Identification of the metabolites regulated in soybean-Rhizobia symbiosis through solid phase microextraction coupled with LC-MS.

Legumes provide one of the uniquely nutrient-rich food sources to the population and are one of the primary field crops that play significant roles in agricultural sustainability. Inoculation with Bradyrhizobium japonicum is necessary for the high yield of leguminous crops, i.e. soybean. Nodulation of soybean by Bradyrhizobium japonicum is a complex process that is essential for cultivation of these legumes and external stress factors, such as draught and soil acidity, that influence the nodulation and crop yield. Alterations in the nodule metabolites are known to identify the type of stress that mitigates nodulation and lowers crop yield. Current techniques aimed at understanding the metabolic activities in the symbiont, such as in the case of metabolic regulations in varying nodule growth phases, rely on exhaustive techniques based on the removal of nodules or other plant tissue. Aiming to capture a more in-depth, accurate profile of this system without quenching the metabolic activity in the nodules, or removing the nodules, a workflow was prepared for the metabolite sampling through in vivo solid phase microextraction in thin film format (TF-SPME). This technique was followed by LC-QTOF-MS instrumental analysis with subsequent metabolite annotation and reference standard validation. Our approach is unique in terms of eliminating the effects that arise due to analyte partition coefficients. We show that the symbiont undergoes metabolic regulations throughout the cultivation period, displaying the efficacy of TF-SPME as a non-exhaustive sampling method that can be used as a tool to investigate the metabolic alterations in nodules. These alterations would potentially fingerprint the environmental effects on soybean yield.