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

138j 7 Quorum Sensing in Bacteria: Potential in Plant Health Protection
to various commercial crops such as onion, potato, carrot, celery, cucumber and pine
apple. Some other species are also known to be pathogenic/opportunistic pathogens
to insects and animals [103].
The soft rot Erwinia spp. includes E. chrysanthemi, E. carotovora sub sp. carotovora
(Ecc), E. atroseptica (Eca) and E. betavascolorum (Ecb). These organisms characteristically
produced an abundance of exoenzymes including pectin methyl esterase, pectate
lyase, pectin lyase, polygalacturonase, cellulose and proteases [107,108]. These enzymes
are virulence factors of the bacterium. Disruption of genes encoding above
enzymes often leads to reduction of virulence in planta [109,110]. Other secondary
metabolites widely studied in Erwinia sp. are the production and QS-based regulation
of carbapenem, a b-lactam antibiotic. The cell density-dependent production of exoenzymesinEccisalsodependentonthesynthesisofOHHLbyCar1,whichencodesaluxI
homologue in Ecc. Disruption of carI leads to a diminution of exoenzyme synthesis
and a consequent reduction of virulence in planta [69,111]. It has been reported that
CarR protein has the ability to sequester OHHL away from an additional LuxR homologue
responsible for induction of exoenzymes synthesis. A second LuxR homologue
expR (eccR/rexR) has been identifiedin ECC. Anderson et al. [112] deduced that control
of exoenzyme production takes place through the input of many regulators, some of
which interact with components of the QS system.
Other species of Erwinia have been found to produce acyl-HSLs; Eca, Ebc and E.
chrysanthemum synthesize different acyl-HSLs (OHHL, HHL and N-decanoyl-Lhomoserine
lactone, DHL) [32,35,41].
7.4.2
R. solanacearum
R. solanacearum causes vascular wilt disease of many plants primarily due its ability
to produce an acidic exopolysaccharide and plant cell wall degrading extracellular
enzymes [113]. Expression of these virulence factors occurs in an apparent cell
density dependent manner, with maximum expression at high cell densities
[114]. The lysR type regulator, PhcA, is central to the complex regulation of EPS
and extracellular enzymes, and hence, pathogenicity in R. solanacearum. PhcA activity
is regulated by a two-component regulatory system, which, in turn, is responsive
to the QS signal molecules, 3-hydroxy palmitic acid methyl ester (3OHPAME).
Exogenous addition of 3OHPAME to cultures at low cell density does induce precocious
production of EPS and enzymes [114–116] PhcS and PhcR that makeup the
two-component system responsive to 3OHPAME. It has been shown that PhcS and
PhcR act together to negatively regulate the expression of PhcA-regulated genes in
the absence of 3OHPAME [117].
7.4.3
Xanthomonas campestris
Cell-to-cell communication in X. campestris (Xcc), the causative agent of the black rot
of cruciferous plants, has been described. This organism also produces a variety of