# The proteasome acts as a hub for local and systemic plant immunity in Arabidopsis thaliana and constitutes a virulence target of Pseudomonas syringae type-III effector proteins

## TL;DR

The proteasome is crucial for plant immunity in Arabidopsis and is targeted by bacterial effectors to weaken plant defenses.

## Contribution

The study identifies proteasome subunits as key players in plant immunity and reveals Pseudomonas effectors that inhibit proteasome activity.

## Key findings

- Proteasome activity is essential for local immune responses and systemic-acquired resistance in Arabidopsis.
- Pseudomonas syringae type-III effectors HopM1, HopAO1, HopA1, and HopG1 inhibit proteasome activity.
- HopM1 interacts with proteasome subunits and ubiquitin ligases, suggesting a direct mechanism of inhibition.

## Abstract

Recent evidence suggests that the ubiquitin-proteasome system (UPS) is involved in several aspects of plant immunity and a range of plant pathogens subvert the UPS to enhance their virulence. Here, we show that proteasome activity is strongly induced during basal defense in Arabidopsis and mutant lines defective in proteasome subunits RPT2a and RPN12a support increased bacterial growth of virulent Pseudomonas syringae DC3000 (Pst), strains in local leaves. Both proteasome subunits are required for PTI events such as production of reactive oxygen species and mitogen-activated protein kinases signaling as well as for defense gene expression. Furthermore, analysis of bacterial growth after a secondary infection of systemic leaves revealed that the establishment of systemic-acquired resistance (SAR) is impaired in proteasome mutants, suggesting that the proteasome plays an important role in defense priming and SAR. In addition, we show that Pst inhibits proteasome activity in a type-III secretion dependent manner. A systematic screen for type-III effector proteins from Pst for their ability to interfere with proteasome activity revealed HopM1, HopAO1, HopA1 and HopG1 as candidates. Identification of proteins interacting with HopM1 by mass-spectrometry indicate that HopM1 resides in a complex together with several E3 ubiquitin ligases and proteasome subunits, supporting the hypothesis that HopM1 associates with the proteasome leading to its inhibition. We conclude that the proteasome is an essential component of the plant immune system and that some pathogens have developed a general strategy to overcome proteasome-mediated defense.

The proteasome is required for local and systemic immune responses and is targeted by Pseudomonas type-III effectors