Histology-guided high-resolution AP-SMALDI mass spectrometry imaging of wheat-Fusarium graminearum interaction at the root–shoot junction

Fungal pathogens like Fusarium graminearum can cause severe yield losses and mycotoxin contamination of food and feed worldwide. We recently showed its ability to systemically colonize wheat via root infection. However, the molecular response of wheat to Fusarium root rot (FRR) infection and systemic spread is still unknown. As a molecular camera, mass spectrometry (MS) imaging combines label-free and multiplex metabolite profiling with histopathology. Atmospheric-pressure (AP)-SMALDI-MS imaging was combined with optical microscopy to study wheat- F. graminearum interaction at the root–shoot junction, which is a crucial line of defense against a pathogen that can invade all distal plant parts. To scope the functional, temporal and local aspects of FRR disease spread, metabolic changes were simultaneous visualized in diseased and healthy stem bases of the resistant cultivar Florence-Aurore at 10, 14 and 21 days after root inoculation. Histological information was used to identify disease relevant tissues and to assist the interpretation of molecular images. Detected mycotoxin compounds secreted by F. graminearum showed a route of stem infection that was consistent with observations made by microscopy. The outer epidermis and vasculature of leaf sheath were, at different disease stages, identified as prominent sites of pathogen migration and wheat protection. Wheat metabolites mapped to these relatively small tissues indicated cell wall strengthening and antifungal activity as direct defenses as well as conservation in the wheat reactions to F. graminearum diseases that affect different plant organs. AP-SMALDI-MS imaging at high spatial resolution is a versatile technique that can be applied to basic and applied aspects of agricultural research. Combining the technology with optical microscopy was found to be a powerful tool to gain in-depth information on almost unknown crop disease. Moreover, the approach allowed studying metabolism at the host–pathogen interface. The results provide important hints to an understanding of the complex spatio-temporal organization of plant resistance. Defense-on-demand responses to pathogen ingress were found, which provide opportunities for future research towards an improved resistance that does not negatively impact yield development in the field by saving plant resources and, moreover, may control different Fusarium diseases.