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Vol. 102, No. 5, 2012 469
Loss of Virulence of the Phytopathogen Ralstonia solanacearum
Through Infection by φ φRSM Filamentous Phages
Hardian S. Addy, Ahmed Askora, Takeru Kawasaki, Makoto Fujie, and Takashi Yamada
Department of Molecular Biotechnology, Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8530, Japan.
Accepted for publication 27 January 2012.
Addy, H. S., Askora, A., Kawasaki, T., Fujie, M., and Yamada, T. 2012.
Loss of virulence of the phytopathogen Ralstonia solanacearum through
infection by φRSM filamentous phages. Phytopathology 102:469-477.
φRSM1 and φRSM3 (φRSM phages) are filamentous phages (inoviruses)
that infect Ralstonia solanacearum, the causative agent of bacterial wilt.
Infection by φRSM phages causes several cultural and physiological
changes to host cells, especially loss of virulence. In this study, we
characterized changes related to the virulence in φRSM3-infected cells,
including (i) reduced twitching motility and reduced amounts of type IV
pili (Tfp), (ii) lower levels of β-1,4-endoglucanase (Egl) activity and
extracellular polysaccharides (EPS) production, and (iii) reduced expres-
sion of certain genes (egl, pehC, phcA, phcB, pilT, and hrpB). The
significantly lower levels of phcA and phcB expression in φRSM3-
infected cells suggested that functional PhcA was insufficient to activate
many virulence genes. Tomato plants injected with φRSM3-infected cells
of different R. solanacearum strains did not show wilting symptoms. The
virulence and virulence factors were restored when φRSM3-encoded
orf15, the gene for a putative repressor-like protein, was disrupted.
Expression levels of phcA as well as other virulence-related genes in
φRSM3-ΔORF15-infected cells were comparable with those in wild-type
cells, suggesting that orf15 of φRSM3 may repress phcA and, con-
sequently, result in loss of virulence.
Ralstonia solanacearum is a widely distributed soilborne
phytopathogen belonging to the β subdivision of Proteobacteria.
It causes lethal bacterial wilt of >200 plant species, including
economically important crops (16,17). During infection, R. so-
lanacearum cells express various virulence and pathogenicity
factors resulting in typical wilting symptoms in host plants. The
virulence factors produced by R. solanacearum consist of a
consortium of plant cell-wall-degrading enzymes (CWDEs)
secreted via the type II secretion system. These CWDEs include
β-1,4-endoglucanase (Egl), endopolygalacturonase (PehA), exo-
polygalacturonases (PehB and PehC), β-1,4-cellobiohydrolase
(CbhA), and a pectin methyl esterase (Pme) (8,15,20,36). Secre-
tion of effector proteins via the type III secretion system (T3SS)
is also an important process in bacterial pathogenesis. Bacteria
that lose the ability to produce these secretion systems cannot
infect host plants (13).
Recently, we isolated and characterized various phages that
infect R. solanacearum strains (40). One of these phages,
φRSM1, is a filamentous phage (inovirus) with a circular single-
stranded DNA genome of 9,004 nucleotides (nt) encoding 14
open reading frames (ORFs) (23). Sometimes, φRSM1-related
DNA sequences are integrated into the genome of certain strains
of R. solanacearum. Askora et al. (3) characterized one such
prophage sequence (φRSM3, 8,929 nt long) and found that
φRSM3 is viable and produces phage particles when introduced
into different strains, including MAFF 106603. The genomes of
φRSM1 and φRSM3 are very similar to each other (93% nucleo-
tide identity) except for two ORFs, one of which (ORF9) encodes
the host recognition protein (pIII). φRSM1 and φRSM3 showed
different host ranges and all 15 strains of R. solanacearum tested
were sensitive to one or the other of the phages (3). Infection by
φRSM1 or φRSM3 (φRSM phage) does not kill host cells but
establishes a persistent association between the host and the
phage. Upon infection by φRSM phages, the host cells showed
some abnormal behaviors and characteristics, such as frequent
aggregation, dark coloration, and relatively small colony size.
