Protective Plant Immune Responses are Elicited by Bacterial Outer Membrane Vesicles

Abstract

SummaryBacterial outer membrane vesicles (OMVs) perform a variety of functions in bacterial survival and virulence. In mammalian systems, OMVs activate immune responses and have been exploited as vaccines. However, little work has focused on the role that OMVs play during interactions with plant hosts. Here we report that OMVs from the pathogenic Pseudomonas syringae and the beneficial Pseudomonas fluorescens activate plant immune responses that protect against bacterial and oomycete pathogens. OMVs from these two species display different sensitivity to biochemical stressors, which could indicate differences in OMV cargo packaging. Furthermore, our study shows that OMV-induced protective immune responses are T3SS- and protein-independent, while OMV-mediated seedling growth inhibition largely depends on protein cargo. Importantly, OMV-mediated plant responses are distinct from those triggered by PAMP/MAMPs or effector molecules alone. OMVs provide a unique opportunity to study virulence factors in combination and add a new layer of interaction and complexity to host-microbe interactions.

Biophysical and proteomic analyses suggest functions ofPseudomonas syringaepvtomatoDC3000 extracellular vesicles in bacterial growth during plant infection

“…These results suggest that the immunogenic activities of the EVs, but not a longer-term trade-off with growth, may be relevant in their interaction with plants. In contrast to our results, McMillan et al reported significant seedling growth repression in response to Pto DC3000 EVs (McMillan et al , 2020). This disparity in results may be due to several factors, including differences in the growth conditions of both the bacterial cultures and the A. thaliana seedlings, the type of biochemical isolation of EVs and vesicle dose.…”

“…4A, S4A). Thus, the immunogenic potential of Pto DC3000 EVs is sufficient to restrict bacterial colonization, consistent with recent observations (McMillan et al , 2020). Since MAMP treatment mediates anti-bacterial protection through the induction of plant immune reactions (Zipfel et al , 2004), we next evaluated defence gene expression to EV treatment using pFRK1::GUS reporter lines (Kunze et al , 2004).…”

“…pauca 9a5c and subsp. Fb7 (Bahar et al , 2016, Feitosa-Junior et al , 2019, Chowdhury & Jagannadham, 2013, Ionescu et al , 2014, Knoke et al , 2020, McMillan et al , 2020, Sidhu et al , 2008, Sole et al , 2015). In this study, we provide several pieces of evidence for the role of EVs in the interaction of Pto DC3000 with plants: i) the bacteria produce EVs in planta during infection; ii) proteins responding to the plant environment are enriched in EVs; iii) known MAMPs and effectors are associated with EVs; and iv) plants respond to EVs with prototypic PTI reactions.…”

“…They also perform immunomodulatory functions by delivering virulence factors to recipient cells resulting in immune-suppression (Kaparakis-Liaskos & Ferrero, 2015), despite having the capacity to activate defences due to their immunogenic cargoes (Kaparakis-Liaskos & Ferrero, 2015). More recently, a number of studies provide evidence that plant pathogenic bacteria, including cultured Pto bacteria release EVs (Bahar et al , 2016, Chowdhury & Jagannadham, 2013, McMillan et al , 2020). Although insights into both immunogenic and virulent roles have been achieved, little is currently known about the importance of EV production in bacterial infection success.…”

“…Both represent MAMPs, with EV-associated EF-Tu shown to activate a prototypic PTI response in a receptor-dependent manner (Bahar et al , 2016). Further results indicate that EV-induced immunity, triggered with EVs recovered from pathogenic Pto DC3000 and the commensal P. fluorescens , protects plants against Pto DC3000 infection (McMillan et al , 2020). These studies hint at some contrasting roles that EVs from bacterial phytopathogens could play during plant infection.…”