The plant endoplasmic reticulum is both receptive and responsive to pathogen effectors

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. Comprised of a network of interconnected tubules and cisternae, the architecture of the ER is highly pleomorphic and dynamic, rapidly remodelling to meet new cellular requirements. During infection with the hemi-biotrophic phytopathogen, Pseudomonas syringae pv. tomato DC3000, the ER in cells immediately adjacent to established bacterial colonies condenses into ‘knot-like’ structures, reminiscent of fenestrated sheets. Based on known temporal dynamics of pathogen effector delivery and initial bacterial multiplication, the timing of these observed morphological changes is rapid and independent of classical elicitor activation of pathogen-triggered immunity. To further investigate a role for ER reconfiguration in suppression of plant immunity we identified a conserved C-terminal tail-anchor domain in a set of pathogen effectors known to localize to the ER and used this protein topology in an in silico screen to identify putative ER-localised effectors within the effectorome of the oomycete Phytophthora infestans. Subsequent characterization of a subset of 15 candidate tail- anchored P. infestans effectors revealed that 11 localised to the ER and/or Golgi. Notably, transient expression of an ER-localised effector from the closely related oomycete, Plasmopara halstedii, reconfigured the ER network, revealing intimate association of labelled ER with perinuclear chloroplasts and clusters of chloroplasts, potentially facilitating retrograde signalling during plant defence

A tell tail sign : a conserved C-terminal tail-anchor domain targets a subset of pathogen effectors to the plant endoplasmic reticulum

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-localized 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-localized NAC transcription factors, indicating that this family is a critical host target for multiple pathogens