PLANT-PATHOGEN INTERACTIONS The GABA Defense

During infection of plants by Agrobacterium tumefaciens, plants are wounded and then a tumor is induced, which is a source of opines, chemicals that stimulate the production of the quorum-sensing (QS) signal N-(3-oxooctanoyl)homoserine lactone (OC8-HSL). -aminobutyric acid (GABA) is produced by plants as part of the response to wounding. Chevrot et al. show that GABA stimulates expression of the attKLM operon in A. tumefaciens, which produces a lactonase that inactivates OC8-HSL. OC8-HSL was undetectable in cultures of A. tumefaciens exposed to GABA. Proteins encoded by the attKLM operon were identified in a proteomic screen for proteins synthesized in response to the addition of GABA to cultures of A. tumefaciens. The induction of the attKLM operon was also monitored using a reporter assay, and in A. tumefaciens deficient for the GABA transporter system, GABA did not induce the reporter. The importance of GABA for the plant response was verified using a transgenic tobacco that expressed a glutamic acid decarboxylase (GAD, the enzyme responsible for GABA synthesis) that was not inhibited by Ca²⁺/calmodulin (GADC). Compared with wild-type tobacco, the plants with the GADC mutant developed less severe disease symptoms in two different virulence assays. Thus, GABA appears to serve as a communication signal between the plant and the pathogen.

R. Chevrot, R. Rosen, E. Haudecoeur, A. Cirou, B. J. Shelp, E. Ron, D. Faure, GABA controls the level of quorum-sensing signal in Agrobacterium tumefaciens. Proc. Natl. Acad. Sci. U.S.A. 103, 7460-7464 (2006). [Abstract] [Full Text]