Elevated CO alters soybean physiology and defense responses, and has disparate effects on susceptibility to diverse microbial pathogens.

Increasing atmospheric CO levels have a variety of effects that can influence plant responses to microbial pathogens. However, these responses are varied, and it is challenging to predict how elevated CO (eCO) will affect a particular plant-pathogen interaction. We investigated how eCO may influence disease development and responses to diverse pathogens in the major oilseed crop, soybean. Soybean plants grown in ambient CO (aCO, 419 parts per million (ppm)) or in eCO (550 ppm) were challenged with bacterial, viral, fungal, and oomycete pathogens. Disease severity, pathogen growth, gene expression, and molecular plant defense responses were quantified. In eCO, plants were less susceptible to Pseudomonas syringae pv. glycinea (Psg) but more susceptible to bean pod mottle virus, soybean mosaic virus, and Fusarium virguliforme. Susceptibility to Pythium sylvaticum was unchanged, although a greater loss in biomass occurred in eCO. Reduced susceptibility to Psg was associated with enhanced defense responses. Increased susceptibility to the viruses was associated with reduced expression of antiviral defenses. This work provides a foundation for understanding how future eCO levels may impact molecular responses to pathogen challenges in soybean and demonstrates that microbes infecting both shoots and roots are of potential concern in future climatic conditions.