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Title: Mutations in y-aminobutyric acid (GABA) transaminase genes in plants or Pseudomonas syringae reduce bacterial virulence

Author
item PARK, DUCK HWAN - Cornell University
item MIRABELLA, ROSSANA - University Of Amsterdam
item Bronstein, Philip
item PRESTON, GAIL - Oxford University
item LIM, CHUN KEUN - Kangwon National University
item COLLMER, ALAN - Cornell University
item SCHUURINK, ROBERT - University Of Amsterdam

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2010
Publication Date: 10/20/2010
Citation: Park, D., Mirabella, R., Bronstein, P., Preston, G.M., Lim, C., Collmer, A., Schuurink, R.C. 2010. Mutations in gamma-aminobutyric acid (GABA) transaminase genes in plants or Pseudomonas syringae reduce bacterial virulence. Plant Journal. 64(2):318-330.

Interpretive Summary: Pseudomonas syringae pv. tomato DC3000 is a bacterial pathogen of tomatoes and the model plant Arabidopsis. Within plants the amino acid '-amino butyric acid (GABA) is the most abundant amino acid at the site of bacterial infections. The DC3000 has three genes, gabT1-3, that can be used to integrate GABA into its metabolism. A DC3000 mutant lacking all three gabT genes was constructed and found to be unable to utilize GABA as a sole carbon and nitrogen source. In media supplemented with GABA, the mutant grew less well than wild-type DC3000 and was unable to produce important bacterial virulence factors. The growth of this gabT mutant was weakly reduced in Arabidopsis and strongly reduced in Arabidopsis plants that accumulate higher levels of GABA. Further plant assays showed that the ability of DC3000 gabT mutant to cause disease associated symptoms was reduced by adding additional GABA to the infections. Therefore GABA may have multiple effects on P. syringae-plant interactions, with elevated levels of the compound in plants leading to increased disease resistance.

Technical Abstract: Pseudomonas syringae pv. tomato DC3000 is a bacterial pathogen of Arabidopsis and tomato that grows in the apoplast. The non-protein amino acid '-amino butyric acid (GABA) is produced by Arabidopsis and tomato and is the most abundant amino acid in the apoplastic fluid of tomato. The DC3000 genome harbors three genes annotated as gabT GABA transaminases. A DC3000 mutant lacking all three gabT genes was constructed and found to be unable to utilize GABA as a sole carbon and nitrogen source. In complete minimal media supplemented with GABA, the mutant grew less well than wild-type DC3000 and showed strongly reduced expression of hrpL and avrPto, which encode an alternative sigma factor and effector, respectively, associated with the type III secretion system. The growth of the gabT triple mutant was weakly reduced in Arabidopsis Ler and strongly reduced in the Ler pop2-1 GABA transaminase-deficient mutant that accumulates higher levels of GABA. Much of the ability to grow on GABA-amended minimal media or in Arabidopsis pop2-1 leaves could be restored to the gabT triple mutant by expression in trans of just gabT2. The ability of DC3000 to elicit the hypersensitive response in tobacco leaves is dependent upon deployment of the type III secretion system, and the gabT triple mutant was less able than wild-type DC3000 to elicit the hypersensitive response in tobacco when bacteria were infiltrated along with GABA at levels of 1 mM or more. GABA may have multiple effects on P. syringae-plant interactions, with elevated levels increasing disease resistance.