A family of conserved bacterial effectors inhibits salicylic acid-mediated basal immunity and promotes disease necrosis in plants

Proc Natl Acad Sci U S A. 2004 Jun 29;101(26):9927-32. doi: 10.1073/pnas.0401601101. Epub 2004 Jun 21.


Salicylic acid (SA)-mediated host immunity plays a central role in combating microbial pathogens in plants. Inactivation of SA-mediated immunity, therefore, would be a critical step in the evolution of a successful plant pathogen. It is known that mutations in conserved effector loci (CEL) in the plant pathogens Pseudomonas syringae (the Delta CEL mutation), Erwinia amylovora (the dspA/E mutation), and Pantoea stewartii subsp. stewartii (the wtsE mutation) exert particularly strong negative effects on bacterial virulence in their host plants by unknown mechanisms. We found that the loss of virulence in Delta CEL and dspA/E mutants was linked to their inability to suppress cell wall-based defenses and to cause normal disease necrosis in Arabidopsis and apple host plants. The Delta CEL mutant activated SA-dependent callose deposition in wild-type Arabidopsis but failed to elicit high levels of callose-associated defense in Arabidopsis plants blocked in SA accumulation or synthesis. This mutant also multiplied more aggressively in SA-deficient plants than in wild-type plants. The hopPtoM and avrE genes in the CEL of P. syringae were found to encode suppressors of this SA-dependent basal defense. The widespread conservation of the HopPtoM and AvrE families of effectors in various bacteria suggests that suppression of SA-dependent basal immunity and promotion of host cell death are important virulence strategies for bacterial infection of plants.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Arabidopsis / genetics
  • Arabidopsis / immunology*
  • Arabidopsis / microbiology
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Cell Wall / physiology
  • Conserved Sequence* / genetics
  • Erwinia amylovora / genetics
  • Erwinia amylovora / pathogenicity
  • Erwinia amylovora / physiology
  • Gene Expression Regulation, Plant
  • Genes, Plant / genetics
  • Genetic Complementation Test
  • Glucans / metabolism
  • Immunity, Innate / physiology*
  • MAP Kinase Signaling System
  • Malus / genetics
  • Malus / immunology*
  • Malus / microbiology
  • Models, Biological
  • Mutation / genetics
  • Necrosis
  • Pantoea / genetics
  • Pantoea / pathogenicity
  • Pantoea / physiology
  • Plant Diseases / genetics
  • Plant Diseases / microbiology*
  • Pseudomonas syringae / genetics
  • Pseudomonas syringae / pathogenicity
  • Pseudomonas syringae / physiology
  • Salicylic Acid / antagonists & inhibitors*
  • Salicylic Acid / metabolism
  • Virulence / genetics


  • Bacterial Proteins
  • Glucans
  • callose
  • Salicylic Acid