Most importantly, φRSM-infected cells lost their virulence against
tomato plants (3). This virulence-reducing effect of φRSM phage
infection contrasts with some other previously reported cases. For
example, infection of Xanthomonas campestris pv. oryzae
NP5850 by the filamentous phages Xf and Xf2 resulted in en-
hanced virulence, possibly because of overproduction of extra-
cellular polysaccharides (EPS) by the phage-infected bacterial
cells (21). Tseng et al. (37) also reported that infection of X.
campestris pv. campestris by the filamentous phage φLf increased
virulence via promoting EPS production. Therefore, the changes
in R. solanacearum cells caused by φRSM infection are worthy of
investigating in relation to their pathogenicity.
In this study, we have further characterized the changes in
phage-infected R. solanacearum cells that result in the reduction
of virulence. We hypothesized that φRSM infection might cause
(i) reduction of cell motility, especially twitching motility; (ii)
reduction of virulence factors such as Egl and EPS; and (iii) re-
duced expression of specific genes involved in virulence and
MATERIALS AND METHODS
Bacterial strains and bacteriophage. R. solanacearum strains
MAFF 106603 (race 1, biovar 3, and phylotype I) and MAFF
106611 (race 1, biovar 4, and phylotype I) were obtained from the
National Institute of Agrobiological Sciences (Japan). Avirulent
strain M4S (race 1, biovar 3, and phylotype 1) was obtained from
the Leaf Tobacco Research Center, Japan Tobacco Inc. (35).
Strain MAFF 106603 was used for all experiments, and strains
Corresponding author: T. Yamada; E-mail address: firstname.lastname@example.org
*The e-Xtra logo stands for “electronic extra” and indicates that the online version
contains three supplemental figures.
© 2012 The American Phytopathological Society
MAFF 106611 and M4S were used as controls. The bacterial cells
were cultured in casamino acid-peptone-glucose (CPG) medium
containing 0.1% casamino acids, 1% peptone, and 0.5% glucose
(18) at 28°C with shaking at 200 to 300 rpm. Strain MAFF
106603 carrying a green fluorescent protein (GFP)-expressing
plasmid pRSS12 was described previously (24), and was culti-
vated in CPG containing kanamycin (50 µg/ml). In some
cases, bacterial cells were cultivated in minimal medium (MM)
containing 1.75 g of K2HPO4, 0.75 g of KH2PO4, 0.15 g of Na-
citrate, 0.25 g of MgSO4, and 1.25 g of (NH4)2SO4 (5) per liter.
For an antibiotic sensitivity assay, exponentially growing cells
(107 CFU/ml) in CPG medium were streaked for growth to single
colonies on CPG plates containing an antibiotic (kanamycin,
chloramphenicol, or ampicillin) at the concentration of 10, 20, 30,
40, 60, or 100 µg/ml. Bacteriophage φRSM3 was described previ-
ously (3). φRSM3 was routinely propagated using strain MAFF
106603 as the host. To collect sufficient phage particles, a total of
2 liters of bacterial culture was grown. When the cultures reached
0.1 unit at an optical density of 600 nm (OD600), the phage was
added at a dose of 0.01 to 0.05 PFU/host cell. After further
growth for 16 to 18 h, cells were removed by centrifugation in an
R12A2 rotor in a Hitachi Himac CR21E centrifuge (Hitachi Koki
Co. Ltd., Tokyo), at 8,000 × g for 15 min at 4°C. The supernatant
was passed through a 0.2-µm membrane filter and then phage
particles were precipitated in the presence of 0.5 M NaCl and 5%
polyethylene glycol 6000. Phage preparations were stored at 4°C
until use. To isolate single colonies of MAFF 106603 infected
with φRSM3, single φRSM3 plaques picked from assay plates
covered with a MAFF 106603 lawn were streaked onto CPG
plates. Single colonies were repeatedly purified. The phage ge-
nomic DNA was isolated from cells in its replicative form and
confirmed by restriction enzyme digestion.
