Abstract
Background Pseudomonas syringae is a widespread bacterial species complex that includes a number of significant plant pathogens. Amongst these, P. syringae pv. actinidiae (Psa) initiated a worldwide pandemic in 2008 on cultivars of Actinidia chinensis var. chinensis. To gain information about the expression of genes involved in pathogenicity we have carried out transcriptome analysis of Psa during the early stages of kiwifruit infection.
Results Gene expression in Psa was investigated during the first five days after infection of kiwifruit plantlets, using RNA-seq. Principal component and heatmap analyses showed distinct phases of gene expression during the time course of infection. The first phase was an immediate transient peak of induction around three hours post inoculation (HPI) that included genes that code for a Type VI Secretion System and nutrient acquisition (particularly phosphate). These genes are probably involved in immediate adaption to the surface of the plant. This was followed by a significant commitment, between 3 and 24 HPI, to the induction of genes encoding the Type III Secretion System (T3SS) and Type III Secreted Effectors (T3SE). Expression of these genes collectively accounted for 6.3% of the bacterial transcriptome at this stage. There was considerable variation in the expression levels of individual T3SEs but all followed the same temporal expression pattern, with the exception of HopAS1, which peaked in expression at 48 HPI. As infection progressed over the time course of five days, there was an increase in the expression of genes with roles in sugar, amino acid and sulfur transport and the production of alginate and colanic acid. These are both polymers that are major constituents of extracellular polysaccharide substances (EPS) and are involved in biofilm production.
Conclusions The results from this study indicate that there is a complex remodeling of the transcriptome during the early stages of infection, with at least three distinct phases of coordinated gene expression. These include genes induced during the immediate contact with the host, those involved in the initiation of infection, and finally those responsible for nutrient acquisition.
Abbreviations
- CFU
- Colony-forming units
- EEL
- Exchangeable effector locus
- GABA
- γ-aminobutyric acid
- HPCP
- Hierarchical clustering on principal components
- HPI
- Hours post inoculation
- EPS
- Extracellular polysaccharide substances
- LB
- Lysogeny broth
- LCA
- Last common ancestor
- PCA
- Principal component analysis
- Psa
- Pseudomonas syringae pv. actinidiae
- Pss
- Pseudomonas syringae pv. syringae
- Pto
- Pseudomonas syringae pv. tomato
- NRP
- Non-ribosomal peptide
- RPKM
- Reads per kilobase per million
- SNP
- Single nucleotide polymorphism
- T2SS
- Type II secretion system
- T2SP
- Type II secreted proteins
- T3SS
- Type III Secretion System
- T3SE
- Type III Secreted Effectors
- T6SS
- Type VI Secretion System
- TraDIS
- Transposon directed insertion-site sequencing