Anti- and pro-microbial roles of autophagy in plant-bacteria interactions

ABSTRACT

Macroautophagy/autophagy and the ubiquitin-proteasome system (UPS) are major proteolytic pathways that are increasingly recognized as battlegrounds during host-microbe interactions in eukaryotes. In plants, the UPS has emerged as central component of innate immunity and is manipulated by bacterial pathogens to enhance virulence. Autophagy has been ascribed a similar importance for anti-bacterial immunity in animals, but the contribution of autophagy to host-bacteria interactions remained elusive in plants. Here, we present and discuss our recent findings that revealed anti- and pro-bacterial roles of autophagy pathways during bacterial infection in the model plant Arabidopsis thaliana. We discovered that selective autophagy mediated by the autophagy cargo receptor AT4G24690/NBR1 limits growth of Pseudomonas syringae pv. tomato DC3000 (Pst) by suppressing the establishment of an aqueous extracellular space (‘water-soaking’). In turn, Pseudomonas employs the effector protein HopM1 to activate autophagy and proteasome degradation (‘proteaphagy’), thereby enhancing its pathogenicity. Thus, our study demonstrates that distinct selective autophagy pathways contribute to host immunity and bacterial pathogenesis during Pst infection and provide evidence for an intimate crosstalk between the proteasome and autophagy system in plant-bacterial interactions.

KEYWORDS:

• Innate immunity
• NBR1
• plant bacterium
• proteaphagy
• Pseudomonas syringae
• selective autophagy

During the long-lasting evolutionary battle between host organisms and their associated pathogens both sides have acquired sophisticated weapons to trick their counterpart. Eukaryotes rely on a multilayered immune system to combat microbes, whereas adapted pathogens subvert defense responses by delivering so-called effector proteins into the host cell. The intricate molecular mechanisms underlying the cellular changes during immune reactions require a high degree of proteomic plasticity. Thus, it is not surprising that the 2 major proteolytic degradation pathways in the cell, autophagy and the ubiquitin-proteasome system (UPS), play essential roles during plant-microbe interactions.

Research in recent years has highlighted the significance of the UPS in the regulation of plant and animal immunity and revealed that various pathogens manipulate the host UPS for their own benefit. Similar to the UPS, autophagy is subverted and hijacked by different pathogens to enhance pathogenicity. The anti- and pro-microbial roles of autophagy in host-bacteria interactions are well established in animals. However, the molecular mechanisms of how autophagy is modulated by phytopathogenic bacteria has remained elusive until our recent work on the interaction of Pseudomonas syringae pv. tomato DC3000 (Pst) with the model plant Arabidopsis thaliana.

Intriguingly, we discovered that distinct selective autophagy pathways have opposing anti- and pro-bacterial roles during Pseudomonas infection [1]. We found that the plant cargo receptor NBR1 counteracts bacterial pathogenicity by suppressing disease-promoting water-logging (‘water-soaking’) in the extracellular space. In contrast, Pst evolved measures to activate autophagy and to trigger proteaphagy, eventually leading to enhanced bacterial pathogenicity. Initially, proteaphagy was discovered in plants as a selective autophagy mechanism to recycle proteasomes during nitrogen starvation and in response to chemical or genetic proteasome inhibition. Our work showed that Pst is able to hijack this pathway to impair proteasome function. It will be interesting to investigate whether other plant pathogens (fungi, oomycetes, viruses) use a similar mechanism to impair proteolytic dynamics. Furthermore, as the proteaphagy pathway is conserved from yeast to animals, it is likely that its activation is a general strategy also used by animal pathogens.

In the Pseudomonas-Arabidopsis system, we found that autophagy is activated by the type-III effector protein HopM1, which is delivered by the bacterium into the plant cell upon successful colonization of the plant extracellular space. We discovered that HopM1 associates with the autophagy and proteasome machinery and is responsible for proteaphagy induction. Notably, this finding is in agreement with the observation that HopM1 suppresses proteasome activity in plants. However, it seems to be rather contradictory to the earlier proposed HopM1-induced proteasomal degradation of AT3G43300/AtMIN7, a host ADP ribosylation factor guanine nucleotide exchange factor, which localizes to the trans-Golgi network (TGN) and is required for vesicle trafficking during plant defense.

Hence, the mechanistic details of how HopM1 is activating autophagy in this complex scenario remain an outstanding question. Initially, we assumed that HopM1 might only activate proteaphagy, as it interacts with proteasome subunits and eventually suppresses proteasome activity similar to the chemical inhibitor MG132. However, HopM1 strongly stimulates autophagy levels and NBR1 flux, arguing against specificity toward the proteaphagy pathway. Based on the emerging crosstalk between autophagy and endosomal trafficking pathways, we speculate that HopM1 might target components in the TGN such as AtMIN7 to promote autophagy. However, it remains to be clarified if HopM1-induced loss of AtMIN7 is linked to enhanced autophagy activity and flux.

Importantly, the HopM1-mediated targeting of AtMIN7 was earlier shown to be required for the induction of ‘water-soaking,’ which promotes bacterial disease progression. Our results indicate that NBR1-mediated selective autophagy counteracts this process and limits the formation of water-soaked lesions by as yet unknown mechanisms. NBR1 in plants is involved in the removal of ubiquitinated aggregates and the xenophagic breakdown of viral proteins and particles. Similar to its role in anti-viral immunity, the anti-bacterial effect of NBR1 might be linked to a direct elimination of the pathogen effector HopM1. Indeed, NBR1 and HopM1 seem to associate in planta; however, protein levels of HopM1 in the presence of NBR1 remain stable. It is therefore tempting to speculate that NBR1-dependent autophagy degrades negative immune regulators and/or other specific components required for HopM1/AtMIN7-mediated water-soaking.

The parallel operation of pro- and anti-bacterial autophagy pathways during Pseudomonas infection provides another example for the multifaceted outcome of the evolutionary arms race in plant-microbe interactions. Given the role of NBR1 in anti-bacterial autophagy, it is evident that plants acquired this selective autophagy receptor to combat microbes. Our data indicate that bacteria developed the capacity to enhance autophagy by the action of bacterial effectors to promote their virulence. We speculate that this bacterial strategy contributed to the evolution of the NBR1-mediated block of disease progression before the Pseudomonas effector HopM1 acquired the function to exploit the autophagy pathway for proteasome suppression. The ongoing arms race between plants and bacteria implies the presence of other yet unknown anti-bacterial autophagy mechanisms as well as pro-bacterial autophagic targets. As such, the identification of the pathogen-triggered autophagy degradome will further reveal how microbes modulate plant immunity.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Federation of European Biochemical Societies (FEBS); Swedish Research Council FORMAS; Swedish University of Agricultural Sciences (SLU).