Characterization of the hrpC and hrpRS Operons of Pseudomonas syringae Pathovars Syringae, Tomato, and Glycinea and Analysis of the Ability of hrpF , hrpG , hrcC , hrpT , and hrpV Mutants To Elicit the Hypersensitive Response and Disease in Plants

Abstract

The species Pseudomonas syringae encompasses plant pathogens with differing host specificities and corresponding pathovar designations. P. syringae requires the Hrp (type III protein secretion) system, encoded by a 25-kb cluster ofhrp and hrc genes, in order to elicit the hypersensitive response (HR) in nonhosts or to be pathogenic in hosts. DNA sequence analysis of the hrpC and hrpRSoperons of P. syringae pv. syringae 61 (brown spot of beans), P. syringae pv. glycinea U1 (bacterial blight of soybeans), and P. syringae pv. tomato DC3000 (bacterial speck of tomatos) revealed that the 13 genes comprising the right half of the hrp cluster (including those in the previously sequenced hrpZ operon) are conserved and identically arranged. The hrpC operon is comprised of hrpF,hrpG, hrcC, hrpT, and hrpV. hrcC encodes a putative outer membrane protein that is conserved in all type III secretion systems. The other four genes appear to be characteristic of group I Hrp systems, such as those possessed byP. syringae and Erwinia amylovora. The predicted products of these four genes in P. syringae pv. syringae 61 are HrpF (8 kDa), HrpG (15.4 kDa), HrpT (7.5 kDa), and HrpV (13.4 kDa). HrpT is a putative outer membrane lipoprotein. HrpF, HrpG, and HrpV are all hydrophilic proteins lacking N-terminal signal peptides. The HrpG, HrcC, HrpT, and HrpV proteins of P. syringae pathovars syringae and tomato (the two most divergent pathovars) had at least 76% amino acid identity with each other, whereas the HrpF proteins of these two pathovars had only 36% amino acid identity. The HrpF proteins of P. syringae pathovars syringae and glycinea also showed significant similarity to the HrpA pilin protein of P. syringae pathovar tomato. Functionally nonpolar mutations were introduced into each of the genes in thehrpC operon of P. syringae pv. syringae 61 by insertion of an nptII cartridge lacking a transcription terminator. The mutants were assayed for their ability to elicit the HR in nonhost tobacco leaves or to multiply and cause disease in host bean leaves. Mutations in hrpF, hrcC, andhrpT abolished or greatly reduced the ability of P. syringae pv. syringae 61 to elicit the HR in tobacco. ThehrpG mutant had only weakly reduced HR activity, and the activity of the hrpV mutant was indistinguishable from that of the wild type. Each of the mutations could be complemented, but surprisingly, the hrpV subclone caused a reduction in the HR elicitation ability of the ΔhrpV::nptIImutant. The hrpF and hrcC mutants caused no disease in beans, whereas the hrpG, hrpT, and hrpV mutants had reduced virulence. Similarly, thehrcC mutant grew little in beans, whereas the other mutants grew to intermediate levels in comparison with the wild type. These results indicate that HrpC and HrpF have essential functions in the Hrp system, that HrpG and HrpT contribute quantitatively but are not essential, and that HrpV is a candidate negative regulator of the Hrp system.

Erwinia amylovora pyrC mutant causes fire blight despite pyrimidine auxotrophy

“…'Glurk' plants, the wild-type and strains pyrC244::Tn5, pyrC244::Tn5(pCR2.1-pyrC) and pyrC244:: Tn5(pCR2.1) all caused tissue collapse typical of the hypersensitive response (HR, Fig. 4b), while the type III protein secretion system mutant hrcC118::Tn5 did not trigger an HR, as expected (Deng et al 1998). This indicated that the pyrC244::Tn5 mutant triggered typical nonhost defence responses in tobacco (Morel and Dangl 1997).…”

First report on the whole genome sequence of Pseudomonas cichorii strain JBC1 and comparison with other Pseudomonas species

“…Among the virulence related genes, hrp and hrc genes play an important role in pathogenicity. The hrp genes such as hrpT, hrpF and hrcC are essential for P. syringae 61 to induce the hypersensitive response in tobacco (Deng et al, 1998). Hojo et al (2008) also confirmed that hrp genes are involved in determining the pathogen's host range after analysing a mutant library of P. cichorii SPC9018.…”

“…Pseudomonas species have been classified into various pathovar groups based on their host specificity (Deng et al, 1998). Sets of hrp and hrc genes are essential and contribute to host specificity and disease development in host plants.…”

Bacterial non‐host resistance: interactions of Arabidopsis with non‐adapted Pseudomonas syringae strains

“…We have shown that the triggered SA and PR gene accumulation at inoculation sites is largely dependent on a TTSS. This finding, together with the fact that the type II non-host HR also requires type III secretion (Deng et al 1998), emphasizes that non-adapted bacteria build up a functional TTSS also in non-host plants and deliver effector proteins into the plant cell. The TTSS effectors subsequently might interact with cellular targets to initiate SA-associated defence responses.…”