Metabolomic profiling of wild-type and mutant soybean root nodules using laser-ablation electrospray ionization mass spectrometry reveals altered metabolism

The establishment of the nitrogen-fixing symbiosis between soybean andBradyrhizobium japonicumis a complex process. To document the changes in plant metabolism as a result of symbiosis, we utilized laser ablation electrospray ionization-mass spectrometry (LAESI-MS) forin situmetabolic profiling of wild-type nodules, nodules infected with aB. japonicum nifHmutant unable to fix nitrogen, nodules doubly infected by both strains, and nodules formed on plants mutated in thestearoyl-acyl carrier protein desaturase(sacpd-c) gene, which were previously shown to have an altered nodule ultrastructure. The results showed that the relative abundance of fatty acids, purines, and lipids was significantly changed in response to the symbiosis. ThenifHmutant nodules had elevated levels of jasmonic acid, correlating with signs of nitrogen deprivation. Nodules resulting from the mixed inoculant displayed similar, overlapping metabolic distributions within the sectors of effective (fix(+)) and ineffective (nifHmutant, fix(-)) endosymbionts. These data are inconsistent with the notion that plant sanctioning is cell autonomous. Nodules lackingsacpd-cdisplayed an elevation of soyasaponins and organic acids in the central necrotic regions. The present study demonstrates the utility of LAESI-MS for high-throughput screening of plant phenotypes. Overall, nodules disrupted in the symbiosis were elevated in metabolites related to plant defense.