A unique signal degradation system has recently been discovered in Agrobacterium tumefaciens. Upon entering stationary phase, A. tumefaciens terminates quorum sensing-dependent Ti-plasmid conjugation by degradation of acyl homoserine lactone (AHL) quormone via the enzyme AttM (AHL-lactonase). attM, together with attK and attL, constitute one transcriptional unit subjected to the control of a common promoter. AttJ, the other member of the signal degradation system, is an IclR-like negative transcriptional factor, which tightly represses the expression of AttM at the early stage of bacterial growth. In this study, we found that this quormone degradation system is activated by either carbon or nitrogen starvation. Quormone degradation was significantly delayed when bacterial culture was supplemented with extra carbon or nitrogen source in the nutrient-limited minimal medium before the onset of stationary phase. To identify the signalling pathway and regulatory mechanisms that mediate quormone degradation, we constructed a reporter strain A6(attKLM::lacZ) in which the promoterless lacZ was transcriptionally fused to the attKLM promoter. Transposon mutagenesis of strain A6(attKLM::lacZ) led to identification of the relA gene, which encodes the stress alarmone (p)ppGpp synthetase. Tn5 knock-out of relA abolished the stationary phase-dependent expression of attM. We concluded that the A. tumefaciens quormone degradation system is coupled to and regulated by the generic (p)ppGpp stress response machinery.