Altering plant-microbe interaction through artificially manipulating bacterial quorum sensing

Abstract

Many bacteria regulate diverse physiological processes in concert with their population size. Bacterial cell-to-cell communication utilizes small diffusible signal molecules, which the bacteria both produce and perceive. The bacteria couple gene expression to cell density by eliciting a response only when the signalling molecules reach a critical threshold (a point at which the population is said to be 'quorate'). The population as a whole is thus able to modify its behaviour as a single unit. Amongst Gram-negative bacteria, the quorum sensing signals most commonly used are N-acylhomoserine lactones (AHLs). It is now apparent that AHLs are used for regulating diverse behaviours in epiphytic, rhizosphere-inhabiting and plant pathogenic bacteria and that plants may produce their own metabolites that interfere with this signalling. Transgenic plants that produce high levels of AHLs or which can degrade bacterial-produced AHLs have been made. These plants have dramatically altered susceptibilities to infection by pathogenic Erwinia species. In addition, such plants will prove useful tools in determining the roles of AHL-regulated density-dependent behaviour in growth promoting, biological control and pathogenic plant-associated bacterial species.

Fig. 1. Thin layer chromatogram showing the range of AHLs produced by various Erwinia carotovora species. After chromatography, AHLs were located by overlaying the chromatogram with agar seeded with the Chromobacterium violaceum strain mutant for cviI (the gene required for AHL synthesis). The presence of AHLs is indicated by complementation of the mutation and restoration of the production of the purple pigment, violacein. A, 3‐oxo‐C6‐HSL standard; B, C6‐HSL standard; C–F, spent bacterial culture supernatants of Erwinia carotovora subsp. carotovora SCRI 193 (C); Erwinia carotovora subsp. atroseptica SCRI 1043 (D); Erwinia carotovora subsp. atroseptica SCRI 1039 (E); and Erwinia chrysanthemi SCRI 1043 (F). Most strains produce both 3‐oxo‐C6‐HSL and C6‐HSL as well as additional AHLs in some cases. The sensitivity of the C. violaceum reporter strain varies according to the AHL being detected, thus the intensity of pigment is not a direct indicator of the relative abundance of each AHL.

Fig. 2. AHL‐producing transgenic tobacco plants restore pathogenicity to an avirulent AHL‐deficient Erwinia carotovora subsp. carotovora mutant (PNP22). The photograph shows the leaves 4 d after infection. 1, Wild‐type tobacco inoculated with wild‐type Erwinia carotovora; 2, Wild‐type tobacco inoculated with AHL‐negative Erwinia carotovora mutant PNP22; 3, AHL‐producing tobacco line inoculated with Erwinia carotovora PNP22.

Fig. 3. Inhibition of quorum sensing responses. A, In a reverse assay, 3‐oxo‐C6‐HSL was added to a top agar containing the Chromobacterium violaceum CviI^– biosensor strain. Inhibition of quorum sensing by the long chain 3‐oxo‐C12‐HSL, or by dilutions of a crude grape extract (60–7·5 µl made up to 60 µl with H₂0), results in inhibition of violacein production but not bacterial growth. B and C, Other plant extracts also inhibit AHL responses. Antibiotic production by wild‐type Erwinia carotovora is revealed by inhibition of growth in a lawn of an Escherchia coli strain sensitive to carbabenem. In B, the well contains water whilst in C it contains a crude strawberry extract. Inhibition of antibiotic production (an AHL response) is revealed by growth of the E. coli lawn adjacent to the well containing the fruit extract.

Table 1. Examples of a number of AHLs and some of the density‐dependent behaviours that they are known to regulate

Table 2. Quorum sensing‐related signal molecules A, The basic N‐acyl homoserine lactone structure (see also Table 1). B, Halogenated furanones produced by a marine alga block bacterial AHL responses. C and D, Some cyclic dipeptides produced by both bacteria and fungi can activate or repress the bacterial AHL receptor. E, The volatile quorum‐sensing molecule produced during infections by Ralastonia solanacearum. F, Proposed structure for one of the quorum‐sensing molecules used by the plant pathogen Xanthomonas campestris. G and H, Examples of butyrolactone signal molecules produced by some soil inhabiting Streptomyces species.