MALDI mass spectrometry‐assisted molecular imaging of metabolites during nitrogen fixation in the Medicago truncatula–Sinorhizobium meliloti symbiosis

Abstract

SUMMARYSymbiotic associations between leguminous plants and nitrogen-fixing rhizobia culminate in the formation of specialized organs called root nodules, in which the rhizobia fix atmospheric nitrogen and transfer it to the plant. Efficient biological nitrogen fixation depends on metabolites produced by and exchanged between both partners. The Medicago truncatula-Sinorhizobium meliloti association is an excellent model for dissecting this nitrogen-fixing symbiosis because of the availability of genetic information for both symbiotic partners. Here, we employed a powerful imaging technique -matrix-assisted laser desorption/ionization (MALDI)/mass spectrometric imaging (MSI) -to study metabolite distribution in roots and root nodules of M. truncatula during nitrogen fixation. The combination of an efficient, novel MALDI matrix [1,8-bis (dimethyl-amino) naphthalene, DMAN] with a conventional matrix 2,5-dihydroxybenzoic acid (DHB) allowed detection of a large array of organic acids, amino acids, sugars, lipids, flavonoids and their conjugates with improved coverage. Ion density maps of representative metabolites are presented and correlated with the nitrogen fixation process. We demonstrate differences in metabolite distribution between roots and nodules, and also between fixing and non-fixing nodules produced by plant and bacterial mutants. Our study highlights the benefits of using MSI for detecting differences in metabolite distributions in plant biology.

Grapevine colonization by endophytic bacteria shifts secondary metabolism and suggests activation of defense pathways

“…Metabolic profiling of bacterial colonization is a new and interesting area of research (Allwood et al 2008), given the advances on metabolite detection techniques including Matrix Assisted Laser Desorption/IonizationTime of Flight Mass spectrometry (MALDI-ToF MS), Ultraperformance Liquid Chromatography-tandem mass spectrometry (UPLC-MS/MS) and MALDI/MS assisted imaging (Bajad et al 2006;Ye et al 2013;Ziegler et al 2012). Analysis of metabolites with these techniques permits the quick assessment of molecules accumulated during host-symbiont interactions.…”

MALDI Mass Spectrometry Imaging for the Simultaneous Location of Resveratrol, Pterostilbene and Viniferins on Grapevine Leaves

“…For mass spectrometry imaging purpose, MALDI has already been used to map different metabolites on plant organs [28] such as lipids on leaf surface [29], flavonoids or glycoalkaloid in roots and root nodules [30,31], sugars within seed or stem sections [32,33]. Different matrices are described in the literature for MALDI MS analysis of vegetable material such as 1,5 diamino naphthalene (DAN) or 9-amino acridine (9-AA) in negative ion mode, and tri-hydroxy acetophenone (THAP), α-cyano-4-hydroxy cinamic acid (CHCA) or 2,5 dihydroxy-benzoic acid (DHB) in positive ion mode.…”

Bacterial Biosensors for in Vivo Spatiotemporal Mapping of Root Secretion

“…7). Although nodules formed on pea and vetch have not been analyzed by metabolomic studies, matrix-assisted laser-desorption ionization mass spectrometric analysis of Medicago truncatula nodules formed by S. meliloti (Ye et al, 2013) revealed many of these same compounds present: Suc, C4-dicarboxylates (succinate/malate/Asp), and GABA; however, salicylic acid also was detected, which, if present in pea or vetch nodules, is below the limit of detection (#1 mM) of Lux-based salicylic acid detection. In pea, hesperetin ($1 mM) was detected in mature nodules (Fig.…”