15The endoplasmic reticulum (ER) is the entry point to the secretory pathway and, as such, is 16 critical for adaptive responses to biotic stress, when the demand for de novo synthesis of 17 immunity-related proteins and signalling components increases significantly. Comprised of a 18 network of interconnected tubules and cisternae, the architecture of the ER is highly 19 pleomorphic and dynamic, rapidly remodelling to meet new cellular requirements. During 20 infection with the hemi-biotrophic phytopathogen, Pseudomonas syringae pv. tomato 21 DC3000, the ER in cells immediately adjacent to established bacterial colonies condenses 22 into 'knot-like' structures, reminiscent of fenestrated sheets. Based on known temporal 23 dynamics of pathogen effector delivery and initial bacterial multiplication, the timing of these 24 observed morphological changes is rapid and independent of classical elicitor activation of 25 pathogen-triggered immunity. To further investigate a role for ER reconfiguration in 26 suppression of plant immunity we identified a conserved C-terminal tail-anchor domain in a 27 set of pathogen effectors known to localize to the ER and used this protein topology in an in 28 silico screen to identify putative ER-localised effectors within the effectorome of the oomycete 29 Phytophthora infestans. Subsequent characterization of a subset of 15 candidate tail-30 anchored P. infestans effectors revealed that 11 localised to the ER and/or Golgi. Notably, 31 transient expression of an ER-localised effector from the closely related oomycete, 32 Plasmopara halstedii, reconfigured the ER network, revealing intimate association of labelled 33 ER with perinuclear chloroplasts and clusters of chloroplasts, potentially facilitating retrograde 34 signalling during plant defence. 37As the gateway to the cell's secretory pathway, the endoplasmic reticulum (ER) provides the 38 environment for secretory protein production, folding and quality control. The ER is a highly 39 dynamic, interconnected network of tubules and cisternae (sheets) that extends throughout 40 the cytoplasm, associating with the plasma membrane and other organelles, and adjacent 41 cells via plasmodesmata (Hawes et al., 2015). Hence, the ER is central to the maintenance of 42 cellular homeostasis and can facilitate intra-and intercellular communication.

The endoplasmic reticulum (ER) is the entry point to the secretory pathway and, as such, is critical for adaptive responses to biotic stress, when the demand for de novo synthesis of immunity-related proteins and signalling components increases significantly. Successful phytopathogens have evolved an arsenal of small effector proteins which collectively reconfigure multiple host components and signalling pathways to promote virulence; a small, but important, subset of which are targeted to the endomembrane system including the ER. We identified and validated a conserved C-terminal tail-anchor motif in a set of pathogen effectors known to localize to the ER from the oomycetes Hyaloperonospora arabidopsidis and Plasmopara halstedii (downy mildew of Arabidopsis and sunflower, respectively) and used this protein topology to develop a bioinformatic pipeline to identify putative ER-localised effectors within the effectorome of the related oomycete, Phytophthora infestans, the causal agent of potato late blight. Many of the identified P. infestans tail-anchor effectors converged on ER-localised NAC transcription factors, suggesting this family is a critical host target for multiple pathogens.