Abstract
Pseudomonas cannabina pv. alisalensis (Pcal) causes bacterial leaf spot and blight of Brassicaceae and Poaceae. We previously identified several potential Pcal virulence factors with transposon mutagenesis. Among these a trpA mutant disrupted the tryptophan synthase alpha chain, and had an effect on disease symptom development and bacterial multiplication. To assess the importance of TrpA in Pcal virulence, we characterized the trpA mutant based on inoculation test and Pcal gene expression profiles. The trpA mutant showed reduced virulence when dip- and syringe-inoculated on cabbage and oat. Moreover, epiphytic bacterial populations of the trpA mutant were also reduced compared to the wild-type (WT). These results suggest that TrpA contributes to bacterial multiplication on the leaf surface and in the apoplast, and disease development. Additionally, several Brassicaceae (including Japanese radish, broccoli, and Chinese cabbage) also exhibited reduced symptom development when inoculated with the trpA mutant. Moreover, trpA disruption led to downregulation of bacterial virulence genes, including type three effectors (T3Es) and the phytotoxin coronatine (COR), and to upregulation of tryptophan biosynthesis genes. These results indicate that a trade-off between virulence factor production and Pcal multiplication with tryptophan might be regulated in the infection processes.
Highlights
The foliar plant bacterial pathogen Pseudomonas syringae causes economically important diseases in a wide range of plants (Agrios, 2005)
Epiphytic bacterial populations of the trpA mutant were reduced compared to the wild-type (WT). These results suggest that TrpA contributes to bacterial multiplication on the leaf surface and in the apoplast, and disease development
TrpA disruption led to downregulation of bacterial virulence genes, including type three effectors (T3Es) and the phytotoxin coronatine (COR), and to upregulation of tryptophan biosynthesis genes. These results indicate that a trade-off between virulence factor production and Pcal multiplication with tryptophan might be regulated in the infection processes
Summary
The foliar plant bacterial pathogen Pseudomonas syringae causes economically important diseases in a wide range of plants (Agrios, 2005). Pst DC3000 is a highly aggressive pathogen once inside host tissue, and uses many type three effectors (T3Es) and the phytotoxin coronatine (COR; Buell et al, 2003; Feil et al, 2005) The function of these two Pst DC3000 virulence factors have been well characterized at the molecular level. Virulence associated genes related to type three secretion system (T3SS) and COR have been investigated well in the P. syringae virulence, functions of other genes including amino acid metabolism remain largely unclear. TrpA mutant expression profiles analysis showed downregulation of virulence related genes, including T3Es and COR, and upregulation of tryptophan biosynthesis related genes compared to WT. These results suggest that trade-off between virulence factor production and tryptophan biosynthesis might be present in bacterial infection processes
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