DNA and RNA isolation and manipulation. Standard mo-
lecular biological techniques for DNA isolation and digestion
with restriction enzymes and other nucleases were as described by
Sambrook and Russell (33). Phage DNA was isolated from puri-
fied phage particles by phenol extraction. In some cases, extra-
chromosomal DNA [replicative form (RF) DNA] was isolated
from phage-infected R. solanacearum cells by the mini-prepa-
ration method (4). Total bacterial RNA was isolated from 3 ml of
a culture of φRSM3-infected MAFF 106603 cells at the expo-
nential phase (1 × 108 CFU/ml) in MM using an RNAprotect
Bacteria Reagent kit (Qiagen K.K., Tokyo) according to the
manufacturer’s protocol. Total RNA was treated with 10 U of
RNase-free DNaseI (TakaraBio, Kyoto, Japan) for 30 min at 37°C
to remove any genomic DNA contaminants. DNase I was in-
activated by phenol/chloroform extraction. The absence of DNA
contaminants in RNA preparations was confirmed by polymerase
chain reaction (PCR) with gene-specific primers (Table 1). Thirty-
five rounds of PCR were performed under standard conditions in
a MY Cycler (Bio-Rad Laboratories, Hercules, CA). The genomic
DNA of MAFF 106603 was used as a positive control in the PCR
Construction of φ φRSM3-Δ ΔORF15. To know the role of
ORF15 found in the φRSM3 genome, we constructed a φRSM3
mutant lacking ORF15 (designated as φRSM3-ΔORF15). The
φRSM3-ΔORF15 DNA construct was generated from φRSM3
DNA by PCR using forward primer 5′-GAT GAG AAC TCC TAT
CAT GGC GAA ACA CTT-3′ (corresponding to φRSM3 DNA
position 8821 to 8850) and reverse primer 5′-ACA AGG TGT
GCC CGG CAC GCT GAA CG-3′ (corresponding to φRSM3
DNA position 8549 to 8521). With these primers and φRSM3
DNA template, PCR produces φRSM3 DNA fragments lacking
ORF15 (positions 8527 to 8820). The PCR product (≈8.66 kbp)
was extracted and purified from agarose gel after electrophoretic
separation and then circularized with T4 DNA ligase (Ligation
High; Toyobo, Osaka, Japan) overnight at 16°C. The resulting
DNA was introduced into cells of R. solanacearum MAFF
106603 by electroporation. After incubation for 2 h at 28°C, bac-
terial cells were subjected to plaque assay. Single plaques were
isolated and phage-containing cells were cultivated to obtain RF
DNA. The φRSM3-ΔORF15 DNA sequence was confirmed by
entire DNA sequencing.
Real-time quantitative reverse-transcription PCR. Real-time
quantitative reverse-transcription (qRT)-PCR was performed as
described previously (2). First-strand cDNAs were synthesized
from 1 µg of total RNA with a ReverTraAce reverse-transcriptase
kit (Toyobo) and gene-specific primers according to the
manufacturer’s instructions. Specific gene primers were designed
using Primer 3 (v. 0.4.0) software (http://frodo.wi.mit.edu/
primer3/#PRIMER_MAX_TEMPLATE_MISPRIMING). The nega-
tive control (to eliminate the possibility of residual DNA
amplification) consisted of the same reaction except that the RT
was omitted from the reaction mixture.
Real-time PCR was performed with a SYBR premix Ex Taq kit
(TakaraBio) using a LineGene fluorescence quantitative detection
system (BioFlux, Tokyo). The 10-µl reaction mixture contained
5 µl of SYBR premix Ex Taq, 1 µl of diluted cDNA, and 0.5 µM
each gene primer (Table 1). PCR was performed under the
following conditions: initial heating for 3 min at 95°C and 45
cycles of 95°C for 10 s, 62°C for 10 s, and 72°C for 15 s. At the
end of the program, the specificity of the primer set was con-
firmed by melting curve analysis (65 to 95°C with a heating rate
of 0.5°C/min). Relative expression levels were calculated as the
TABLE 1. List of primers for Ralstonia solanacearum used for reverse-transcriptase polymerase chain reaction
Primer namea Oligo sequences (5′→3′) Amplified gene Product (bp)
a F = forward, R = reverse, and * = gene-specific primer used for first-strand cDNA synthesis.
Vol. 102, No. 5, 2012 471
ratio of expression of each gene against that of the 16S rRNA
gene in R. solanacearum.
Assays of Egl activity and EPS. Total Egl activity was
determined by measuring the reducing sugars (30) released during
incubation of 20% (vol/vol) culture supernatant in 120 mM phos-
phate buffer (pH 7.0) with carboxymethylcellulose at 15 mg/ml as
a substrate at 50°C for 4 h according to Addy et al. (1). One unit
of enzyme activity was defined as releasing glucose at 1 nmol/min.
