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

136j 7 Quorum Sensing in Bacteria: Potential in Plant Health Protection
luxS (E.c), respectively; these are highly homologous to one another but not to any
other identified gene, indicating that luxS genes define a new family of autoinducerproducing
genes.
Freeman et al. [86] showed that V. harveyi control light production using two parallel
QS systems. It produces two autoinducers, AI-1 and AI-2, which are recognized by
cognatemembrane-boundtwo-componenthybridsensorkinasescalledluxNandluxQ,
respectively.TheyhavealsoshowedthattheAI-1andLuxNhaveamuchgreatereffecton
the level of LuxO phosphate and therefore on Lux expression than do AI-2 and LuxQ.
LuxO functions as an activator protein via interaction with alternative sigma factor
sigma 54. LuxO together with sigma 54 activates the expression of negative regulator of
luminescence;phenotypesotherthanluxareregulatedbyLuxOandsigma54[87].Inthe
same year, Miyamoto et al. [88] observed that LuxO is involved in control of luminescence
in V. fischeri but luxO was stimulated by N-acyl-HSL autoinducer, indicating that
luxO is part of a second signal transduction system controlling luminescence.
McKnight et al. [89] revealed that a second type of intercellular signal is involved in
lasB induction (elastase). This signal was identified as two heptyl-3-hydroxy-4quinolone,
designated as Pseudomonas quinolone signal (PQS). Its production and
bioactivity depend on the las and rhl QS system, respectively. This signal is not
involved in sensing cell density. Zhang and Pierson [90] reported that a second QS
system, CsaR–CsaI, is involved in regulating biosynthesis of cell surface components
in P. aureofaciens 30–84. Smith et al. [91] described that QS of P. aeruginosa
contribute to its pathogenesis both by regulating expression of virulence factors
(exoenzymes and toxins) and by inducing inflammation. 3-oxo-C 12-HSL activates T
cells to produce the inflammatory cytokine gamma interferon [91]. It also induces
cyclooxygenase 2 (Cox-2) expressions. Sinorhizobium meliloti required exopolysaccharides
(EPSs) for efficient invasion of root nodules on the host plant: alfalfa. S.
meliloti ExpR activates transcription of genes involved in EPSII production in a
density-dependent manner [92].
von Bodman et al. [93] described how phytopathogenic bacteria have incorporated
QSmechanisms intocomplexregulatorycascades thatcontrol genesfor pathogenicity
and colonization of host surface. QS involves the production of extracellular polysaccharide-degradative
enzymes, antibiotics, siderophores and pigments as well as Hrp
protein secretion, Ti plasmid transfer, mobility, biofilm formation and epiphytic fitness.
Wagner et al. [94] investigated global gene expression patterns modulated by QS
regulons. Pseudomonas quinolone signal is also an integral component of the circuitry
and is required for the production of rhl-dependent exoproduct at the onset of stationary
phase [95]. Twenty novel QS-regulated proteins were identified, many of which are
involved in iron utilization, suggesting a link between QS and the iron regulatory
system. PhuP and HasR are components of the two distinct heme uptake systems
present in P. aeruginosa, both proteins are positively regulated by QS cascade.
McGrath et al. [96] reported that PQS production was dependent on the ratio of
3-oxo-C12-HSL and C4-HSL, suggesting a regulatory balance between the QS systems.
Juhas et al. [97] identified the gene PA2591 as a major virulence regulator,
vqsR, in the QS hierarchy. Sircili et al. [98] reported that QS activates that expression
of the lee genes in EPEC (enteropathogenic Escherichia coli) with QseA activating