Nutrient assimilation and amino acid utilisation in Pseudomonas syringae

The aim of this study was to increase understanding of amino acid utilisation and amino acid permease (aa_permease) genes in the plant pathogen Pseudomonas syringae, which inhabits the nutrient-limited environment of the plant surface and intracellular apoplast. aa_permease genes are significantly reduced in P. syringae genomes compared to non plant-pathogenic Pseudomonas. Accordingly, this work demonstrates that P. syringae can utilise a restricted number of amino acids for growth compared to non plant-pathogenic Pseudomonas. The remaining aa_permease genes in Pseudomonas syringae pv. tomato are annotated as transporting GABA, ethanolamine, proline and aromatic amino acids. Sequence analyses, phylogenetic analyses, chemotaxis assays and gene expression analyses supported the functional annotations of the GABA, ethanolamine and aromatic aa_permeases, but showed that expression of the proline permease was specifically induced by histidine, which suggests that this encodes a histidine transporter. Expression of the GABA and histidine permeases was also induced in the presence of tomato apoplast, indicating that these amino acids may be assimilated during apoplast colonisation. P. syringae pv. tomato exhibited chemotaxis towards tomato apoplast and several constituent amino acids, which may be important for invasion of the apoplast from the plant surface. Uptake of several amino acids including GABA and histidine appeared to be negatively affected in alkaline media. The aa_permeases use electrochemical potential energy to transport substrates and this mechanism may be affected by pH. The plant apoplast becomes increasingly alkaline during infection and this may select against the retention of particular transporters in P. syringae. As GABA is an abundant amino acid in the apoplast and can be used by P. syringae strains for growth, the growth of P. syringae pv. tabaci was monitored in transgenic tobacco plants with increased GABA. Although growth was similar in all plants, significantly fewer symptoms and more callose deposition was observed upon infection