Agriculture Reference
In-Depth Information
host interactions, the ability to interfere bacterial communication is crucial in the prevention
of infection by pathogens that use QS to coordinate expression of virulence factors. These
tactics of signal interference are termed quorum-quenching.
The quorum-quenching strategies take place at both the eukaryote-to-prokaryote and
prokaryote-to-prokaryote levels [Waters and Bassler, 2005]. The main mechanisms of
quorum quenching identified so far are: (i) inhibition of the biosynthesis of the signal
molecule, (ii) premature degradation of the signal molecule or the LuxR homologue
regulatory protein (in the case of AHLs), (iii) competitive antagonism due to the synthesis of
a compound structurally similar to the signal molecule, and (iv) inhibition of sensor kinases.
Several recent papers thoroughly review the details of these quenching mechanisms [Zhang
and Dong, 2004, Waters and Bassler, 2005, Bjarnsholt
et al
., 2007, Dong
et al.
, 2007].
A number of bacterial species are able to enzymatically inactivate QS signals. AHLs are
degraded by AHL-lactonases and AHL-acylases. AHL-lactonases cleave the HSL lactone
rings, while AHL-acylases hydrolyze the amide bond, releasing the corresponding fatty acid
and inactive HSL [Zhang and Dong, 2004, Waters and Bassler, 2005, Dong
et al.
, 2007].
These enzymes are one of the most promising tools to develop new control strategies for
infectious diseases.
Structural mimics of QS signals act outcompeting the legitimate signal for receptor
binding [Dong
et al.
, 2007]. The best known structural analogues of AHLs are furanones (see
section 5., Figure 4.), either natural or synthetically produced.
Furanones also act accelerating
the degradation of LuxR [Manefield
et al.
, 2002]. Synthetic analogues of naturally produced
furanones have been developed which are potent inhibitors of QS in
P. aeruginosa
[Bjarnsholt and Givskov, 2007].
II.
QS
IN
P
LANT
-B
ACTERIAL
A
SSOCIATIONS
QS is a common feature of bacteria (either beneficial or deleterious) that live in
association with plants, regulating a wide range of phenotypes involved in the plant-bacteria
interactions, such as plasmid transfer, virulence factors, competence, and production of
antifungal compounds [Newton and Fray, 2004]. Production of AHLs has been reported for
plant-associated strains of the genera
Agrobacterium
(
A. radiobacter, A. rhizogenes, A.
tumefaciens, A. vitis
)
, Azospirillum
(
A. lipoferum
)
, Burkholderia
(
Burkholderia
sp.)
, Erwinia
(
E. amylovora, E. carotovora, E. chrysantemi, E. herbicola
),
Pantoea
(
P. stewartii
)
,
Pseudomonas
(
P. aureofaciens, P. chloraphis, P. corrugata, P. fluorescens, P. putida, P.
savastanoi, P. syringae
)
, Ralstonia
(
R. solanacearum
)
, Rhizobium
(
R. etli
,
R. fredii
,
R.
leguminosarum
bv.
viciae
, bv.
phaseoli
, bv.
trifolii
)
, Serratia
(
S. plymuthica
),
Sinorhizobium
(
S. meliloti
)
,
and
Xanthomonas
(
X. campestris, X. oryzae
) [Cha
et al
., 1998, Elasri
et al.
,
2001, Lithgow
et al.
, 2001, Vial
et al.
, 2006, Barnard
et al.
, 2007, Licciardello
et al
., 2007,
Liu
et al.
, 2007, Poonguzhali
et al.
, 2007].
Elasri
et al.
(2001) tested the ability to produce AHLs of a collection of 137
Pseudomonas
spp. strains isolated form soil and rhizosphere of plants, and interestingly
concluded that the percentage of AHL-producers was significantly correlated with the degree
of relationship with the plant, decreasing from 49% among plant-pathogenic bacteria to 28%
and 0% among nonpathogenic bacteria associated with the plant and soilborne bacteria,
