Membrane contact sites between chloroplasts and the pathogen interface underpin plant focal immune responses

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Abstract
Communication between cellular organelles is essential for mounting effective innate immune responses. The transport of organelles to pathogen penetration sites and their assembly around the host membrane, which delineates the plant-pathogen interface, are well-documented. However, whether organelles associate with these specialized interfaces, and the extent to which this process contributes to immunity, remain unknown. Here, we discovered defense-related membrane contact sites (MCS) comprising a membrane tethering complex between chloroplasts and the extrahaustorial membrane (EHM) surrounding the haustorium of the pathogen Phytophthora infestans in Nicotiana benthamiana. The assembly of this complex involves association between the chloroplast outer envelope protein CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1) and its plasma membrane-associated partner KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT 1 (KAC1). Our biochemical assays revealed that CHUP1 and KAC1 interact, and infection cell biology assays demonstrated their co-accumulation in foci where chloroplasts contact the EHM. Genetic depletion of CHUP1 or KAC1 reduces the focal deposition of callose around the haustorium without affecting other core immune processes. Our findings suggest that the chloroplast-EHM attachment complex promotes plant focal immunity, revealing key components and their potential roles in the deposition of defense materials at the pathogen interface. These results advance our understanding of organelle-mediated immunity and highlight the significance of MCS in plant-pathogen interactions.
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