Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

extracellular enzymes and EPS, which are the virulence factors of this bacterium. In
Xcc 8004, production of extracellular enzymes and EPS has been shown to be subject
to regulation by the rpf (regulation of pathogenicity factor) cluster comprising some
nine genes (rpfA-I). It has indicated that Xcc appears to have a unique system for
cell-to-cell signaling; it shares similarities with the system employed by V. harveyi
and R. solanacearum. Most evident is that all three species appears to contain
specialized two-component regulators to integrate and/or sense their respective QS
signals [49,118–121].
7.4.4
Other Bacteria
7.5 Quorum-Sensing Signal Molecules in Gram-Negative Bacteriaj139
More than 30 species of the genus Burkholderia are described, many of which are
human pathogens [68]. It has been indicated that all Burkholderia sp. investigated so
far employ QS systems that relay on AHL signal molecules to express certain
phenotypic traits in a population density manner.
The role of the quorum-sensing system in the expression of a variety of traits was
found in Pseudomonas corrugate that produce short-chain AHL quorum-sensing
signal molecules. The main AHL produced was N-hexanoyl-L-homoserine lactone
(C 6-AHL). These bacteria also possess an AHL quorum-sensing system designated
PcoI/PcoR [122].
Pomianek and Semmelhack [123] have discovered effective modulators of the
autoinducer-1 circuit for bacterial quorum sensing by the synthesis and evaluation of
a small library of aryl-substituted acyl-homoserine lactone analogues. This series
highlights the sensitivity to structure of the contrasting responses of agonism
and antagonism of the natural signal and identifies an analogue that provokes the
same response as the natural signal but at 10-fold lower concentration, a
superagonist .
AHL signal molecules are utilized by Gram-negative bacteria to regulate gene
expression in a density-dependent manner, as for example, S. liquefaciens MG1 and
P. putida IsoF colonize tomato roots, produce AHL in the rhizosphere and increase
systemic resistance of tomato plants against the fungal leaf pathogen, Alternaria
alternata. The AHL-negative mutant S. liquefaciens MG44 was less effective in reducing
symptoms and A. alternata growth. Salicylic acid (SA) levels were increased
in leaves when AHL-producing bacteria colonized the rhizosphere [131].
7.5
Quorum-Sensing Signal Molecules in Gram-Negative Bacteria
Three types of autoinducers are reported in literature (i) AHL, acyl-HSL or HSL)
found in Gram-negative bacteria, autoinducer peptides (AIP) in Gram-positive
bacteria and autoinducer-2 compounds (AI-2s), which are found in Gram-negative
and Gram-positive bacteria. Some of the common signal molecules are listed in
Table 7.2.