Resolving spatially distinct phytohormone response zones in Arabidopsis thaliana roots colonized by Fusarium oxysporum.

Jasmonic acid (JA), ethylene (ET), and salicylic acid (SA) are the three major phytohormones coordinating plant defense responses, and all three are implicated in the defense against the fungal pathogen Fusarium oxysporum. However, their distinct modes of action and possible interactions remain unknown, in part because all spatial information on their activity is lacking. Here, we set out to probe this spatial aspect of plant immunity by using live microscopy with newly developed fluorescence-based transcriptional reporter lines. We have created a GreenGate vector collection of Plant Immune system Promoters (GG-PIPs) that allow us to image local activation of immune pathways with single-cell resolution. Using this system, we demonstrated that SA and JA act spatially separately from each other in distinct sets of root cells neighboring the fungal colonization site, while ET contributed to both sets. SA and ET induced the hypersensitive response as a first line of defense, while JA and ET governed active defense against the pathogen in a separate, second line of defense. Such an approach to resolve plant immune responses on an individual cell level has been lacking, and this work demonstrates that this microscopy-based approach can contribute to understanding plant immune responses in detail.