Decoupling Community Complexity From Efficacy: A Meta-Analysis of Synthetic Microbial Communities for Plant Stress...

Drought, salinity, pollution and nutrient deficiency substantially impair plant growth by disrupting water balance, ion homoeostasis, redox regulation and metabolism. Synthetic microbial communities (SynComs) represent a frontier in microbiome engineering, theoretically offering superior stability over single-strain inoculants through functional redundancy. However, the design rules governing their efficacy - specifically the trade-off between community complexity and phenotypic output - remain empirically unresolved. This meta-analysis evaluated how synthetic microbial communities (SynComs) affect plant growth and stress-related physiological responses under different stress conditions and examined whether their efficacy was associated with community complexity or specific microbial functional traits. Data from 34 studies (433 independent comparisons) were collected. Then, the natural log response ratios (lnRR) of plant morphological and physiological traits were calculated. A random-effects model was used to estimate global effect sizes and quantify between-study heterogeneity. Plant performance was significantly enhanced following SynComs inoculation, evidenced by a 111.93% increase in total biomass (mean lnRR = 0.751). The highest efficacy was observed under biotic stress (total plant dry weight +191.57%) and pollution (underground dry weight +154.23%), with biomass accumulation being more pronounced in belowground traits than in aboveground components. Physiologically, this growth promotion was associated with the alleviation of oxidative stress in plants, as manifested by the significant upregulation of antioxidant enzymes (CAT + 47.09%, POD + 41.55%, and SOD + 44.71%) and a concurrent reduction in lipid peroxidation. Furthermore, the weighted meta-analysis revealed that single-genus communities significantly outperformed multi-genus communities in the present dataset (p = 0.004). However, efficacy was entirely decoupled from community richness, measured as strain number (R² = 0.000, p = 0.490). These findings suggest that greater taxonomic breadth or higher strain richness was not necessarily associated with improved SynCom performance. Rather, efficacy appeared to be driven by specific microbial functional traits (such as IAA production and phosphorus solubilisation). We propose that the rational design of SynComs should prioritise strain function over community size to maximise agricultural sustainability and reduce metabolic burden.