Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2013)

Many Gram-negative phytopathogenic bacteria inject an array of type III secreted effectors (T3SEs) into plant cells via the type III secretion system (T3SS). These effectors play an essential role in bacterial infection mainly by targeting host immunity. However, the molecular basis of their functionalities remains largely unknown.

My research focuses on a T3SE produced by Pseudomonas syringae, called HopZ1, which is a member of the widely distributed YopJ effector family. Our lab identified several HopZ1 interacting proteins (ZINPs) from soybean using yeast two-hybrid screening, among which GmJAZ1 is homologous to Arabidopsis jasmonate ZIM-domain (AtJAZ) proteins. JAZs are key negative regulators of the jasmonate (JA) signaling and major components of the JA receptor complex. The main goal of my thesis is to investigate the function of HopZ1 in plant hosts by directly targeting the JAZ proteins.

In Chapter one, I proved that HopZ1 interacts with both GmJAZ1 and multiple AtJAZs in vitro and in planta. Upon interaction, JAZs can be acetylated by HopZ1a through a putative acetyltransferase activity. In addition, during bacterial infection of Arabidopsis, P. syringae producing wild type, but not the catalytic mutant, of HopZ1a induces the degradation of JAZs, activates the expression of JA-responsive genes, and suppresses the salicylic acid (SA) signaling pathway. As a result, HopZ1a promotes P. syringae infection in Arabidopsis.

In Chapter two, I determined that HopZ1a can acetylate AtJAZ6, AtJAZ8, AtJAZ9 and AtJAZ10 in vitro. Mass spectrometry analyses suggest that multiple serine/threonine residues within the TSYDSDSSDTTS peptide in the N-terminal region of AtJAZ8 are potentially acetylated by HopZ1a. AtJAZ8 mutants with this sequence deleted, or with all the eight serine and threonine residues within this sequence substituted with alanines, are no longer acetylated by HopZ1a, suggesting that one or more serine/threonine residues within this sequence are the acetylation sites.

My research provides the first example of a bacterial effector that subverts host immunity by directly targeting the receptor complex of a defense-associated hormone in plants. This work also revealed a novel mechanism to regulate the JA signaling in plants through posttranslational modification of JAZs.

    Cover page: Functional Study of HopZ1, a Type III Secreted Effector of the Bacterial Pathogen Pseudomonas syringae
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2015)
    Gram-negative bacteria inject type III secreted effectors (T3SEs) into host cells to manipulate the immune response. The YopJ family effector HopZ1a produced by the plant pathogen Pseudomonas syringae possesses acetyltransferase activity and acetylates plant proteins to facilitate infection. Using mass spectrometry, we identified a threonine residue, T346, as the main autoacetylation site of HopZ1a. Two neighboring serine residues, S349 and S351, are required for the acetyltransferase activity of HopZ1a in vitro and are indispensable for the virulence function of HopZ1a in Arabidopsis thaliana. Using proton nuclear magnetic resonance (NMR), we observed a conformational change of HopZ1a in the presence of inositol hexakisphosphate (IP6), which acts as a eukaryotic co-factor and significantly enhances the acetyltransferase activity of several YopJ family effectors. S349 and S351 are required for IP6-binding-mediated conformational change of HopZ1a. S349 and S351 are located in a conserved region in the C-terminal domain of YopJ family effectors. Mutations of the corresponding serine(s) in two other effectors, HopZ3 of P. syringae and PopP2 of Ralstonia solanacerum, also abolished their acetyltransferase activity. These results suggest that, in addition to the highly conserved catalytic residues, YopJ family effectors also require conserved serine(s) in the C-terminal domain for their enzymatic activity.
      Cover page: Two serine residues in Pseudomonas syringae effector HopZ1a are required for acetyltransferase activity and association with the host co-factor.
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2013)
      Gram-negative bacterial pathogens deliver a variety of virulence proteins through the type III secretion system (T3SS) directly into the host cytoplasm. These type III secreted effectors (T3SEs) play an essential role in bacterial infection, mainly by targeting host immunity. However, the molecular basis of their functionalities remains largely enigmatic. Here, we show that the Pseudomonas syringae T3SE HopZ1a, a member of the widely distributed YopJ effector family, directly interacts with jasmonate ZIM-domain (JAZ) proteins through the conserved Jas domain in plant hosts. JAZs are transcription repressors of jasmonate (JA)-responsive genes and major components of the jasmonate receptor complex. Upon interaction, JAZs can be acetylated by HopZ1a through a putative acetyltransferase activity. Importantly, P. syringae producing the wild-type, but not a catalytic mutant of HopZ1a, promotes the degradation of HopZ1-interacting JAZs and activates JA signaling during bacterial infection. Furthermore, HopZ1a could partially rescue the virulence defect of a P. syringae mutant that lacks the production of coronatine, a JA-mimicking phytotoxin produced by a few P. syringae strains. These results highlight a novel example by which a bacterial effector directly manipulates the core regulators of phytohormone signaling to facilitate infection. The targeting of JAZ repressors by both coronatine toxin and HopZ1 effector suggests that the JA receptor complex is potentially a major hub of host targets for bacterial pathogens.
        Cover page: Bacterial Effector Activates Jasmonate Signaling by Directly Targeting JAZ Transcriptional Repressors
        Top