Arabidopsis immune secretory pathways to powdery mildew fungi

Innate immune responses in host plants begin with the recognition of pathogen-specific nonself molecules and terminate with the secretion of immune molecules. In the dicotyledonous model plant, Arabidopsis thaliana, two distinct secretory pathways required for disease resistance to powdery mildew fungi have been identified so far. One is an exocytic pathway consisting of PEN1, SNAP33 and VAMP721/722 SNARE proteins, but the other is an efflux-mediated one composed of PEN2 atypical myrosinase and PEN3 ABC transporter. Based on the conservation of the mechanically same exocytic pathway in the monocotyledonous plant barely, the former is regarded as an ancient secretory pathway, whereas the latter is considered as a newly evolved one in the Brassicaceae family including Arabidopsis. We recently identified synaptotagmin 1 as an additional regulator of these two secretory pathways. With current results, we discuss how these two secretory pathways contribute to Arabidopsis immunity depending on fungal adaptedness to Arabidopsis.