Chemical mapping of the ectomycorrhizal interface using optical photothermal infrared spectroscopy

The ectomycorrhizal (ECM) symbiosis underpins forest nutrient cycling through tightly regulated exchange of carbon and nutrients at the plant–fungal interface. Despite their ecological importance, the spatial chemistry of this interface remains poorly characterised. Here, we apply optical photothermal infrared (O-PTIR) spectroscopy to chemically map ECM root tip cryosections, with the aim of visualising potential transfer or storage compounds directly at the mycorrhizal interface.Spectral mapping reveals consistent spatial patterns across multiple ECM root tip cross sections. Distinct spectral bands are associated with the plant stele and the hyphal mantle, suggesting the presence of tissue-specific spectral bands at the ECM interface. Correspondingly, multivariate analysis shows a clear separation between plant and fungal tissues. In contrast, spectra from a putative Hartig net region overlap with both domains, consistent with a chemically heterogeneous interface where plant and fungal molecular signatures converge.These first data demonstrate the feasibility of O-PTIR for resolving chemically distinct domains within ECM root tips. This approach provides a promising foundation for investigating the spatial organisation of metabolites at the ECM interface and highlights the potential of high-resolution vibrational spectroscopy for studying nutrient exchange at biologically complex interfaces.