3. Results
3.1. Colonization experiments. G. diazotrophicus
PAL 5 or UAP 5541/pRGS561 were isolated from surface
disinfected and non-disinfected plant tissues and enumerated
in specific media to estimate the epi- and endophytic colonization.
In situ microscopic localization of G. diazotrophicus
UAP 5541/pRGS561 allowed detection of colonization sites.
Tomato plants colonization was not accompanied by inhibition
of plant growth or other macroscopically visible disease
symptom.
3.1.1. Control of disinfection treatment of plant tissues
(1) No zones of blue staining were observed in disinfected
samples from plants previously inoculated with the gusA-marked
strain, (2) no bacterial growth was observed in LGI plates mounted
with disinfected tissues, (3) bacterial growth was rarely observed
when the wash solution from the last tissue rinse was plated on LGI.
Similar to the root tests, no bacterial colonies were found on aerial
parts neither in post disinfection wash tests nor in incubations of
whole aerial tissues. Based on these results, we can assume that the
disinfection procedure employed in this work was sufficient for the
elimination of the surface-associated bacteria, in agreement with
Dong et al. (2003).
3.1.2. Root colonization
Colony counting of plants inoculated with either G. diazotrophicus
PAL 5 or UAP 5541/pRGS561 showed a similar behavior for both
strains (Fig. 1).
The rhizoplane of tomato plantlets was rapidly colonized by
G. diazotrophicus after seedling inoculation: bacterial concentrations
of 7.0–7.5 log CFU g−1 (fw) were observed since 2 days P.I. and
remained almost constant along the 25 days period of observation
(Fig. 1).
Endophytic population remained at 5.0–6.0 log CFU g−1 (fw) for
the first 7 days (Fig. 1). After that, the population showed fluctuations
varying between 4.0 and 6.0 log CFU g−1 (fw) along the
observation period. No bacteria could be isolated from the control
plants (non-inoculated).
Tomato roots recovered at different times revealed the presence
of G. diazotrophicus after GUS technique application. Seedlings
inoculated with strain UAP 5541/pRGS561 showed intense blue
staining in root-stem junctions and in sites of emergence of lateral
roots after 2 days P.I. (Fig. 2). Microscopic observations showed
blue color on root hairs, sub-stomatal cavities of root–stem junctions,
epidermal regions and on the external cell layer as shown in
root transversal sections (Fig. 2). Root tip cells were rarely stained.
Non-inoculated roots did not develop blue zones after GUS staining,
confirming the lack of endogenous -glucuronidase activity
and providing the specificity of the visualization procedure (data
not shown).
3.1.3. Stems colonization
Surface disinfected stems from inoculated tomato seedlings
showed significant bacterial populations after 2 days P.I., varying
around 3.5–5.0 log CFU g−1 (fw) throughout the experiment (Fig. 1).
Stem endophytic bacterial population, although significant, was
lower than the one found in roots (Fig. 1).