Root Endophytic Synthetic Communities Promote Oat Seedling Growth Through Functional Complementarity and Root Transcriptional Reprogramming

Synthetic microbial communities (SynComs) offer a sustainable strategy for promoting crop growth, yet SynComs derived from oat endophytes and their mechanisms remain poorly understood. Here, endophytic bacteria were isolated from oat roots to construct SynComs based on functional complementarity. The optimal combination was screened by pot experiments, and its growth-promoting mechanisms were elucidated by integrating reference genome annotation with root transcriptomic profiling. Among 22 isolated strains, 10 SynComs were constructed, of which SynCom7, composed of Glutamicibacter arilaitensis G55, Pseudomonas bijieensis G229, Acinetobacter calcoaceticus J158, and Peribacillus simplex G18, showed the strongest growth promotion. Compared with the control, SynCom7 significantly promoted oat seedling growth, increasing shoot fresh weight, total root length, root surface area, and root tip number by 47.7%, 21.6%, 75.6%, and 56.1%, respectively. Genomic prediction based on reference type strains suggested that the four strains putatively carried complementary genes involved in siderophore biosynthesis, IAA secretion, phosphate solubilization, and carbon source utilization. Transcriptomic analysis further showed that SynCom7 activated brassinosteroid and auxin signaling, promoted cell wall biosynthesis gene expression, and moderately downregulated stress responses, thereby reprogramming the growth–defense trade-off. Notably, expression of cell wall biosynthesis genes (CslF6, MAP65-6), hormone signaling genes (LAX, DET2), a root development transcription factor (NAC090), and aquaporin genes (TIP1-2, PIP2-2) was positively correlated with root tip number and root surface area. Collectively, SynCom7 promotes oat seedling growth through strain functional complementarity and host root transcriptional reprogramming, providing microbial resources for developing oat-specific fertilizers.