Abstract
The phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) requires type III effector proteins (T3Es) for virulence. After translocation into the host cell, T3Es are thought to interact with components of host immunity to suppress defence responses. XopJ is a T3E protein from Xcv that interferes with plant immune responses; however, its host cellular target is unknown. Here we show that XopJ interacts with the proteasomal subunit RPT6 in yeast and in planta to inhibit proteasome activity. A C235A mutation within the catalytic triad of XopJ as well as a G2A exchange within the N-terminal myristoylation motif abolishes the ability of XopJ to inhibit the proteasome. Xcv ΔxopJ mutants are impaired in growth and display accelerated symptom development including tissue necrosis on susceptible pepper leaves. Application of the proteasome inhibitor MG132 restored the ability of the Xcv ΔxopJ to attenuate the development of leaf necrosis. The XopJ dependent delay of tissue degeneration correlates with reduced levels of salicylic acid (SA) and changes in defence- and senescence-associated gene expression. Necrosis upon infection with Xcv ΔxopJ was greatly reduced in pepper plants with reduced expression of NPR1, a central regulator of SA responses, demonstrating the involvement of SA-signalling in the development of XopJ dependent phenotypes. Our results suggest that XopJ-mediated inhibition of the proteasome interferes with SA-dependent defence response to attenuate onset of necrosis and to alter host transcription. A central role of the proteasome in plant defence is discussed.
Highlights
Plants have to protect themselves from a plethora of microbial enemies
The Arabidopsis genome encodes two RPT6 genes (RPT6a, At5g19990 and RPT6b, At5g20000) which are directly arranged in tandem and expression of both genes is supported by cDNAs
Both protein sequences share 97% identity with each other (Figure S1); screening the Arabidopsis two-hybrid library recovered only RPT6a as an in XopJ interacting protein and the ability of RPT6b to bind XopJ was investigated in a direct interaction assay in yeast
Summary
In a first layer of defence, conserved microbial molecules called PAMPs/MAMPs (pathogen/microbe-associated molecular patterns) are recognized on the cell surface which leads to the induction of a number of defence responses, including the generation of reactive oxygen species, the initiation of MAP kinase signalling, PR-gene expression, and callose depositions at the cell wall [1] These responses are sufficient to prevent multiplication and spread of a broad range of potential pathogens and mostly result in PTI (PAMP-triggered immunity). To overcome this barrier many gram-negative plant pathogenic bacteria have acquired a highly conserved type III secretion system (T3SS) which enables them to inject so called type III effector proteins (T3Es) into the plant cell. In this second layer of defence effector recognition results in an effective immune response which is often accompanied by rapid, localized cell death, termed the hypersensitive response (HR), eventually restricting bacterial spread and leading to effector triggered immunity (ETI) [6]
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