Silencing the mob: disrupting quorum sensing as a means to fight plant disease

Abstract

Bacteria are able to sense their population's density through a cell-cell communication system, termed 'quorum sensing' (QS). This system regulates gene expression in response to cell density through the constant production and detection of signalling molecules. These molecules commonly act as auto-inducers through the up-regulation of their own synthesis. Many pathogenic bacteria, including those of plants, rely on this communication system for infection of their hosts. The finding that the countering of QS-disrupting mechanisms exists in many prokaryotic and eukaryotic organisms offers a promising novel method to fight disease. During the last decade, several approaches have been proposed to disrupt QS pathways of phytopathogens, and hence to reduce their virulence. Such studies have had varied success in vivo, but most lend promising support to the idea that QS manipulation could be a potentially effective method to reduce bacterial-mediated plant disease. This review discusses the various QS-disrupting mechanisms found in both bacteria and plants, as well as the different approaches applied artificially to interfere with QS pathways and thus protect plant health.

Keywords: acylhomoserine lactone; lactonase; plant-pathogenic bacteria; quorum quenching; quorum sensing.

Figure 1

Overview of the different quorum sensing (QS) manipulation mechanisms occurring in various plant–microbe interactions. The following examples are indicated. (1) Ajoene for natural QS‐inhibiting substances produced by plants. (2) Karakin1 for natural QS‐inducing substances produced by plants. (3) Dimethyl disulfide for bacterial volatiles inhibiting QS. (4, 5) N‐Acylhomoserine lactone (AHL) produced by epiphytic bacteria or transgenic plants, respectively. Pointed arrows show QS‐inducing mechanisms; T‐shaped signs show QS‐inhibiting mechanisms. Grey omnomnomagons indicate AHL‐degrading enzymes; brown elliptic shapes indicate affected bacteria (e.g. phytopathogens); purple elliptic shapes indicate plant‐associated bacteria (e.g. plant growth‐promoting rhizobacteria, PGPR). Figure was drawn by Rachel Dror.

Figure 2

Degradation of N‐acylhomoserine lactone (AHL) by lactonase and acylase enzymes.