Determination of Root Exudate Concentration in the Rhizosphere Using ¹³C Labeling

One of the most remarkable metabolic features of plant roots is their ability to secrete a wide range of compounds into the rhizosphere, defined as the volume of soil around living roots. Around 5%-21% of total photosynthetically fixed carbon is transferred into the rhizosphere through root exudates. Until recently, studies on the quantity and quality of root exudates were conducted mostly under axenic or monoxenic in vitro conditions. Today, in situ assays are required to provide a better understanding of root exudates dynamics and role in plant-microbe interactions. By incubating plants with ¹³CO₂ in situ for one week and quantifying ¹³C enrichment from the root-adhering soil using mass spectrometry, we were able to determine root exudate levels. Indeed, labeled substrate ¹³CO₂ is converted into organic carbon via plant photosynthesis and transferred into the soil through root exudation. We assume that all ¹³C increases above natural abundance are mainly derived from exudates produced by ¹³C-labeled plants.