For EPS production, cells were grown in BG broth for 3 days at
28°C (8). To precipitate EPS, NaCl was added to the culture
TABLE 2. Changes in Ralstonia solanacearum cells caused by φRSM3 infection
Feature Uninfected cells
Color of culture
Edge of the colony
Antibiotic sensitivity (minimum inhibitory concentration)
White to yellow
Yellow to brown
Reduced or no twitching
Fig. 1. Morphology of Ralstonia solanacearum colonies. Colonies of strain MAFF 106603 A and C, uninfected and B and D, infected with φRSM3 were
observed A and B, 20 h and C and D, 30 h after streaking on minimal medium (25). Bar = 100 µm.
supernatant to a final concentration of 0.1 M, and four volumes of
acetone was added. After standing overnight at 4°C, precipitated
materials were recovered by centrifugation (8,000 × g, 10 min,
4°C), dissolved in 500 µl of double-distilled (dd)H2O, heated for
10 min at 65°C, and centrifuged at 8,000 × g for 5 min to remove
insoluble material. The concentration of hexosamine in the
culture supernatant was estimated using a modified Elson and
Morgan reaction (12). Appropriately diluted samples (0.45 ml)
were mixed with 0.15 ml of concentrated HCl, hydrolyzed in
sealed tubes at 110°C for 30 min, and then the colorimetric assay
was conducted. The absorbance at 530 nm was determined, and
the hexosamine concentration was calculated from an N-acetyl D-
glucosamine standard curve. The background due to residual
media components was subtracted. For a control, N-acetyl D-
glucosamine standards were subjected to the entire analysis and
were added before the hydrolysis step. For each assay, three
independent experiments were repeated, and mean value and
standard deviation value were calculated. The significance of
observed differences was judged by statistical analysis (Student’s
Cell motility and movement monitoring. R. solanacearum
cells were cultured in CPG broth for 1 day at 28°C. After
centrifugation at 8,000 × g for 2 min at 4°C, cells were washed
twice with ddH2O and resuspended in ddH2O (OD600 = 1.0). For
each assay, 5 µl of the suspension was dropped onto the test
medium: MM for twitching motility (25), swimming medium
(SWM) for swimming motility (19), and swarming medium
(SRM) for swarming motility (19). Motility was observed by
measuring the diameter of the dropped culture for 6 days. To
monitor the movement of bacterial cells in tomato stems, we used
cells of a GFP-expressing strain (φRSM3-infected or uninfected
MAFF 106603 harboring pRSS12) (11,24). For inoculation into
plants, the bacterial cells grown in CPG medium for 1 to 2 days
were washed and resuspended in ddH2O at a density of 108
CFU/ml. The suspension (1 µl) was injected with a needle into the
major stem (between the cotyledon and the first leaf) of tomato
plants (Solanum lycopersicum L. ‘Oogata Fukuju’, 4 weeks old,
with four to six leaves) and incubated in a Sanyo growth cabinet
(Sanyo, Osaka, Japan) at 28°C (16 h of light and 8 h of darkness).
After incubation for 1 week, the plant stem was cut into slices
20 µm in thickness with a microtome and then observed under a
Leica MZ16 microscope equipped with a GFP3 filter (11,24).
Bacterial surface appendages. Cells of R. solanacearum
strains were streaked heavily onto MM plates and incubated for
≈22 to 24 h. The colonies were suspended in a small volume of
10 mM Tris-HCl buffer at pH 8, and the cell suspension (same
cell density in each sample) was forced five times through a 25-
gauge hypodermic needle (6). Bacterial cells were removed by
centrifugation at 8,000 × g for 20 min at 4°C. The bacterial
surface appendages were collected by centrifugation at 136,000 ×
g for 60 min. Precipitated materials were subjected to Tris-Tricine
sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-
PAGE) according to Schagger and von Jagow (34). For protein
identification, separated protein bands were transferred to poly-
vinylidene difluoride nylon membranes (Immobilion; Nihon
Millipore, K.K., Kyoto, Japan) using a semi-dry transfer cell
(Bio-Rad Laboratories). Each protein band was subjected to N-
terminal peptide sequence analysis on a protein sequencer (Model
492; Applied Biosystems, Foster City, CA) as described by
Askora et al. (3).
