Immune recognition of the secreted serine protease ChpG restricts the host range of Clavibacter michiganensis from eggplant varieties

Abstract

Bacterial wilt and canker caused by Clavibacter michiganensis (Cm) inflict considerable damage in tomato‐growing regions around the world. Cm has a narrow host range and can cause disease in tomato but not in many eggplant varieties. The pathogenicity of Cm is dependent on secreted serine proteases, encoded by the chp/tomA pathogenicity island (PI), and the pCM2 plasmid. Screening combinations of PI deletion mutants and plasmid‐cured strains found that Cm‐mediated hypersensitive response (HR) in the Cm‐resistant eggplant variety Black Queen is dependent on the chp/tomA PI. Singular reintroduction of PI‐encoded serine proteases into Cm∆PI identified that the HR is elicited by the protease ChpG. Eggplant leaves infiltrated with a chpG marker exchange mutant (CmΩchpG) did not display an HR, and infiltration of purified ChpG protein elicited immune responses in eggplant but not in Cm‐susceptible tomato. Virulence assays found that while wild‐type Cm and the CmΩchpG complemented strain were nonpathogenic on eggplant, CmΩchpG caused wilt and canker symptoms. Additionally, bacterial populations in CmΩchpG‐inoculated eggplant stem tissues were c.1000‐fold higher than wild‐type and CmΩchpG‐complemented Cm strains. Pathogenicity tests conducted in multiple Cm‐resistance eggplant varieties demonstrated that immunity to Cm is dependent on ChpG in all tested varieties, indicating that ChpG‐recognition is conserved in eggplant. ChpG‐mediated avirulence interactions were disabled by alanine substitution of serine231 of the serine protease catalytic triad, suggesting that protease activity is required for immune recognition of ChpG. Our study identified ChpG as a novel avirulence protein that is recognized in resistant eggplant varieties and restricts the host range of Cm.

Deletion of pbpC Enhances Bacterial Pathogenicity on Tomato by Affecting Biofilm Formation, Exopolysaccharides Production, and Exoenzyme Activities in Clavibacter michiganensis

“…It was revealed that the plasmid genes (including pat-1 , ppaJ , CelA , phpA , and phpB ), plasmid quantity, PAI on the chromosome, and transcription regulatory factors are involved in the pathogenicity of C. michiganensis [ 43 , 44 , 45 ]. ChpG encodes a transmembrane protein that mediates the immune response of multiple eggplant varieties, whereas some virulence factors remain undiscovered and unevaluated [ 25 ]. The high relative expression levels of CelA , xysA , and xysB , which encode cellulase or xylanase, in the △ pbpC mutants indicated that the canker symptom is primarily caused by cell-wall-degrading enzymes.…”

“…The second factor contributing to virulence is pat-1 , which encodes a putative serine protease and has been identified on plasmid pCM2 [ 23 , 24 ]. A number of transcriptional regulators also associated with virulence, such as Vatr1 and Vatr2 , are located in chromosomal regions [ 25 ].…”

Clavibacter nebraskensis causing Goss's wilt of maize: Five decades of detaining the enemy in the New World

“…Among the plant‐pathogenic members of Clavibacter , putative genes associated with pathogenicity may carry different roles, which may depend on their plant host. In C. michiganensis , some chp serine proteases are known to not affect virulence when deleted (Peritore‐Galve et al, 2021 ) and some are recognized to restrict pathogen growth and disease in related plant species outside of tomato (Park et al, 2022 ; Verma & Teper, 2022 ). Conversely, in C. capsici , one chp protease ( chpG ) is a putative pathogenicity factor critical for disease as knockouts and natural variants lacking chpG are less virulent (Hwang et al, 2020 ).…”

Genetic and transcriptomic dissection of host defense to Goss's bacterial wilt and leaf blight of maize

“…Consequently, recognition of pathogen-associated molecular patterns (PAMPs) is thought to play an important role in plant defense to Cn and Cm. Proteases have been shown to elicit a nonhost resistance in the case of Cm and Clavibacter sepedonicus (Cs) ( Nissinen et al 2009 ; Lu et al 2015 ; Verma and Teper 2022 ). Bacterial cold shock proteins, cell wall components, exopolysaccharides, extracellular cell wall degrading enzymes secreted by bacteria, and even degraded fragments from the host cell wall may function as PAMPs to elicit plant defense ( Balaji et al 2008 ).…”