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

transcription of ler, hence QS is involved in modulating the regulation of the EPEC
virulence gene. QS regulates type III secretion (TTS) in V. parahemolyticus. At higher
cell density (in the presence of autoinducers), QS represses TTS in V. harveyi and
V. parahaemolyticus [99]. One of the possible QS-regulated phenotype is swarming: a
flagella-driven movement of differentiated swarmer cells (hyperflagellated, elongated,
multinucleated) by which bacteria can spread as a biofilm over a surface. The
glycolipid or lipopeptide biosurfactants thereby produced function as wetting agents
by reducing the surface tension. Surface migration is regulated by AHL and also
other low-molecular-mass signal molecules (such as furanosyl borate diester AI-2) in
biosurfactant production of different bacteria [100]. The first evidence for autoinduction
in E. amylovora and a role of an AHL-type signal were reported. Further, two
major plant virulence traits, production of extracellular polysaccharide (amylovora
and levan) and tolerance to free oxygen radicals were controlled in a bacterial cell
density dependent manner [101]. In V. harveyi Hfq together with sRNAs create an
ultrasensitive regulatory switch that controls the critical transition into higher cell
density, QS mode [102]. In V. harveyi, three-way coincidence detectors in the regulation
of a variety of genes including those responsible for bioluminescence, type III
secretion and metalloprotease (VVP) production [99]. Metalloprotease (VVP) production
by V. vulnificus is known to be regulated by quorum sensing, supported by
the fact that expression of vvp gene was closely related with expression of the luxS
gene [104].
Chatterjeeetal.[105]reportedthatE.carotovorasubspeciespossesstwoclassesofQS
signaling system, since AHL control the expression of various traits including extracellular
enzyme/protein production and pathogenicity. The AHL response correlates
with expR-mediated inhibition of exoprotein and secondary metabolite production.
PQSproductionisnegativelyregulatedbytherhlQSsystemandpositivelyregulatedby
the las QS system. For more detailed information on QS in specific bacteria, see the
excellent review articles on Vibrios (V. harveyi, V. cholerae, V. fisheri, V. anghillarium,
V. vulnificus) by Milton [106]. It is concluded that AHL-dependent QS in Vibrio fischeri
(LuxI/R system) is not found in all Vibrios. A more complex system is found in
V. harveyi. Three parallel systems transmit signals via phosphorelase that converge
on to one regulatory protein LuxO. Components of the three systems are found in
Vibrios. However, the number and types of signal circuits found each strain indicates
the diversity and complexity of the Vibrio QS systems [106].
7.4
QS in Certain Phytopathogenic Bacteria
7.4.1
E. carotovora
7.4 QS in Certain Phytopathogenic Bacteriaj137
Erwinia are the only members of the family Enterobacteriaceae that are pathogenic
to plants. Three major species among these are E. amylovora, E. carotovora and
E. herbicola. They have gained economic significance due to the diseases they cause