Immune surveillance at plasmodesmata restricts the movement of a fungal effector

To establish infection, phytopathogens deploy effectors to compromise host defences and facilitate invasive growth. As part of this, the battle for control of symplastic connectivity via plasmodesmata is a key determinant of infection outcomes, yet little is known about how fungal effectors directly exploit these channels, and in turn, how hosts defend them. Here, we have identified ChEC108 as a plasmodesmal-targeting, cell-to-cell mobile effector from the anthracnose fungus, Colletotrichum higginsianum. ChEC108 binds the plasmodesmal protein HEAVY METAL-ASSOCIATED (HMA) ISOPRENYLATED PLANT PROTEIN 6 (HIPP6) from Arabidopsis via a tetrahedral metal ion coordination site with either of its HMA domains. This binding event induces plasmodesmal closure, impairing the cell-to-cell mobility of ChEC108. Alongside the finding that loss of either HIPP6 from the host or ChEC108 from C. higginsianum favours infection, we conclude that HIPP6 detects the presence of an invading fungus and elicits defence.