Comparison of Vacuum MALDI and AP-MALDI Platforms for the Mass Spectrometry Imaging of Metabolites Involved in Salt Stress in Medicago truncatula.

Caitlin Keller,Jean-Michel Ané,Lingjun Li,Michael R Sussman,Dhileepkumar Jayaraman,Sanhita Chakraborty,Jeanne M Harris,Junko Maeda

doi:10.3389/fpls.2018.01238

Abstract

Matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) is routinely used to determine the spatial distributions of various biomolecules in tissues. Recently, there has been an increased interest in creating higher resolution images using sources with more focused beams. One such source, an atmospheric pressure (AP) MALDI source from MassTech, has a laser capable of reaching spatial resolutions of 10 μm. Here, the AP-MALDI source coupled with a Q Exactive HF Orbitrap platform is compared to the commercial MALDI LTQ Orbitrap XL system using Medicago truncatula root nodules. AP-MALDI parameters, such as the S-lens value, capillary temperature, and spray voltage, were optimized on the Q Exactive-HF platform for optimal detection of plant metabolites. The performance of the two systems was evaluated for sensitivity, spatial resolution, and overall ability to detect plant metabolites. The commercial MALDI LTQ Orbitrap XL was superior regarding the number of compounds detected, as at least two times more m/z were detected compared to the AP-MALDI system. However, although the AP-MALDI source requires a spatial resolution higher than 10 μm to get the best signal, the spatial resolution at 30 μm is still superior compared to the 75 μm spatial resolution achieved on the MALDI platform. The AP-MALDI system was also used to investigate the metabolites present in M. truncatula roots and root nodules under high salt and low salt conditions. A discriminative analysis with SCiLS software revealed m/z ions specific to the control and salt conditions. This analysis revealed 44 m/z ions present at relatively higher abundances in the control samples, and 77 m/z enriched in the salt samples. Liquid chromatography-tandem MS was performed to determine the putative molecular identities of some of the mass ions enriched in each sample, including, asparagine, adenosine, and nicotianamine in the control samples, and arginine and soyasaponin I in the salt treated samples.

Highlights

Matrix-assisted laser desorption/ionization (MALDI) is commonly used as a soft ionization technique to study a wide range of biomolecules
The S-lens is an ion guide behind the heated capillary consisting of a series of stacked rings that operates as a radio frequency (RF) device to capture and focus ions into a beam
Adjusting the S-lens and spray voltage values did not reveal any clear trend during the profiling experiments, most m/z had an increase in signal at 80% for the S-lens RF value

Summary

Introduction

Matrix-assisted laser desorption/ionization (MALDI) is commonly used as a soft ionization technique to study a wide range of biomolecules. A powerful application of MALDI-mass spectrometry (MS) is the ability to determine the spatial distribution of molecules in a tissue slice by mass spectrometry imaging (MSI) (Caprioli et al, 1997). MALDI-MSI has been used to study a wide range of biomolecules, from small molecule metabolites (Lee et al, 2012; Gemperline et al, 2014), to neuropeptides (Chen and Li, 2010), and intact proteins (Chaurand et al, 2006). Tandem MS has been coupled with AP-MALDI ionization, which provides the ability to fragment molecules and use the fragmentation patterns to identify biomolecules (Mayrhofer et al, 2006; Ito et al, 2012). Other applications of AP-MALDIMSI include imaging of secondary metabolites in licorice rhizome (Li et al, 2014) and neuropeptides in crustaceans (Chen et al, 2018)

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