The Pseudomonas syringae Hrp pathogenicity island has a tripartite mosaic structure composed of a cluster of type III secretion genes bounded by exchangeable effector and conserved effector loci that contribute to parasitic fitness and pathogenicity in plants

Abstract

The plant pathogenic bacterium Pseudomonas syringae is divided into pathovars differing in host specificity, with P. syringae pv. syringae (Psy) and P. syringae pv. tomato (Pto) representing particularly divergent pathovars. P. syringae hrp͞hrc genes encode a type III protein secretion system that appears to translocate Avr and Hop effector proteins into plant cells. DNA sequence analysis of the hrp͞hrc regions in Psy 61, Psy B728a, and Pto DC3000 has revealed a Hrp pathogenicity island (Pai) with a tripartite mosaic structure. The hrp͞hrc gene cluster is conserved in all three strains and is flanked by a unique exchangeable effector locus (EEL) and a conserved effector locus (CEL). The EELs begin 3 nt downstream of the stop codon of hrpK and end, after 2.5-7.3 kb of dissimilar intervening DNA with tRNA Leu -queA-tgt sequences that are also found in Pseudomonas aeruginosa but without linkage to any Hrp Pai sequences. The EELs encode diverse putative effectors, including HopPsyA (HrmA) in Psy 61 and proteins similar to AvrPphE and the AvrB͞AvrC͞AvrPphC and AvrBsT͞AvrRxv͞YopJ protein families in Psy B728a. The EELs also contain mobile genetic element sequences and have a G ؉ C content significantly lower than the rest of the Hrp Pai or the P. syringae genome. The CEL carries at least seven ORFs that are conserved between Psy B728a and Pto DC3000. Deletion of the Pto DC3000 EEL slightly reduces bacterial growth in tomato, whereas deletion of a large portion of the CEL strongly reduces growth and abolishes pathogenicity in tomato.

Genomewide identification of Pseudomonas syringae pv. tomato DC3000 promoters controlled by the HrpL alternative sigma factor

“…Kim and G.B.M., unpublished data). Notably missing were members of the AvrBs2 and AvrBs3 families, which are widespread in Xanthomonas spp., or any members of the AvrRxv͞YopJ family, which are found in genera as diverse as Salmonella, Yersinia, Xanthomonas, Erwinia, and Rhizobium, and have also been reported in another strain of P. syringae (i.e., P. s. syringae B728a) (1,9). However, it is important to note that further searches after closure and annotation of the DC3000 genome may yield additional homologs of known effectors.…”

“…Many effectors were initially discovered through their ability to betray the pathogen to the host R (resistance) gene surveillance system, thereby rendering the pathogen avirulent on a test plant (7). Over 25 effector genes have been identified by Avr or Hop phenotypes in various P. syringae pathovars and races (8,9). The encoded effectors seem to determine both basic pathogenicity and host range, but the number of such proteins produced by any single strain has not been systematically investigated.…”

A Conserved Carboxylesterase Is a SUPPRESSOR OF AVRBST-ELICITED RESISTANCE inArabidopsis

“…YopJ-like effectors also exist in several other bacterial plant pathogens, including Erwinia amylovora (Oh et al, 2005), Pseudomonas syringae (Alfano et al, 2000;Arnold et al, 2001), and Ralstonia solanacearum (Deslandes et al, 2003;Lavie et al, 2004), as well as one plant symbiont (Ciesiolka et al, 1999). The maintenance of this class of effectors in diverse pathogens suggests that each may target a similar substrate or use a conserved catalytic mechanism to alter eukaryotic signal transduction.…”

Negative Regulation of Pathogenesis in Pseudomonas syringae pv. tabaci 11528 by ATP-Dependent Lon Protease

“…tabaci causes wildfire diseases in host tobacco plants but induces a hypersensitive response (HR) in other non-host plants (Hirano and Upper, 2000). The ability of P. syringae to cause disease in its hosts and elicit an HR in non-host plants is controlled by the hypersensitive response and pathogenicity (hrp)/ hypersensitive response and conserved (hrc) genes residing in a pathogenicity island also known as Hrp PAI (Alfano et al, 2000). Hrp PAI is conserved among many gram-negative plant pathogenic bacteria, including P. syringae, Ralstonia solanacearum, Xanthomonas campestris, and Erwinia amylovora.…”