Pathogenicity assays. Cells of R. solanacearum were grown in
CPG medium for 1 to 2 days at 28°C. After centrifugation, cells
were resuspended in ddH2O at a density of 108 CFU/ml (OD600 =
0.3). For the virulence assay, the bacterial cell suspension (1 µl)
was injected with a needle into the major stem of tomato plants
(4 to 6 weeks old with four leaves) at a site between the cotyledon
and the first leaf. As a control, Escherichia coli cells at the same
density were injected in the same manner. Each bacterial strain
was injected into five plants. Plants were cultivated in a Sanyo
Growth Cabinet at 28°C (16 h of light and 8 h of darkness) for up
to 1 week before evaluation of disease symptoms. The wilting
symptoms were graded from 0 to 4 as described by Winstead and
Kelman (39) and modified by Poueymiro et al. (31).
Cultural, physiological, and morphological changes in cells
infected with φ φRSM3. Infection by φRSM phages does not cause
host cell lysis but establishes a persistent association between the
host and the phage (3,40). Although φRSM3-infected cells always
showed slightly less cell density compared with wild-type cells,
the growth curves were almost comparable in either rich medium
(CPG) or MM between infected and uninfected cells (Supple-
mental Figure 1). φRSM3-infected cells of R. solanacearum strain
MAFF 106603 produced φRSM3 particles and yielded the repli-
cative form of φRSM3 DNA. Restriction enzyme digestion of the
DNA with ClaI and HincII confirmed the exact genomic structure
of φRSM3 recovered from the cells (data not shown). We also
confirmed the changes in MAFF 106603 cells caused by φRSM3
infection, including frequent aggregation, dark coloration, and
relatively small size of colonies (Table 2; Supplemental Figures 2
and 3), as previously reported by Askora et al. (3). In addition to
these changes, φRSM3-infected cells showed enhanced antibiotic
resistance. Wild-type MAFF 106603 cells could not grow (no
Fig. 2. Comparison of proteins from cell surface structures. Cell surface
appendages were released by passing bacterial cells through a hypoder-
mic needle and their protein components were solubilized, separated by
sodium dodecyl sulfate polyacrylamide gel electrophoresis, and stained with
Coomassie blue. Molecular size of each marker protein (from Amersham
LMW gel filtration kit) is indicated on the left. FliC, PilA, and HrpY proteins
were identified by their N-terminal amino acid sequence as described
Vol. 102, No. 5, 2012 473
single colonies) on CPG plates containing kanamycin (Km) at
30 µg/ml or chloramphenicol (Cm) at 20 µg/ml, whereas, after
φRSM3 infection, cells could grow on CPG plates containing Km
at 60 µg/ml and Cm at 40 µg/ml. Both φRSM3-infected and
uninfected cells were sensitive to ampicillin (Amp) at 40 µg/ml;
no growth, including single colonies, was observed. These results
were similar to those observed in strain MAFF 730138 infected
with φRSM1 (3).
Comparison of twitching motility and cell surface struc-
tures between φ φRSM3-infected and uninfected cells. Colonies
of MAFF 106603 cells on CPG and MM are usually viscous and
glossy; however, after infection with φRSM3, they became
smaller and less viscous. Especially on MM, the irregular and
rough colony margins of uninfected cells became smooth after
φRSM3 infection. R. solanacearum cells show twitching motility
in culture (25), and our results suggested that phage infection
affected this motility; therefore, we examined the micromor-
phology of the colonies. On MM plates, the colony margins of
uninfected MAFF 106603 showed irregularly shaped spear-
heads, and rafts of bacteria were separated from the colonies (Fig.
1A and C), indicating active twitching motility (25). After 30 h
of growth, uninfected colonies typically had thin or layered
edges with multiple irregular projections (Fig. 1C), whereas
colonies of φRSM3-infected cells were round with smooth mar-
gins, and lacked rafts or spearheads (Fig. 1B and D). This colony
morphology resembled that of a pilQ mutant of K60, which lacks
type IV pili and does not twitch (25), suggesting a decrease or
loss of twitching motility of MAFF 106603 cells infected with
It is well known that type IV pili (Tfp) are involved in twitch-
ing motility as well as adhesion, aggregation, and pathogenesis of
various bacteria (27,38). Therefore, we examined whether cell
surface structural components were affected by φRSM3 infection.
