Plants were harvested two days after seedling inoculation
(when it is possible to separate roots and stems) and then at 2–3
days intervals. Roots and stems were then separated. For each tissue
three samples were combined and three replicates of tissue
samples collected (9 plants each time) to determine the average
colonization and fresh weight (fw). To determine endophytic
population, plant tissues were rinsed with sterile distilled water
and disinfected with 2% sodium hypochlorite for three minutes
with constant agitation for roots and 2 min for stems. Samples
were then washed four times with sterilized water and manually
crushed using a mortar and pestle. The homogenates were
resuspended in 1 ml of PBS and vortexed. This suspension was 10-
fold serially diluted and plated on LGI agar plates, containing Nal, Sm and X-Gluc (5-bromo-4-chloro-3-indoxyl-beta-d-glucuronide
cyclohexylammonium salt, 40 g ml−1) for gusA-marked strain or
Nal for strain PAL 5. Colony forming units were counted after incubation
at 28 ◦C for 3 days. As previously described by Luna et al.
(2010) three control procedures were performed to ensure the efficiency
of the surface disinfection method: (1) disinfected plant
tissue samples taken 96 h postinoculation (P.I.) (with the gusAmarked
strain) were observed by optical microscopy after staining,
(2) disinfected tissues were placed for 1 min onto plates containing
LGI, removed and plates were incubated at 28 ◦C, (3) the wash
solution from the last rinse was cultured on LGI plates.
To determine the extent of total colonization (rhizoplane and
endophytic population) of inoculated seedlings, another set of
plantlets were removed from the agar. Roots were rinsed with
sterile distilled water and processed as above, without surface disinfection.
Rhizoplane population was determined by subtracting
the inside population from the total bacterial counts determined
without surface disinfection (Gyaneshwar et al., 2001).
Plants inoculated with strain UAP 5541/pRGS561 were harvested
and separated into roots and stems. GUS activity was tested
daily during the first week after inoculation and at three-day intervals
thereafter. The plants were carefully removed from the growth
medium and roots were gently washed with sterile water in order
to wash the remaining agar away. The staining procedure was carried
out as described by Jefferson et al. (1987). Non-inoculated
plants were analyzed at the same time intervals. Samples were
observed after staining and photographed using a Carl Zeiss Photomicroscope.
Multiple samples were examined either directly or
by using hand-cut sections of plant tissues immersed in agarose
blocks.