Bt Research 2012, Vol.3, No.3, 11
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19
15
Effective colonization by cells of
B. thuringiensis
on
the surface of roots of cabbage seedlings demonstrates
that it is possible for this bacterium to exert a
physiological effect on this host plant, an event that is
essential if this microorganism may act as a growth
promoter.
The treatment of seeds with different strains of
B.
thuringiensis
for 5 minutes did not present a growth-
promoting effect on cabbage seedlings. Nevertheless,
further assays should be carried out, with seeds
immersed for longer in the bacterial solution, or even
direct application on roots. This is because the
adherence of bacteria to plants may increase over time
and depending on the initial concentration of the
inoculums (Reis and Olivares, 2006). Additionally,
there may be an interaction between the application
method and the concentration, because the substances
secreted by
B. thuringiensis
may stimulate plant
development in a plant-microorganism interaction
(
Lee et al., 2009).
Observing the seedlings grown
in vitro
by scanning
electron microscope, 30 days after inoculating the
B.
thuringiensis
strains in the seeds, allowed vegetative
cells, spores and crystals of
B. thuringiensis
to be seen
in various parts of the plants. This observation of
vegetative cells did not only demonstrate the
adherence and penetration of this microorganism in
cabbage seeds, but also the infection, multiplication
and establishment in the interior of cabbage seedlings.
Colonization of the various parts of cabbage seedlings
by
B. thuringiensis
was thus proven.
The fact that vegetative cells were viewed by scanning
microscope means that the bacteria may have used
substances from the plant to develop and keep
themselves alive in the seedlings. According to some
reports, the isolates that have most ability to use seed
exudates have a selective advantage in colonizing
roots, because carbohydrates and amino acids are
released abundantly in the form of exudates during the
seed germination process (Subrahmnyam et al., 1983).
The bacteria may have penetrated by emergence of
primary roots (Reis and Olivares, 2006). They may
also have entered during the development of lateral
roots that grow in the direction of the cortex, tearing
the cell layer of the epidermis and emerging to the
exterior. Large cavities created during this process
may have formed sites of infection for the strains of
B.
thuringiensis,
as have already been observed in
bacteria such as
Gluconacetobacter diazotrophicus
(
James et al., 1994) and
Azospirillium spp.
(
Patriquin
et al
.,
1983).
Infection in the roots can also occur due
to abrasion with the growth medium, which promotes
the formation of injuries during the process of root
growth. These injuries are often the gateway for
microorganisms to enter plants (Reis and Olivares,
2006),
a fact that has been observed in
Gluconacetobacter
diazotrophicus
,
where a large number of bacteria were
noted accumulating in the regions where root epidermis
cells had ruptured (James et al., 1994).
The originality of this work with
B. thuringiensis
lies
in the fact that cabbage seeds were treated in bacterial
culture, rather than using sprayed leaves. It appears
that the bacteria adhered to the surface of the seeds,
penetrating by means of natural openings during the
germination process, such as through roots or injuries,
and finally colonizing the cabbage seedlings. Favorable
conditions probably led to colonization, as well as the
absence of reactions on the part of the plant tissues
(
Reis and Olivares, 2006). The presence of
B.
thuringiensis
spores in the ostile proves again that the
sub-stomata chamber functions as a gateway for entry
and egress of the bacteria, a phenomenon reported
previously by Oliveira (2006) for the bacterium
H.
rubrisubalbicans
in sugarcane (
Saccharum officinarum
)
.
Although interaction between cabbage plants and
B.
thuringiensis
is not completely understood, it is
possible that the bacterium reaches a favorable site
and is able to resist removal, which constitutes a
selective advantage (Reis and Olivares, 2006). This
implies that this bacterial species may thus have
colonized the roots, stems and leaves through natural
openings, arriving on the surface of cabbage leaves
via stomata cells by means of evapotranspiration or
exudation.
These events could be observed for strains S1905,
S2122 and S2124 as well as for the standard strain
S1450. Structures from all the
B. thuringiensis
strains
tested were visible adhering to the surface of leaves
and in their depressions, being concentrated around
the stomata. They were also seen in the stomata
chamber.
The colonization of leaves seedlings of cabbage could