Cell surface structure proteins were prepared as described in
Materials and Methods, separated by SDS-PAGE, and compared
between φRSM3-infected and uninfected cells. Compared with
uninfected cells, φRSM3-infected cells had considerably de-
creased levels of PilA (identified by the N-terminal sequence of
TLIELMI), the major component of Tfp (3), and decreased levels
of HrpY (identified by a trypsin fragment sequence of TGFQAQ),
the major component of type III pili (3) (Fig. 2). These results indi-
cated that infection by φRSM3 resulted in decreased formation of
Tfp in host cells, resulting in decreased twitching motility.
Changes in EPS production and Egl activity. The changes in
colony morphology after phage infection described above sug-
gested that the phage affected EPS production as well as extra-
cellular Egl activity. In uninfected cells of strain MAFF 106603,
EPS production and Egl activity were 965.4 µg/ml and 0.31 U/ml,
respectively. These values are comparable with those reported for
pathogenic strains 82N (9) and U-7R (29). After φRSM infection,
the EPS production and Egl activity were decreased to
674.6 µg/ml (69.9%) and 0.13 U/ml (39.6%), respectively.
Changes in gene expression levels in φ φRSM3-infected cells.
The phenotypic changes observed for the φRSM3-infected cells
described above, some of which seemingly involved in virulence
and pathogenicity, led us to examine expression levels of specific
genes related to pathogenesis and virulence. We targeted six
genes (egl, hrpB, pehC, phcA, phcB, and pilT) because these
Fig. 3. Expression analysis of Ralstonia solanacearum genes involved in virulence. Transcript levels of each gene were determined by quantitative reverse-
transcription polymerase chain reaction from RNA extracted from MAFF 106603 cells uninfected and infected with φRSM3. Expression levels were also analyzed
in cells infected with φRSM3-ΔORF15. For each gene, expression level was normalized to that of 16S rRNA as an internal standard. Mean expression and
standard deviation values were calculated from the results of three independent experiments. Bars within each gene are marked specifically if values differ
significantly at 0.01 ≤ P ≤ 0.05 (*) or at 0.001 ≤ P ≤ 0.01(**), or not significantly at P > 0.05 (ns) compared with uninfected cells, according to the Student’s t test.
might be closely related to the observed changes. Two genes for
housekeeping functions, aceE for pyruvate dehydrogenase subunit
E1 and polA for DNA polymerase, were also included for con-
trols. Exponentially growing cells (OD600 = 1.0) in MM (mimick-
ing the natural environment in plant tissues) (14) were subjected
to qRT-PCR analyses with specific primers for each gene (Table
1), as described in Materials and Methods. All of the genes
examined except for two housekeeping genes showed decreased
expression levels in φRSM3-infected cells compared with un-
infected cells (Fig. 3). Expression levels of housekeeping genes
represented by aceE and polA in φRSM3-infected cells were
comparable with those of wild-type cells, suggesting that the
normal basic metabolism was still functioning after phage infec-
tion. This was consistent with the comparable growth rate
between wild-type and φRSM3-infected cells. The egl and pehC
genes encoding β-1,4-Egl and exopolygalacturonase, respectively,
showed drastically decreased expression levels (≈1/100 to 1/1,000)
compared with uninfected cells, consistent with the observation of
reduced Egl activity in φRSM3-infected cells. The pilT gene,
which has a role in twitching motility (27,38), also showed lower
expression levels in φRSM3-infected cells. Interestingly, φRSM3-
infected cells also showed decreased expression of phcB (to 1/136
of that in uninfected cells), which is responsible for synthesis of
3-OH palmitic acid methyester (3-OH PAME), an autoinducer of
quorum sensing that controls virulence and pathogenicity (10).
The two-component regulatory system PhcS/PhcR responds to
threshold levels of 3-OH PAME, and elevates the level of
functional PhcA, which controls expression of many virulence
genes (7). Furthermore, the level of phcA expression itself was
also significantly reduced in φRSM3-infected cells (to 1/20 that in
uninfected cells). The expression of hrpB, which regulates the
T3SS, was also decreased in infected cells.
