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Direct Imaging of Plant Metabolites in the Rhizosphere Using Laser Desorption Ionization Ultra-High Resolution Mass Spectrometry

Authors
  • Lohse, Martin1
  • Haag, Rebecca1, 2
  • Lippold, Eva3
  • Vetterlein, Doris3, 4
  • Reemtsma, Thorsten1, 5
  • Lechtenfeld, Oliver J.1, 6
  • 1 Department of Analytical Chemistry, Helmholtz Centre for Environmental Research – UFZ, Leipzig , (Germany)
  • 2 Ansbach University of Applied Sciences, Ansbach , (Germany)
  • 3 Department of Soil System Science, Helmholtz Centre for Environmental Research – UFZ, Halle , (Germany)
  • 4 Soil Science, Martin Luther University Halle-Wittenberg, Halle , (Germany)
  • 5 Institute of Analytical Chemistry, University of Leipzig, Leipzig , (Germany)
  • 6 ProVIS – Centre for Chemical Microscopy, Helmholtz Centre for Environmental Research – UFZ, Leipzig , (Germany)
Type
Published Article
Journal
Frontiers in Plant Science
Publisher
Frontiers Media SA
Publication Date
Dec 03, 2021
Volume
12
Identifiers
DOI: 10.3389/fpls.2021.753812
Source
Frontiers
Keywords
Disciplines
  • Plant Science
  • Methods
License
Green

Abstract

The interplay of rhizosphere components such as root exudates, microbes, and minerals results in small-scale gradients of organic molecules in the soil around roots. The current methods for the direct chemical imaging of plant metabolites in the rhizosphere often lack molecular information or require labeling with fluorescent tags or isotopes. Here, we present a novel workflow using laser desorption ionization (LDI) combined with mass spectrometric imaging (MSI) to directly analyze plant metabolites in a complex soil matrix. Undisturbed samples of the roots and the surrounding soil of Zea mays L. plants from either field- or laboratory-scale experiments were embedded and cryosectioned to 100 μm thin sections. The target metabolites were detected with a spatial resolution of 25 μm in the root and the surrounding soil based on accurate masses using ultra-high mass resolution laser desorption ionization Fourier-transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). Using this workflow, we could determine the rhizosphere gradients of a dihexose (e.g., sucrose) and other plant metabolites (e.g., coumaric acid, vanillic acid). The molecular gradients for the dihexose showed a high abundance of this metabolite in the root and a strong depletion of the signal intensity within 150 μm from the root surface. Analyzing several sections from the same undisturbed soil sample allowed us to follow molecular gradients along the root axis. Benefiting from the ultra-high mass resolution, isotopologues of the dihexose could be readily resolved to enable the detection of stable isotope labels on the compound level. Overall, the direct molecular imaging via LDI-FT-ICR-MS allows for the first time a non-targeted or targeted analysis of plant metabolites in undisturbed soil samples, paving the way to study the turnover of root-derived organic carbon in the rhizosphere with high chemical and spatial resolution.

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