Root system growth and development responses to elevated CO2: underlying signalling mechanisms and role in improving plant CO2 capture and soil C storage.

Carbon storage in soils is one of the most promising strategies for mitigating greenhouse gas emissions and the associated climate change. In this context, how plant root systems respond to the elevation of the atmospheric CO2 concentration is of crucial importance because these organs are the main source of C input into the soils. It is expected that root growth will be stimulated by elevated CO2 as a consequence of enhanced photosynthesis, and that this will favour belowground C sequestration. In addition, larger root systems with optimized architecture are also expected to improve water and nutrient acquisition by plants, and to indirectly stimulate photosynthetic CO2 capture. This review critically examines the evidence supporting these expectations from a molecular physiology perspective. We illustrate the strong but highly variable effects of elevated CO2 on root system size and architecture, and provide an update on the signalling mechanisms that may trigger these effects. This highlights the lack of knowledge on the physiological and genetic bases of the root growth and development response to elevated CO2, but shows that candidate genes and genetic resources are largely available to fill this gap.