Distinct membrane trafficking pathways defined by the requirement for <scp>GNOM</scp> or <scp>BIG1</scp> to <scp>BIG4</scp> mediate preinvasive immunity toward filamentous fungal pathogens

In response to attack by filamentous pathogens, including fungi and oomycetes, plants form localized cell wall depositions, called papillae, and encasements of intracellular infection structures that are thought to block pathogen entry and proliferation, respectively. In Arabidopsis thaliana, the syntaxin PENETRATION1 (PEN1;SYP121) mediates preinvasive immunity against nonadapted powdery mildew fungi. In addition, a conserved function shared with its closest paralog, SYP122, enables a general papilla/encasement response and preinvasive immunity toward a wide range of filamentous pathogens. Using genetic analysis, pathogen assays, and endomembrane trafficking studies, we investigated the roles of PEN1 and SYP122 and their associated pathways in preinvasive immunity toward filamentous pathogens. We found that PEN1 and SYP122 operate through two distinct endomembrane routes defined by specic ADP-ribosylation factor guanine nucleotide exchange factors: GNOM and Brefeldin A-Inhibited Guanine nucleotide exchange factor (BIG)1 to BIG4. Yet, inhibition of GNOM and BIG1 to BIG4 does not affect the powdery mildew-specific preinvasive immunity, which instead relies on a preformed endosomal compartment enriched in MILDEW RESISTANCE LOCUS O2 (MLO2). Furthermore, the durable mlo-based resistance requires PEN1 and SYP122. We conclude that plants deploy two independent membrane trafficking pathways for preinvasive immunity. We suggest these pathways integrate with MLO proteins to fine-tune resistance, providing broad and durable defense against filamentous pathogens.