Structural and Functional Analysis of the Type III Secretion System fromPseudomonas fluorescensQ8r1-96

Abstract

Pseudomonas fluorescens Q8r1-96 represents a group of rhizosphere strains responsible for the suppressiveness of agricultural soils to take-all disease of wheat. It produces the antibiotic 2,4-diacetylphloroglucinol and aggressively colonizes the roots of cereal crops. In this study, we analyzed the genome of Q8r1-96 and identified a type III protein secretion system (T3SS) gene cluster that has overall organization similar to that of the T3SS gene cluster of the plant pathogen Pseudomonas syringae. We also screened a collection of 30 closely related P. fluorescens strains and detected the T3SS genes in all but one of them. The Q8r1-96 genome contained ropAA and ropM type III effector genes, which are orthologs of the P. syringae effector genes hopAA1-1 and hopM1, as well as a novel type III effector gene designated ropB. These type III effector genes encoded proteins that were secreted in culture and injected into plant cells by both P. syringae and Q8r1-96 T3SSs. The Q8r1-96 T3SS was expressed in the rhizosphere, but mutants lacking a functional T3SS were not altered in their rhizosphere competence. The Q8r1-96 type III effectors RopAA, RopB, and RopM were capable of suppressing the hypersensitive response and production of reactive oxygen species, two plant immune responses.

Modulation of Host Immunity by Beneficial Microbes

“…However, it is worth noticing that all of these mutants displayed altered in vitro growth compared with the wild-type strain, suggesting a more general role of the TTSS in the physiology of P. fluorescens SBW25 (Jackson et al 2005). On the other hand, P. fluorescens Q8r1-96 mutants lacking functional TTSS are not altered in their rhizosphere competence (Mavrodi et al 2011). Overall, the significance of type III secretion in PGPR remains far from clear.…”

“…The existence of a classical TTSS is not restricted only to pathogenic bacteria and rhizobia but also occurs in other nonpathogenic, root-associated bacteria. TTSS was first described for the PGPR P. fluorescens SBW25 (Rainey 1999) but other fluorescent pseudomonads are also equipped with a TTSS (Mavrodi et al 2011;Mazurier et al 2004;Preston et al 2001;Rezzonico et al 2004). In P. fluorescens SBW25, genes encoding for TTSS reside in a 20-kb tts cluster (named rsc/rsp) and several ORF in the cluster display a significant degree of similarity to the hrc/hrp genes of pathogenic bacteria (Preston et al 2001).…”

“…et al 2011). Interestingly, all three type III effectors were found to be capable of suppressing typical innate immune responses when ectopically expressed in N. benthamiana (Mavrodi et al 2011). Several mutations in components of the TTSS of P. fluorescens SBW25 have been shown to affect the competitive colonization capacity of the bacterium in the rhizosphere of sugar beet (Jackson et al 2005).…”

Virulence Function of Pectobacterium atrosepticum Secretion System Mutants on Evaluation of Some Solanum tuberosum Resistance Genes

“…HrpN from Erwinia amylavora was the first phytopathogenic protein for which T3SS role was shown (6). It is suggested that harpins perform as help-ers in the translocation of Avr-proteins but their exact role and function is still not clear (7,8). A structural similarity has also been shown between the HrpW C-terminal protein and the pectate lyase enzyme (9).…”

Type III Secretion System and Virulence Markers Highlight Similarities and Differences between Human- and Plant-Associated Pseudomonads Related to Pseudomonas fluorescens and P. putida

“…More generally, the presence of this secretion system involved in cell-tocell interactions with eukaryotic organisms appears to be quite a common feature in nonpathogenic strains of plant-associated P. fluorescens (16,23,25).…”