Spatially distinct phytohormone responses of individualArabidopsis thalianaroot cells to infection and colonization byFusarium oxysporum

Abstract

AbstractJasmonic acid (JA), ethylene (ET) and salicylic acid (SA) are the three major phytohormones coordinating a plant’s defense response to pathogenic attack. While JA and ET are assumed to primarily control the defense against necrotrophic pathogens, SA-induced defense responses target mainly biotrophic microbes, and can include drastic measures such as programmed cell death as part of the plant’s hypersensitive response (HR).Fusarium oxysporumis a hemibiotrophic fungal pathogen of several plant species, including many important food crops, and the model plant speciesArabidopsis thaliana. Colonization of the plant’s root vascular tissue by the fungus eventually results in wilting and plant death. A general role for JA, ET and SA in combating infection and colonization of the plant byF. oxysporumhas been demonstrated, but their distinct roles and modes of action have so far not been described. Here, using high resolution microscopy with fluorescent marker lines ofA. thalianaroots infected withF. oxysporumwe show that SA acts spatially separate from JA, in a distinct set of root cells immediately neighboring the fungal colonization site. There, SA induces HR to stop the spread of colonization. JA acts in a different, but also clearly defined set of cells, slightly removed from the colonization site, where it initiates a defense response to actively resist the invader. ET is activated in a stretch of cells that covers both, the cells with activated SA and JA signaling, and may be involved in creating these two distinct zones. These results show how the three phytohormones act together, but spatially and functionally separate from each other, to fight this hemibiotrophic pathogen. Such a high-resolution analysis to resolve the plant’s immune response to pathogenic infection on an individual cell level and in intact tissue has so far been lacking.Graphical AbstractColonization of theA.thalianaroot tip byF.oxysporumstrainFo5176 leads to immediate cell death of the colonized and surrounding tissue.As the colonization front progresses through the vasculature, the cell death front moves along with it through not only the vasculature, but also the surrounding tissues.WRKY70 positively regulates salicylic acid (SA) biosynthesis in cells immediately adjacent to the colonized tissue, inducing a hypersensitive response (HR), thereby killing off the cells deemed lost to the intruder, establishing the cell death front.Slightly further removed from the HR zone, WRKY11 induces jasmonate (JA) biosynthesis in cells of the vasculature to launch a defense response aimed at actively repelling the fungus.