plant surface microbiology.pdf

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Galdino Andrade
decrease in the viable vegetative or spore form Bacillus units in the soil. Reddy
and Rhae (1989) reported that a strain of B. subtilis introduced into an onion
rhizosphere at a concentration of 7.2¥10 5 seed –1 could be recovered at only
7¥10 3 plant –1 after 30 days, despite the decrease in numbers, the B. subtilis was
effective in suppressing indigenous soil microbiota in the rhizosphere. Young
et al. (1995) also observed that B. cereus survival was not influenced by developing
wheat roots and the absence of a rhizosphere effect may be due to the
fact that B. cereus was isolated originally from non-rhizosphere soil. The
Bacillus spp. are often reported to be present in low numbers in the rhizosphere
compared with other bacteria, such as fluorescent pseudomonas (Elliot
Juhnke et al. 1987).
Populations of C, P-cycling microorganisms and formed nodules changed
during the plant growth period and were influenced by B. thuringiensis inoculation
in soybean plants. No differences were found on assemblages of bacteria
and fungi in soil inoculated with B. thuringiensis, but time influenced the
populations. The time corresponded to plant growth, AM root colonisation
and nodule formation. Some physiological changes during plant growth,
including C compounds released to the medium, influenced bacteria growth
(Amora-Lazcano and Azcón 1997). AM colonisation and Bradyrhizobium
japonicum nodulation normally decreased the amount of plant root-derived
and organic matter available for heterotrophic bacteria and other soil
microorganism growth by altering the root cell permeability, thus affecting
exudation (Schwab et al. 1983). The carbon cycling microbiota populations
also decreased their number of cells, possibly because of changes in C concentration
in the rhizosphere. Negative correlation between symbiotic and
cellulolytic, amylolytic and proteolytic microorganisms shows that carbon
compounds from the root are important factors for their proliferation. Deleterious
effects of AM roots on soil bacteria have also been observed, suggesting
C competition (Marschner and Crowley 1996), although AM fungi and
rhizobia do not consume C from the rhizosphere due to their symbiotic condition
(Secilia and Bagyaraj 1987; Paulitz and Linderman 1989). Cellulolytic
and amylolytic microorganisms decreased their cell number during the
experiment, whereas proteolytic microorganisms increased their population
the first time. This result suggested that this group had an extra supply of
nutrients from inoculated crystal protein. The faster decrease in the proteolytic
cell number after day 15 could be explained by the small amount of ICP
free in the soil. Saxena et al. (1999) suggested that ICP binds rapidly and
tightly to clays and humic acids and is protected against microbial degradation
by being bound to soil particles. AM infection and nodule number
increased in the time following the plant growth. The saprophyte fungi population
decreased when the soil was inoculated with Cry- strain, and the same
effect was observed in AM infection. In another experiment carried out under
axenic conditions in Petri dishes, Cry– and Cry+ strains showed an inhibitory
effect against the growth of some saprophytes fungi. This fungistasis effect