Type III secretion in plant growth-promoting Pseudomonas fluorescens SBW25

Preston, GM Bertrand, N Rainey, PB

Gm. Preston et al., Type III secretion in plant growth-promoting Pseudomonas fluorescens SBW25, MOL MICROB, 41(5), 2001, pp. 999-1014

71

INGLESE

Article

Microbiology

MOLECULAR MICROBIOLOGY

0950-382X → ACNP

41

5

2001

999 - 1014

ISI

0950-382X(200109)41:5<999:TISIPG>2.0.ZU;2-E

In vivo expression technology (IVET) analysis of rhizosphere-induced genes in the plant growth-promoting rhizobacterium (PGPR) Pseudomonas fluorescens SBW25 identified a homologue of the type III secretion system (TTSS) gene hrcC. The hrcC homologue resides within a 20-kb gene cluster that resembles the type III (Hrp) gene cluster of Pseudomonas syringae. The type III (Rsp ) gene cluster in P. fluorescens SBW25 is flanked by a homologue of the P. syringae TTSS-secreted protein AvrE. P. fluorescens SBW25 is nonpathogenic and does not I elicit the hypersensitive response (HR) in any host plant te sted. However, strains constitutively expressing the rsp-specific sigma fac tor RspL elicit an AvrB-dependent HR in Arabidopsis thaliana ecotype Col-0, and a host-specific HR in Nicotiana clevelandii. The inability of wild-typ e P. fluorescens SBW25 to elicit a visible HR is therefore partly attributa ble to low expression of rsp genes in the leaf apoplast. DNA hybridization analysis indicates that rsp genes are present in many plant-colonizing Pseu domonas and PGPR, suggesting that TTSSs may have a significant role in the biology of PGPR. However, rsp and rsc mutants retain the ability to reach h igh population levels in the rhizosphere. While functionality of the TTSS h as been demonstrated, the ecological significance of the rhizosphere-expres sed TTSS of P. fluorescens SBW25 remains unclear.