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

11.5 Quorum Sensing in the Rhizospherej227
named PAI-2 or BHL, autoinducer synthase gene and the rhlR gene encoding a
transcriptional activator protein [110,115]. This system regulates the expression
of the rhlAB operon that encodes a rhamnosyltransferase required for rhamnolipid
biosurfactant production [113]. The presence of these compounds reduces
surface tension and thereby allows P. aeruginosa cells to swarm over semisolid
surfaces [141]. The rhl system is also necessary for optimal production of LasB
elastase, Las A protease, pyocyanin, cyanide and alkaline protease [116,117,138].
Significantly, transcription of rhlI is enhanced in presence of RhlR–BHL, creating
a further autoregulatory loop within LasRI/RhlRI regulons. Latifi et al. [116] have
reported that the rhl system also regulates the expression of rpoS, which encodes a
stationary phase sigma factor (s 3 ) involved in the regulation of various stressresponse
genes. However, QS regulation of s 5 in P. aeruginosa has recently been
questioned [142]. According to this group, the sigma factor negatively regulates
rhlI transcription. Like the las cell-to-cell signaling system, the rhl system, also
referred to as VSm (virulence secondary metabolites), regulates the expression of
various extracellular virulence factors of P. aeruginosa. Studies conducted to
determine how the LasRI and RhlRI systems interact with each other demonstrated
the subordinate nature of the RhlRI system in the hierarchy of regulatory
command that exists between two QS regulons. Two independent studies have
shown that the RhlRI system functionally depends on the LasRI system, as transcriptional
activation of rhlR is dependent on LasR–OdDHL [116,123]. Thus,
activation of the Las system leads to subsequent activation of the Rhl system and
together the two LuxR homologues regulate the transcription of genes within their
respective regulons.
Molecules involved in QS have gained special attention in nitrogen fixation and
associated symbiotic processes. Several gene products required in symbiosis are
encoded by the Sym plasmid, which also carries many important AHL synthase
genes. Among these, rhlABC genes, which are implicated in rhizosphere establishment,
are also controlled by the QS system [143]. Also, QS genes, bisR and
triR, are responsible for the transfer of plasmid in Agrobacterium tumefaciens [144],
an organism endowed with excellent properties to serve as a model in gene
transfer.
The operation of the QS system in the rhizosphere appears to hold great promise
in controlling the damping-off in vegetables caused by zoosporic fungi in nurseries
and elsewhere. Rhamnolipid production controlled by the rhlRI system plays a
crucial role in controlling the spread of zoospore in the rhizosphere, which causes
rapid and severe seedling loss [110,114,145].
Indeed, in terms of AHL-mediated communication, the intricacies of a language
that once seemed alien, appears now to enter a new era, with innovative technologies
presenting even more opportunities to rapidly enhance our understanding of
QS systems. Among these innovations, high-throughput analysis of bacterial
genes and proteins that fall under the regulatory umbrella of proteins such as
LuxRI homologues is of particular interest. Further, it is likely that many more
physiological processes, regulated by bacterial QS systems, will be characterized in
future.