Plant heat shock protein Hsp70 is the major target of HopI1, a virulence effector of pathogenic Pseudomonas syringae. Hsp70 is essential for the virulence function of HopI1. HopI1 directly binds Hsp70 through its C-terminal J domain and stimulates Hsp70 ATP hydrolysis activity in vitro. In plants, HopI1 forms large complexes in association with Hsp70 and induces and recruits cytosolic Hsp70 to chloroplasts, the site of HopI1 localization. Deletion of a central P/Q-rich repeat region disrupts HopI1 virulence but not Hsp70 interactions or association with chloroplasts. Thus, HopI1 must not only bind Hsp70 through its J domain, but likely actively affects Hsp70 activity and/or specificity. At high temperature, HopI1 is dispensable for P. syringae pathogenicity, unless excess Hsp70 is provided. A working hypothesis is that Hsp70 has a defense-promoting activity(s) that HopI1 or high temperature can subvert. Enhanced susceptibility of Hsp70-depleted plants to nonpathogenic strains of P. syringae supports a defense-promoting role for Hsp70.T o cause a successful infection, many plant pathogenic bacteria use a type III secretion system through which dozens of different effector proteins are injected into plant cells (1). Pseudomonas syringae, a type III-requiring pathogen, causes disease on foliage and fruits of diverse plants. Some P. syringae effectors can restrict host range and/or disease potential, rendering the pathogen "avirulent," when they are recognized by a plant's defense machinery (2). However, in many cases, effectors suppress plant immune responses (1, 3). These virulence effectors are of intense interest, because their study not only gives insight into pathogenic mechanisms, but they can be used to identify previously unknown defense components (1, 4, 5) whose engineering may lead to novel approaches for creating disease-resistant plants.The P. syringae pv. maculicola ES4326 (Pma) HopI1 effector, a J protein (i.e., one that contains a J domain) suppresses accumulation of the defense regulator salicylic acid (SA) and related plant defenses (6). HopI1 localizes to chloroplasts where SA is synthesized (7) and also affects thylakoid stack structure within chloroplasts (6). HopI1-expressing plants can rescue the virulence defect of PmaΔhopI1 bacteria, indicating that HopI1 exerts its effects from within plant cells. All pathogenic P. syringae examined have a HopI1 allele with a conserved 190-amino acid N-terminal region of unknown function, a middle region with variable numbers of P/Q-rich 37/38 amino acid repeats (1-6) and a conserved 70-amino acid J domain. Several alleles with different repeat numbers can complement the virulence defect of PmaΔhopI1, indicating they all function similarly (6).The J domain of HopI1 provides a clue to HopI1's possible mechanism of action. J proteins bind Hsp70 through the J domain and stimulate Hsp70's ATP hydrolysis activity as well as other activities such as de novo folding of client proteins, intervening when proteins are improperly folded-often during stress conditions...