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

7.3 QS and Bacterial Traits Underregulationj135
The second extracellular factor, Factor 2, present was N-butyryl-homoserine lactone
(BHL), indicating that multiple QS systems can occur and interact with each
other in a single bacterial species. One signal–response system is encoded by the
luxL, luxM and luxN loci [71].
Swift et al. [25] investigated density-dependent multicellular behavior in prokaryotes
such as in bioluminescence, sporulation, swarming, antibiotic biosynthesis,
plasmid conjugal transfer and production of virulence determinants in animals, fish
and plant pathogens. In the same year, Givskov et al. [72] reported swarming motility
of Serratia liquefaciens to be QS controlled. Investigations by Wood and Pierson [73]
led to the conclusion that the production of phenazine (Ph) antibiotics in Pseudomonas
aureofaciens (Pau) 30–84 is regulated by the product of phzI, which is a
member of the LuxI family N-acyl-homoserine lactone (N-acyl-HSL synthases).
QS-regulated phenotypes including the swarming motility of S. liquefaciens, toxin
production of V. harveyi and the bioluminescence of V. fischeri have been documented
[72,74–76].
Pearson et al. [77] described that two QS systems (las and rhl) regulate virulence
gene expression in P. aeruginosa. Rhamnolipid production has been reported to
require both rhl system and rhlAB (encoding a rhamnosyl transferase). The las
system directs the synthesis of the autoinducer N-(3-oxdodecanoyl)-homoserine
lactone (PAl-1), which induces lasB responsible for the production of elastase. Wood
et al. [78] reported that P. aureofaciens 30–84, which colonizes the wheat rhizosphere,
produces three phenazine antibiotics to enhance its survival in competition with
other organisms. Here, N-hexanoyl-homoserine lactone (HHL) serves as an interpopulation
signal molecule in the wheat rhizosphere; HHL is also required for
phenazine expression in situ. Milton et al. [79] investigated that V. anguillarum may
produce multiple AHL signal molecules to control virulence gene expression. Major
AHL identified as N-3-oxodecanoyl-L-homoserine lactone (ODHL) synthesized by
the gene vanI belonging to the luxI family of putative AHL synthases, vanR related to
the luxR family of transcriptional activators. In the same year, Swift et al. [59]
reported that in Aeromonas cell division may be linked to QS and that the major
signal molecule N-butanoyl-L-homoserine lactone (BHL) is synthesized via both
AhyI and AsaI. AhyR and BHL are both required for ahyI transcription. In addition,
a minor AHL, N-hexanoyl-homoserine lactone, was identified.
Quorum sensing plays a role in orchestrating the expression of exoprotease,
siderophores, exotoxins and several secondary metabolites and participates in the
development of biofilms [80–83]. The cviI gene of the soil bacterium Chromobacterium
violaceum encodes the enzyme for N-hexanoyl-L-homoserine lactone. C6-HSL
induces production of the purple pigment violacein as well as antifungal chitinase
[26]. Chernin et al. [84] showed that C. violaceum produces a set of chitinolytic
enzymes, whose production is regulated by HHL. In C. violaceum ATCC 31532 a
number of phenotypic characteristics, including production of the purple pigment
violacein, hydrogen cyanide and exoprotease are also known to be regulated by the
endogenous AHL–HHL.
Surett et al. [85] reported the identification and analysis of the gene responsible for
AI-2 production in V. harveyi, S. typhimurium and E. coli as luxS (V.h), luxS (S.t) and