Chloroplast movement and positioning protein CHUP1 is required for focal immunity against Phytophthora infestans

When a plant detects a pathogen, chloroplasts terminate photosynthetic activity and uptake vital roles in the immune system to help stave off infection, including the production of defense hormone precursors and antimicrobial reactive oxygen species. Additionally, chloroplasts associate with the nucleus and produce greater numbers of tubular extensions called stromules during immune challenge. We previously showed that during infection by the potato blight pathogen Phytophthora infestans, chloroplasts accumulate at the pathogen haustoria, hyphal extensions that are accommodated within the host cell. However, the extent to which chloroplast positioning around haustoria, or at the nucleus, contributes to immunity during infection remains unknown. Here we show a striking increase in the susceptibility to P. infestans of Nicotiana benthamiana CRISPR knock-out lines lacking the chloroplast movement and anchoring gene, CHLOROPLAST UNUSUAL POSITIONING 1 (CHUP1). However, the positioning of chloroplasts around the haustorium or nucleus is not impaired in the absence of CHUP1. Further, loss of CHUP1 leads to an extreme clustering of chloroplasts around the nucleus in the presence and absence of infection, showing that greater chloroplast-nucleus association does not necessarily equate to more robust immunity. While plants lacking CHUP1 have reduced basal stromules, they are still able to induce stromules following immune stimulation, indicating that multiple populations of stromules exist. Lastly, we found that CHUP1 is required for proper deposition of callose - a cell wall material implicated in pathogen penetration resistance - around P. infestans haustorium, but not for other core immune processes. Our results implicate chloroplasts in plant focal immunity and point to a key role of CHUP1 in facilitating the deposition of defense material at the pathogen interface.