Abstract
Pseudomonas fluorescens 2P24 is a plant growth-promoting rhizobacterium (PGPR) isolated from wheat take-all decline soil. Genomic analysis of strain 2P24 revealed the presence of a complete SPI-1 type III secretion system (T3SS) gene cluster on the chromosome with an organization and orientation similar to the SPI-1 T3SS gene clusters of Salmonella enterica and P. kilonensis F113. Phylogenetic analysis revealed that the SPI-1 T3SS gene cluster of strain 2P24 might be obtained from Salmonella and Shigella by horizontal gene transfer. Two transcriptional regulator homologs of HilA and InvF were found from the SPI-1 T3SS gene cluster of strain 2P24. HilA regulated the expression of the structural genes positively, such as invG, sipB, sipD, prgI, and prgK. Prediction of transcriptional binding sites and RNA-seq analysis revealed 14 genes were up-regulated by InvF in strain 2P24. Exploring potential roles of SPI-1 T3SS revealed that it was not associated with motility. However, 2P24ΔinvF reduced resistance against Fusarium graminearum significantly. 2P24ΔhilA enhanced formation of biofilm significantly at 48 h. All three mutants 2P24ΔhilA, 2P24ΔinvF, and 2P24ΔinvE-C reduced the chemotactic responses to glucose significantly. Finally, the determination of SPI-1 mutants to trigger innate immunity in Nicotiana benthamiana showed that 2P24ΔinvE-C reduced the ability to induce the production of reactive oxygen species compared with the wild type strain 2P24.
Highlights
Type III secretion system (T3SS) is a protein secretion apparatus that is widespread in animal and plant pathogenic bacteria
Through genome mining and comparison, a 23 kb Salmonella Pathogenicity Island 1 (SPI-1) T3SS gene cluster comprised of 25 predicted open reading frames (ORFs) was identified in P. fluorescens 2P24, and the gene names were assigned according to the SPI-1 T3SS of S. enterica
Comparing SPI-1 T3SS gene clusters of plant growth-promoting rhizobacterium (PGPR) and mammalian pathogens indicated that the organization and orientation of SPI1 T3SS in strain 2P24 were very close to those in S. enterica and P. kilonensis F113 (Figure 1)
Summary
Type III secretion system (T3SS) is a protein secretion apparatus that is widespread in animal and plant pathogenic bacteria. Ysc, SPI-1, and SPI-2 T3SS usually exist in animal pathogenic bacteria. Animal and plant pathogens deploy T3SS to deliver type III effectors (T3Es) into host cells for pathogenesis. The distribution of T3SS and the mechanisms of T3Es in animal and plant pathogens have been studied well. Preston et al (2001) found that P. fluorescens SBW25 harbored a Hrp T3SS, and expression of the rscC gene, a hrcC homolog, was induced in the sugar beet rhizosphere. Genomic analysis of P. kilonensis F113 revealed a complete Hrp T3SS gene cluster and four T3E homologs RopM, RopAA1, RopAA2, and RopB (Barret et al, 2013). The effects of SPI-I mutation on antagonism, motility, chemotaxis, biofilm formation, and plant immunity were determined
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