Resource exchange in the rhizosphere: Molecular tools and the microbial perspective

The interface between living plant roots and soils (the rhizosphere) is a central commodities exchange, where organic carbon flux from roots fuels decomposers that, in turn, can make nutrients available to roots. This ongoing exchange operates in the path of vast, transpiration-driven water flow. How the spatio-temporal patterning in resource availability around plant roots affects rhizosphere community composition, activity, and nutrient cycling remains unknown. This review considers how molecular approaches contribute to the exploration of rhizosphere resource exchange, highlighting several recently developed methods linking microbial identity with substrate uptake and gene expression. In particular, strengths and weaknesses of genetically engineered bioreporters are discussed, because currently they alone provide in situ spatio-temporal information at scales of rhizosphere organisms. The soil spatial context is an emerging frontier in ecological soils research. We conclude with parallels linking empirical investigation in the rhizosphere with the quest for understanding general rhizosphere function in Earth's diverse ecosystems.