© Benaki Phytopathological Institute
Swarming motility in plant-associated bacteria
19
they colonize different surfaces (Park, 2003).
Many bacteria secrete small diffusible as
quorum sensing (QS) signaling molecules
such as acyl-homoserine lactones (AHLs)
(Rajput
et al.
, 2015) or volatile signal mole-
cules such as terpenoids, alkenes, aldehydes
(Piechulla and Degenhardt, 2014), thus me-
diating different types of cell-to-cell com-
munication among physically separated mi-
croorganisms (Schmidt
et al.
, 2015).
AHLs are commonly synthesized by
members of the LuxI family of proteins and
are sensed by members of the LuxR fami-
ly of transcriptional regulators (Daniels
et
al.
, 2004). Above threshold concentrations
which is dependent by the cell population
density (quorum sense signals), AHLs are in-
volved in the activation of expression of cer-
tain genes which confer the ability to the
bacterium to migrate in a wide range of eco-
logical niches.
The complexity of AHL signal molecules
(QS signals) produced by bacteria are limited,
thus there is considerable opportunity for
cross talk among bacteria, as in most envi-
ronmental niches, multiple bacterial species
coexist as dynamic communities. Gantner
and colleagues, using a reporter engineered
Pseudomonas putida
strain on plant surfaces,
showed that some AHL signals were able to
travel relatively long distances (up to 78 mm)
but was most commonly detected only a few
micrometers away from the producing strain
(Gantner
et al.
, 2006). This was referred to as
the cell-to-cell calling distance. Culturable
rhizosphere bacteria of wheat also produced
signal that could inhibit QS in
P. chlororaphis
(
aureofaciens
) via mechanisms that remain
uncharacterized (Morello
et al.
, 2004).
The ability of QS signals to influence
QS regulated networks is an important
mechanism in modulating the QS-con-
trolled surface motility of physically sepa-
rated bacteria. This conjecture was elegant-
ly demonstrated in recent studies where it
was shown that bacterial epiphytes pro-
duced signals capable of interfering with
the QS system of
Pseudomonas syringae
pv.
syringae
(Pss) affected its swarming motil-
ity (Dulla
et al.
, 2009). The plant epiphytic
pathogenic bacterium Pss grows and sur-
vives on leaf surfaces, invades into the leaf
tissue and contributes to brown spot dis-
ease, thus the assessment of lesion forma-
tion was considered as sensitive marker
of its motility behavior; non-motile bacte-
ria were less virulent (Haefele and Lindow,
1987). In this respect, proficiency in swarm-
ing motility on plant surface has been cate-
gorized as a virulence factors.
QS suppresses swarming in Pss and QS-
deficient hyperswarmer strains invade leaves
more readily than wild-type strains, thereby
causing a higher incidence of brown spot
lesions on bean, suggesting that swarm-
ing motility of Pss strongly contributes to its
ability to invade leaves and incite disease on
the leaves (Quinones
et al.
, 2005). Nonmotile
mutants of Pss are less able to survive des-
iccation stresses on leaves, apparently be-
cause they cannot access protected sites in
or on the leaf surface (Quinones
et al.
, 2004).
Microarray analysis of Pss gene expression
during growth in epiphytic versus apoplas-
tic sites, revealed that genes involved in mo-
tility were relatively expressed at higher lev-
els when bacteria are located in former site,
suggesting that bacteria are requiring active
motility for relocation on leaves surface com-
pared to a the apoplast where these traits
were less expressed (Yu
et al.
, 2013 ). Co-in-
oculation of wild type Pss along with select-
ed AHLs-producing epiphytic bacteria which
produced large amounts of AHLs identical to
those produced by Pss, decreased mobility
Pss B728a on the leaf surface caused by in-
appropriate induction of Pss B728a QS sys-
tem, resulting in less invasion into the tissue
(Dulla
et al.
, 2009). Similarly, premature in-
duction of
Xylella fastidiosa
by its QS diffus-
ible signal factor which however was pro-
duced by the transgenic host plant enforced
the pathogen to prematurely adopt a suite
of phenotypes that would restrict its abili-
ty to move in the plant. Diffusible signal fac-
tor is also produced by other beneficial en-
dophytic bacteria such as
Stenotrophomonas
maltiphila
(Fouhy
et al.
, 2007; Zhu
et al.
, 2012),
thus it may represent an alternative to inhibit
the migration of
X. fastidiosa
.
