Multiple intramolecular trafficking signals in RESISTANCE TO POWDERY MILDEW 8.2 are engaged in activation of cell death and defense

Abstract

The extrahaustorial membrane (EHM) is a host-derived interfacial membrane encasing the haustorium of powdery mildew fungi. Arabidopsis thaliana RESISTANCE TO POWDERY MILDEW 8.2 (RPW8.2) is specifically targeted to the EHM via two EHM-targeting signals. Here, we demonstrate that proper coordination between the trafficking forces engaged via the EHM-targeting signals and the nuclear localization signals (NLSs), as well as the nuclear export signals (NESs), in RPW8.2 is critical for the activation of cell death and defense. We show that in the absence of pathogens, RPW8.2 is partitioned between the cytoplasm and the nucleus, and turned over via both the 26S proteasome- and the vacuole-dependent pathways. Enhanced cytoplasmic localization of RPW8.2 by tagging it with a NES led to lethal cell death. By contrast, enhanced nuclear localization of RPW8.2 by adding an NLS to it resulted in resistance to powdery mildew. Whereas expression of the NES-containing C-terminal domain of RPW8.2 in the cytoplasm is sufficient to trigger cell death, no such cell death-inducing activity is found with RPW8.2 variants that contain the two EHM-targeting signals along with the NES-containing C-terminal domain. In addition, we present evidence for the involvement of a leaf senescence pathway in RPW8.2-mediated cell death and defense. Taken together, our data suggest that RPW8.2 is subject to adjustment by distinct and perhaps coordinated mechanisms for its localization and function via interaction with the multiple intramolecular trafficking signals, which should provide further insights into RPW8.2-activated, EHM-focused resistance against powdery mildew.

Keywords: RPW8.2; cell death; defense; extrahaustorial membrane; nuclear export signal; nuclear localization signal; powdery mildew.