Visualising the ionome in resistant and susceptible plant–pathogen interactions

At the morphological and anatomical level the ionome, or the elemental composition of an organism, is an understudied area of plant biology. In particular, the ionomic responses of plant-pathogen interactions are scarcely described, and there are no studies on immune reactions. In this study we explored two X-ray fluorescence based ionome visualisation methods (benchtop and synchrotron-based micro XRF (µXRF), as well as the quantitative Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) method, to investigate the changes that occur in the ionome of compatible and incompatible plant-pathogen interactions. We utilised the agronomically important and comprehensively studied interaction between potato, Solanum tuberosum, and the late blight oomycete pathogen, Phytophthora infestans, as an example. We used one late blight susceptible potato cultivar and two resistant transgenic plant lines (only differing from the susceptible cultivar in one or three resistance genes) both in control and P. infestans inoculated conditions. In the lesions from the compatible interaction we observed rearrangements of several elements. This included, a decrease of the mobile macro nutrient potassium (K), and an increase of iron (Fe), and manganese (Mn) in the lesion versus the tissue outside the lesion. Interestingly, we observed distinctly different distribution patterns of accumulation at the site of inoculation in the resistant lines for calcium (Ca), Magnesium, Mn and silicon (Si) compared to the susceptible cultivar. The results reveal different ionomes in diseased plants compared to resistant plants. Our results demonstrate a technical advance and pave the way for deeper studies of the plant-pathogen ionome in the future.