A fine control of quorum-sensing communication in Agrobacterium tumefaciens

Abstract

The bacterial pathogen Agrobacterium tumefaciens produces the quorum-sensing (QS) signal 3-oxo-octanoylhomoserine lactone (OC8HSL) for controlling horizontal transfer of its tumor inducing (Ti) plasmid that carries both the T-DNA and the virulence genes. Over-accumulation of OC8HSL also increases severity of plant symptoms (number of emerging tumors at infection site) by an unknown mechanism. A. tumefaciens strain C58 expresses two lactonases, AttM (BlcC) and AiiB, that cleave OC8HSL and are potential modulators of QS. Recent data highlight the direct contribution of lactonases AttM and AiiB in the control of OC8HSL level and QS-regulated functions such as conjugation of Ti plasmid and seriousness of plant symptoms. Expression of the two lactonases is regulated by different plant signals. A working model of QS in the course of the A. tumefaciens-plant host interaction is proposed and discussed.

Keywords: AiiB; AttM; GABA; agrocinopine; lactonase; proline; virulence.

Figure 1

Regulation of AttM-encoding gene by plant signals. The upper part summarizes knowledge on the catabolic and QS-silencing functions of the attKLM operon, its regulation by transcriptional repressor AttJ, as well as the antagonistic activity of Pro for importation of GABA in the bacterial pathogen. The lower part illustrates the synthesis and degradation of GABA in plants. In addition to abbreviations used in the text: αKG, alpha-ketoglutarate; GAD, glutamate decarboxylase; GABA-T, GABA transaminase; GLYR, Glycolate reductase; SSADH, SSA dehydrogenase; Suc, succinate; TCA, tricarboxylic acid cycle. Simple and stopped black arrows represent regulatory pathways in A. tumefaciens; blue arrows, movements across cell compartments; double-line arrows, enzymatic reactions; and cylinder indicates the bacterial ABC-transporter Bra.

Figure 2

A working model for QS regulation in A. tumefaciens C58-plant host interaction. See text for details. Simple and stopped arrows represent regulatory pathways, whereas double-line arrows, cylinders, cross, and pentagons represent enzymatic reactions, bacterial ABC-transporters, VirA/G system, and OC8HSL, respectively.