2.4. Persistence of R. solanacearum in substrate and host tissue
Forty-gram samples of disinfected substrate (mixture of peat
moss and sand as used in the previous assay with tomato plants)
were placed into two sterile glass petri plates, and a 10 ml aliquot of
R. solanacearum inoculum (106 cfu ml1; produced as described
above) was added to each sample and mixed into the medium with
a sterilized spatula. The final water content was around 60% of the
water-holding capacity of the medium. A sample of 1 g was
immediately removed and serially diluted in 9 ml of physiological
water (8.5 g NaCl L1), and 100 mlwas spread onto the surface of the
selective medium (Triphenyl Tetrazolium Chloride, TTC) and the
non-selective medium (Yeast Dextrose Agar, YDA). Five dilutions
and two replications were used for each dilution and growing
medium. Two glass petri plates were sealed with parafilm to
prevent moisture loss and were incubated in the dark at 25 C and
45 C, respectively, for six weeks. At periodic intervals of seven
days, 1 g sample was removed, serially diluted and plated as
described above. All plates were inverted and incubated at 28 C for
one week and colony forming units (cfu) of bacteria and separately
of R. solanaceraum were recorded.
Pure isolates of all R. solanaceraum colonies were analyzed using
the DAS-ELISA technique, as described above. When the result was
positive, intracellular spaces of tobacco plant leaves were infiltrated
with aqueous suspensions of bacteria (108 cfu ml1) to test their
pathogenicity after the thermal treatments.
To determine the persistence of the pathogen in soil in association
with the host tissue (artificially infected tomato plants), three
pieces of infected stem were placed into sterilized tubes containing
6 g of the disinfected growing medium. Twelve tubes with soil were
prepared, sealed with parafilm to avoid water loss, and placed in
two incubators at 25 C and 45 C, respectively. On a weekly basis
over a 6-week period, the plant debriswas removed from two tubes
of soil incubated at each temperature. The plant pieces from each
tube were washed with sterilized water, and were homogenized
with sterilized water in a mortar. To determine the presence of
pathogenic bacteria, part of this bacterial solution was directly
infiltrated into intracellular spaces of tobacco leaves (about 200 ml
in each leaf) to measure phytopathogenicity. The other part of the
suspension was stored at 6 C.
For bacterial suspensions showing negative results in tobacco,
the remaining suspension was serially diluted and plated as
described above to ensure that no R. solanacearum colonies were
obtained.
To determine the persistence of R. solanacearum in association
with the host tissue, one piece of infected stem was put into each
sterilized Eppendorf tube to be processed as described above.
A total of 12 pieces of infected stems were used in the experiment.
The different pure cultures obtained from the above experiments
were identified through sequencing the 16S rDNA gene. DNA
was extracted using the EaZy Nucleic Acid Isolation kit (E.Z.N.A.
Omega Bio-Tek Inc., USA). Oligonucleotides pA-pH (Edwards et al.,
1989) were used in the PCR and for sequencing. The thermal
profile consisted of 36 cycles including denaturation at 94 C for
1 min, primer annealing at 55 C for 2 min and an extension at 72 C
for 6 min. Amplification products were purified using the High Pure
PCR Product Purification kit (Roche). Sequencing was done at the
Instituto de Biología Molecular y Celular de Plantas de la Universidad
Politécnica de Valencia in Spain (ABI-337 Perkin Elmer)
using the Big Dye Terminator Cycle Sequencing kit (version 2.0)
(PE Biosystem) and the program CHROMAS (version 1.45).
 
2.4. Persistence of R. solanacearum in substrate and host tissue
Forty-gram samples of disinfected substrate (mixture of peat
moss and sand as used in the previous assay with tomato plants)
were placed into two sterile glass petri plates, and a 10 ml aliquot of
R. solanacearum inoculum (106 cfu ml1; produced as described
above) was added to each sample and mixed into the medium with
a sterilized spatula. The final water content was around 60% of the
water-holding capacity of the medium. A sample of 1 g was
immediately removed and serially diluted in 9 ml of physiological
water (8.5 g NaCl L1), and 100 mlwas spread onto the surface of the
selective medium (Triphenyl Tetrazolium Chloride, TTC) and the
non-selective medium (Yeast Dextrose Agar, YDA). Five dilutions
and two replications were used for each dilution and growing
medium. Two glass petri plates were sealed with parafilm to
prevent moisture loss and were incubated in the dark at 25 C and
45 C, respectively, for six weeks. At periodic intervals of seven
days, 1 g sample was removed, serially diluted and plated as
described above. All plates were inverted and incubated at 28 C for
one week and colony forming units (cfu) of bacteria and separately
of R. solanaceraum were recorded.
Pure isolates of all R. solanaceraum colonies were analyzed using
the DAS-ELISA technique, as described above. When the result was
positive, intracellular spaces of tobacco plant leaves were infiltrated
with aqueous suspensions of bacteria (108 cfu ml1) to test their
pathogenicity after the thermal treatments.
To determine the persistence of the pathogen in soil in association
with the host tissue (artificially infected tomato plants), three
pieces of infected stem were placed into sterilized tubes containing
6 g of the disinfected growing medium. Twelve tubes with soil were
prepared, sealed with parafilm to avoid water loss, and placed in
two incubators at 25 C and 45 C, respectively. On a weekly basis
over a 6-week period, the plant debriswas removed from two tubes
of soil incubated at each temperature. The plant pieces from each
tube were washed with sterilized water, and were homogenized
with sterilized water in a mortar. To determine the presence of
pathogenic bacteria, part of this bacterial solution was directly
infiltrated into intracellular spaces of tobacco leaves (about 200 ml
in each leaf) to measure phytopathogenicity. The other part of the
suspension was stored at 6 C.
For bacterial suspensions showing negative results in tobacco,
the remaining suspension was serially diluted and plated as
described above to ensure that no R. solanacearum colonies were
obtained.
To determine the persistence of R. solanacearum in association
with the host tissue, one piece of infected stem was put into each
sterilized Eppendorf tube to be processed as described above.
A total of 12 pieces of infected stems were used in the experiment.
The different pure cultures obtained from the above experiments
were identified through sequencing the 16S rDNA gene. DNA
was extracted using the EaZy Nucleic Acid Isolation kit (E.Z.N.A.
Omega Bio-Tek Inc., USA). Oligonucleotides pA-pH (Edwards et al.,
1989) were used in the PCR and for sequencing. The thermal
profile consisted of 36 cycles including denaturation at 94 C for
1 min, primer annealing at 55 C for 2 min and an extension at 72 C
for 6 min. Amplification products were purified using the High Pure
PCR Product Purification kit (Roche). Sequencing was done at the
Instituto de Biología Molecular y Celular de Plantas de la Universidad
Politécnica de Valencia in Spain (ABI-337 Perkin Elmer)
using the Big Dye Terminator Cycle Sequencing kit (version 2.0)
(PE Biosystem) and the program CHROMAS (version 1.45).
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