Impacts of Atmospheric CO and Soil Nutritional Value on Plant Responses to Rhizosphere Colonization by Soil Bacteria.

Concerns over rising atmospheric CO concentrations have led to growing interest in the effects of global change on plant-microbe interactions. As a primary substrate of plant metabolism, atmospheric CO influences below-ground carbon allocation and root exudation chemistry, potentially affecting rhizosphere interactions with beneficial soil microbes. In this study, we have examined the effects of different atmospheric CO concentrations on Arabidopsis rhizosphere colonization by the rhizobacterial strain WCS417 and the saprophytic strain KT2440. Rhizosphere colonization by saprophytic KT2440 was not influenced by sub-ambient (200 ppm) and elevated (1,200 ppm) concentrations of CO, irrespective of the carbon (C) and nitrogen (N) content of the soil. Conversely, rhizosphere colonization by WCS417 in soil with relatively low C and N content increased from sub-ambient to elevated CO. Examination of plant responses to WCS417 revealed that plant growth and systemic resistance varied according to atmospheric CO concentration and soil-type, ranging from growth promotion with induced susceptibility at sub-ambient CO, to growth repression with induced resistance at elevated CO. Collectively, our results demonstrate that the interaction between atmospheric CO and soil nutritional status has a profound impact on plant responses to rhizobacteria. We conclude that predictions about plant performance under past and future climate scenarios depend on interactive plant responses to soil nutritional status and rhizobacteria.