Allelochemicals Biologica... - Name

BACTERIAL ROOT ZONE COMMUNITIES, BENEFICIAL ALLELOPATHIES AND PLANT DISEASE CONTROL
2.4. Microbe-microbe Disruption of Quorum Sensing Mechanisms
It should now be apparent that quorum sensing plays a significant role in the biology
and regulation of both plant-microbe and microbe-microbe interactions. And while
pathogenic bacteria depend significantly on quorum sensing regulation to coordinate
the saprophytic and parasitic phases of their life cycles, plants and their adherent root
zone microbial communities have evolved mechanisms by which to disrupt this strategy.
For example, Variovorax paradoxus, a relatively common soil organism, is able
to utilize (degrade) acyl-HSL signaling molecules as an energy source (Leadbetter
and Greenberg, 2000) with the effect that those classes of bacteria relying on acyl-
HSLs for cell-to-cell signaling molecules will be kept ‘unaware’ of their own presence
and population density. Pierson et al. (1998) demonstrated that approximately 8% of
rhizobacteria recovered at random from the surfaces of wheat roots could specifically
stimulate quorum sensing gene regulated expression in adjacent P. aureofaciens
bacteria. Thus different bacteria appear able to exchange quorum sensing signals,
with the possibility of forming functional mixed communities (Bauer and Robinson,
2002).
Several instances have been reported of soil bacteria in possession of enzymes
designed to degrade or inactivate acyl-HSLs (Bauer and Robinson, 2002; Dong et al.,
2001, 2002; Leadbetter, 2001; Whitehead et al., 2001). A case in point is the lactonase
enzyme, AiiA, from Bacillus cereus, which, it is believed, opens the lactone ring in
acyl-HSLs, thereby reducing signal strength in the order of a 1000 fold (Dong et al.,
2000). In circumstances where B. cereus and E. carotovora co-exist as commensals
in field soils, AiiA is able to inactivate the acyl-HSL autoinducer in E. carotovora
rendering the pathogen avirulent.
Clearly, understanding the mechanism of signal synthesis, and being able to
identify signal synthesis inhibitors, has implications for developing quorum sensingtargeted
antivirulence molecules, or engineering beneficial communities which utilize
acyl-HSL signals as an energy source and so inhibit pathogenic trait expression. In
the former case, AHL signal-inactivating molecules (the so-called ‘quorum quashing’
moieties) - namely AHL-lactonases and AHL-acylases - have already been identified
in a Ralstonia sp. isolated from a mixed-species biofilm (Lin et al., 2003).
However, while selection pressures upon component bacterial populations in a
community will include biological pressures created by microbially mediated
moderation of habitats, external abiotic pressures created by environmental
perturbations - such as climate or crop management practices - also exist. As a result,
even though “... microorganisms are potentially everywhere, [the] environment
selects...” (Alexander, 1997).
3. MICROBIAL ANTAGONISM AND DISEASE CONTROL
3.1. Microbially Induced Biological Control in Soils
Since every living soil sample will yield organisms with antagonistic activity to some
other organism, or group of organisms, it has almost become axiomatic that
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