Functional Study of HopZ1, a Type III Secreted Effector of the Bacterial Pathogen Pseudomonas syringae
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.