Location: Sunflower and Plant Biology ResearchTitle: An Arabidopsis lipid flippase is required for timely recruitment of defenses to the host-pathogen interface at the plant cell surface Author
Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2017
Publication Date: 6/5/2017
Citation: Underwood, W., Ryan, A., Somerville, S.C. 2017. An Arabidopsis lipid flippase is required for timely recruitment of defenses to the host-pathogen interface at the plant cell surface. Molecular Plant. 10(6):805-820. Interpretive Summary: Plant immune responses are activated upon perception of conserved non-self elicitor signals derived from microbial pathogens. A major component of the plant immune response is local reinforcement of the cell wall at sites of pathogen attack to restrict access of the invading microorganism to the plant cell. Reinforcement of the wall involves deposition of polysaccharide and phenolic polymers that are presumed to act as a physical barrier and is also thought to involve targeted accumulation of antimicrobial metabolites that act as a chemical barrier. The cellular processes involved in constructing these wall reinforcing barriers, including the spatial targeting mechanisms, trafficking events, and associated regulatory controls are poorly understood. Using a marker protein that accumulates within cell wall reinforcements, we conducted a forward genetic screen and isolated mutants mis-targeting this marker protein during interaction of plant cells with a powdery mildew fungus. One of these mutants was found to be disrupted in a lipid flippase that plays a role in vesicle formation within the plant endomembrane system. Our findings suggest that defense proteins involved in wall reinforcement continuously cycle from the cell surface to the trans-Golgi network and, upon detection of an invading pathogen, are redirected to the host-pathogen interface.
Technical Abstract: Deposition of cell wall-reinforcing papillae is an integral component of the plant immune response. The Arabidopsis PENETRATION 3 (PEN3) ATP binding cassette (ABC) transporter plays a role in defense against numerous pathogens and is recruited to sites of pathogen detection where it accumulates within papillae. However, the trafficking pathways and regulatory mechanisms contributing to recruitment of PEN3 and other defenses to the host-pathogen interface are poorly understood. Here, we report a confocal microscopy-based screen to identify mutants with altered localization of PEN3-GFP after inoculation with powdery mildew fungi. We identified a mutant, aberrant localization of PEN3 3 (alp3), displaying accumulation of the normally plasma membrane (PM)-localized PEN3-GFP in endosomes. The mutant was found to be disrupted in the P4-ATPase AMINOPHOSPHOLIPID ATPASE 3 (ALA3), a lipid flippase which plays a critical role in vesicle formation. We provide evidence that PEN3 undergoes continuous endocytic cycling from the PM to the trans-Golgi network (TGN). In alp3, PEN3 accumulates in the TGN, causing delays in recruitment to the host-pathogen interface. Our results indicate that PEN3 and other defense proteins continuously cycle through the TGN and timely exit of these proteins from the TGN is critical for effective pre-invasive immune responses against powdery mildews.