Microorganisms associated with plants can affect nutrient and water acquisition, plant defenses, and ecological interactions, with effects on plant growth that range from beneficial to antagonistic. In Glycine max (soybean), many studies have examined the soil microbiome and the legume-rhizobium relationship, but little is known about foliar endophytes, their effects on plant biomass and fitness, and how plants respond to their presence. To address these questions, we inoculated Glycine max with field-collected isolates of previously isolated, dominant strains of Methylobacterium and Colletotrichum in either sterile or non-sterile soil. We then used RNAseq to compare the transcriptomic responses of plants to single- and co-inoculation of endophytes. We found that all endophyte treatments increased soybean growth compared to control, but only in sterile soil. These results suggest context-dependency, with endophytes serving as facultative mutualists under stress or nutrient deprivation. Similarly, transcriptomic analyses revealed that soybean defense and stress responses depended on the interaction of endophytes
Methylobacterium elicited the strongest response but was modulated by the presence of Colletotrichum. Our findings highlight the environmentally dependent effects of co-existing endophytes within soybean leaves.