Better together? Diverse microbial communities promote stable soybean resistance to simultaneous drought and herbivore stress

Agroecosystems experience stress from drought and insect pests, which threatens crop production and poses a significant challenge to crop management strategies. Soybeans (Glycine max) rely on symbiotic rhizobial bacteria, which are facultative legume mutualists that infect and colonize soybean roots and conduct symbiotic biological nitrogen (N) fixation in exchange for carbon (C). This relationship is crucial for ecosystem functioning, as plants require N for growth, reproduction, and defense. Within soils, rhizobial populations are exceptionally genetically diverse, with different strains possessing unique traits across environments. Legumes can simultaneously associate with multiple strains, and diversity in rhizobial partners may be one potential method for introducing plant resistance to simultaneous stressors into monoculture crop plantings. We inoculated a commercial soybean variety with eight unique rhizobial strains in single-strain monocultures, two-strain bicultures, and four-strain polyculture combinations under drought and ambient conditions in a greenhouse. We then performed feeding assays using the generalist legume pest, Chrysodeixis includens. We quantified herbivore damage, herbivore growth rates, root nodulation, and plant functional and fitness traits. As rhizobial diversity increased, the variation in caterpillar growth rates between ambient and droughted treatments decreased. Also, increased rhizobial diversity decreased variation in multivariate functional plant trait expression. Understanding how rhizobial diversity influences plant-insect interactions in combined abiotic and biotic stress environments advances fundamental entomological knowledge. This work offers novel insight into management strategies that promote the stability of soybean performance in increasingly variable climates. Future work will address the “winners” and losers” of rhizobial competition within biculture and polyculture mixtures.