Loss of virulence in φ φRSM3-infected R. solanacearum. The
reduced expression of many virulence genes in φRSM3-infected
cells suggested a potentially decreased ability by the pathogen to
cause disease. When 1 µl of cell suspension containing 105 CFU
Fig. 4. Effects of φRSM3 infection on virulence of Ralstonia solanacearum. Tomato plants (4 weeks old) were injected with cells of MAFF 106603 uninfected or
infected with A, φRSM3 or B, φRSM3-ΔORF15. As a control, plants were injected with cells of Escherichia coli JM109. Each bacterial strain was injected into
20 plants (5 are shown for each experiment in the figure). All plants injected with φRSM3-uninfected or φRSM3-ΔORF15-infected R. solanacearum cells showed
wilting symptoms 1 week after injection. All plants injected with φRSM3-infected cells or with E. coli cells (control) failed to show any symptoms. Pictures were
taken A, 3 weeks and B, 1 week after infection.
Vol. 102, No. 5, 2012 475
of MAFF 106603 was injected into the major stem of tomato
plants, all plants showed wilting symptoms as early as 3 days
postinfection (p.i.) (wilting grade 1) and died 5 to 7 days p.i.
(wilting grade 4) (Fig. 4A). In contrast, all 20 plants injected with
φRSM3-infected MAFF 106603 cells did not show any wilting
symptoms until 4 weeks p.i. This was also the case with other
host strains such as MAFF 106611; φRSM3-infected MAFF
106611 completely lost its virulence against tomato plants (data
not shown). To compare the bacterial behavior in tomato plants,
φRSM3-infected or uninfected MAFF 106603 cells harboring a
GFP-expressing plasmid pRSS12 were injected into the stem, as
described in Materials and Methods. Stem slices at intervals of
10 mm above and below the injection point were examined
1 week after the bacterial injections. As indicated by GFP fluores-
cence, the bacterial cells accumulated in the xylem vessels and
moved both upward and downward in wilted tomato plants
inoculated with phage-uninfected cells (Fig. 5A), whereas most of
the φRSM3-infected bacterial cells remained around the injection
point, and their movement and growth were severely limited
ORF15 encoded on the φ φRSM3 genome is involved in the
loss of virulence in infected cells. To understand the basis for the
reduced virulence of φRSM-infected cells (in other words, reduce
the expression levels of virulence-related genes), we considered
the possibility that some gene encoded by φRSM3 may directly
affect host gene expression. Fourteen ORFs were identified on the
φRSM3 genome, three of which (ORF2, ORF3, and ORF13) are
with unknown functions (without any DNA-binding motifs) and
variable among φRSM phages (3). Recently, we identified
ORF15, located upstream of ORF14 (Int) on the φRSM3 genome
(nucleotide positions 8527 to 8820, accession number AB434711).
This ORF encodes a protein of 98 amino-acid residues (also
corresponding to ORF15 of φRSM1, accession number A0JC19)
with sequence similarity to putative phage repressors (ex.
Pelobacter propionicus DSM2379, E value = 0.004). When strain
MAFF 106603 was infected with a φRSM3 mutant whose ORF15
was removed (φRSM3-ΔORF15), the cells caused wilting on
inoculated tomato plants as efficiently as wild-type cells (Fig.
4B). It was found that the expression level of phcA in φRSM3-
ΔORF15-infected cells was comparable with that of wild-type
cells (Fig. 3). The other virulence genes reduced in φRSM3-
infected cells, including hrpB and pilT, were also recovered to
almost the same levels as the wild-type levels, except phcB. The
potential repressor function of ORF15 suggests that it causes,
directly or indirectly, the repression of phcA and other virulence
genes and, therefore, loss of virulence in infected cells. We
attempted to directly introduce ORF15 into host cells but failed to
stably maintain it when ligated to pRSS12 (under the control of
the lac promoter) (24), which was transformed into strain MAFF
106603 and other strains.
Fig. 5. Cross-sections of tomato plants injected with green fluorescent protein (GFP)-expressing Ralstonia solanacearum cells. Tomato seedlings (4 weeks old)
were injected with uninfected cells or φRSM3-infected cells. After 1 week, stem slices were cut at intervals of 10 mm A, above and B, below the injection point
(0 mm). In tomato plants injected with uninfected cells, GFP fluorescence was observed in xylem vessels in sections from above and below the injection point
whereas, in those injected with φRSM3-infected cells, GFP fluorescence remained around the injection point. Arrowhead indicates a specific site of the tissue as
position marker. The area indicated by a square is enlarged at the upper right. Numbers; distance from the injection point (+ = upward and – = downward).
