biomass. However, under nutrient-limited conditions, a combined root and leaf application of N-fixing PGPR may be more beneficial. Root application of diazotrophs can also be advantageous when utilizing their additional traits, such as biocontrol activity, independent of their N-fixing capabilities. The choice of an appropriate approach ensures that PGPR applications are targeted and efficient, maximizing their potential to promote plant growth and health while minimizing environmental impact. Further research into the molecular mechanisms underlying these interactions, particularly the role of microbial signaling molecules and their impact on plant hormone regulation and nutrient assimilation, is essential to refine PGPR inoculation practices and realize their full potential in enhancing crop productivity. While this study focused on a hydroponic system to evaluate SynCom's efficacy under controlled conditions, it is important to investigate its performance in soil-based systems, considering its application in different plant systems. Further research is therefore needed to fully understand the long-term effects of the SynCom on plant health and productivity in soil-based cultivations and soil biota and the implications of inoculation on the overall sustainability of agricultural practices. Conceptualization, R.A.J. and M.R. ; Methodology, R.A.J., F.L. and M.R. ; Validation, R.A.J., F.L., A.G.F. and M.R. ; Formal Analysis, R.A.J. and M.R. ; Investigation, R.A.J., F.L., A.G.F. and M.R. ; Resources, M.R. ; Data Curation, R.A.J., F.L., A.G.F. and M.R. ; Writing--Original Draft Preparation, R.A.J. ; Writing--Review and Editing, R.A.J., F.L., A.G.F. and M.R. ; Visualization, R.A.J. ; Supervision, M.R. ; Project Administration, M.R. ; Funding Acquisition, M.R. All authors have read and agreed to the published version of the manuscript.