Quorum Quenching in Agrobacterium tumefaciens : Chance or Necessity?

Abstract

Cell-cell communication or “quorum sensing” (QS) between members of a population is an established phenomenon that has been described for many different bacterial species. A number of different types of QS systems have been discovered; however, a unifying theme is the synthesis of a small signal molecule, often called an autoinducer or pheromone, which activates a specific response when it accumulates to a threshold concentration within a population. A relatively new and exciting aspect of the field of QS that has received much recent attention is “quorum quenching” (QQ), or interference of a QS signaling system. This occurs through either the inhibition of a QS component or the depletion of the signal itself, resulting in an attenuation of the response. In the plant pathogen Agrobacterium tumefaciens, an enzyme (BlcC) that destroys the bacterium's QS signal has been recently described, prompting much speculation that this enzyme is specifically involved in the quenching of the QS system. A variety of explanations for the adaptive significance of QQ in the QS system of A. tumefaciens and implications for the bacterium's role as a plant pathogen have been suggested in the literature (for example, see references 4 and 27). However, the role of BlcC in QQ was never directly addressed. In A. tumefaciens, the QS system regulates Ti (tumor-inducing) plasmid conjugation. In this issue, Khan and Farrand (12) directly address the biological significance of BlcC by examining its effect on Ti plasmid conjugation both in culture and in planta. Their study has implications for our understanding of the possible roles in Agrobacterium and other bacteria of BlcC-like enzymes, which are generally thought to function as quorum quenchers of proteobacteria.