Because filamentous phages such as φRSM assemble on the
host cell membrane and protrude from the cell surface, the nature
of the host cell surface may change drastically during phage
production. Among the changes observed in φRSM-infected cells,
the reduction of Tfp formation and decreased twitching motility
are especially important. Filamentous phages infect via Tfp on the
host cell surface (26,28). Askora et al. (3) suggested that minor
components of Tfp of R. solanacearum might be involved in the
host discrimination by φRSM1 and φRSM3. Frequent protrusion
of φRSM particles from the infected cell surface may somehow
compete with the formation of Tfp. As reported by Kang et al.
(22), Tfp is responsible for twitching motility and adherence to
multiple surfaces and is required for virulence. Therefore, the loss
of virulence in the φRSM-infected cells seems to be at least partly
due to the reduction of Tfp formation and decreased twitching
However, the concomitant multiple changes in φRSM-infected
cells suggest that there are other complex mechanisms involved in
the loss of virulence. In φRSM3-infected cells, the expression of
certain genes involved in virulence was reduced. Especially, the
significantly lower levels of phcA and phcB expression suggested
insufficient amounts of PhcA in the cells. The transcriptional
regulator PhcA plays a critical role in the regulatory network of R.
solanacearum pathogenicity (7). Abundant functional PhcA
activates production of multiple virulence factors such as Egl,
PehC, and EPS. φRSM3 infection leads the loss of virulence in
host bacterial cells while cells infected with φRSM3-ΔORF15
lacking solely ORF15 caused wilting on inoculated tomato plants
as efficiently as wild-type cells. The expression levels of phcA
and other genes in φRSM3-ΔORF15-infected cells were compar-
able with those of wild-type cells. The expression level of
phcB was exceptionally retained at lower levels in φRSM3-
ΔORF15-infected cells. This may be caused by changes in the cell
surface nature during phage production. The gene for ORF15 is
highly expressed in φRSM3-infected cells (A. Askora, unpub-
lished data). These data suggested that ORF15 of φRSM3
may repress phcA and the other virulence genes directly or
indirectly, consequently resulting in loss of virulence in infected
cells. It is noteworthy that φRSM3-related prophages are inte-
grated in the genome of some R. solanacearum strains, including
UW551 (3), IPO1609 (32), and CMR15 (32). These prophages
lack an ORF15 homolog and, thus, apparently do not affect host
The loss of virulence in host cells caused by φRSM3 infection
is in contrast with the previously observed effects of infection
with φRSS1, another inovirus infecting R. solanacearum cells
(40). φRSS1 infection enhanced the virulence of R. solanacearum
strain C319 on tobacco (40) and strains MAFF 106603 and
MAFF 106611 on tomato (1). Recently, Addy et al. (1) revealed
that infection with φRSS1 induced early expression of phcA. The
surface-associated phage particles may change cell surface nature
(ex. hydrophobicity) and enhance cell-to-cell interactions, result-
ing in high local cell densities and early activation of phcA.
φRSS1, that has a small genome (6,662 nt) and lacks a regulatory
gene (23), grows very abundantly up to ≈10
under usual culture conditions, whereas φRSM3 with a genome of
8,929 nt grows less abundantly (titer of 1/100 compared with
φRSS1), so that the surface effects on host cells caused by phage
particles may be less prominent in φRSM infection. Recently, it
has been found that φRSS1 was derived from a larger phage
φRSS0 (GenBank accession number JQ408219) by losing a 662-
nt region containing ORF13 (a putative regulatory gene) (Tasaka,
unpublished data). It is noteworthy that φRSS0 infection caused
loss of virulence in host cells, just as observed in φRSM3-infected
cells. In this case, ORF13 of φRSS0 may function like ORF15 of
φRSM3 and φRSS1 may correspond to φRSM3-ΔORF15. There-
11 to 10
fore, both filamentous phages φRSM-type and φRSS-type appear
to exert the same general effect on the host physiology.
Because the φRSM-infected cells can grow and continue to
produce infectious phage particles under appropriate conditions,
φRSM phages may serve as an efficient tool to control bacterial
wilt in crops by decreasing the virulence of the pathogen.
This study was supported, in part, by a Grant-in-Aid from the Ministry
of Education, Science, Sport and Culture of Japan (21580095 to T.
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