plant surface microbiology.pdf

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Galdino Andrade
significant quantities of bio-insecticide crystals in pollen, leaves and roots,
their effect on the functional groups of soil microorganisms is little understood.
The influence of the mineralogical composition of the soil on the stability
of the bio-insecticide crystals has been reported by several authors. Tapp et al.
(1995a) showed that the toxin is rapidly adsorbed or linked to clay minerals in
the soil, remaining protected from degradation by soil microorganisms. In
another study, Tapp et al. (1995b) showed that the toxin adsorbed by clay minerals
becomes resistant to hydrolytic action of the enzymes produced by the
soil microbiota. It has also been shown that soils with high levels of organic
matter have a high protein crystal adsorption capacity (Palm et al. 1994). Sims
et al. (1996) observed that B. thuringiensis var kurstaki Cry1Ab toxin present
in Bt-transgenic maize tissues incorporated in the soil can be detected by
bioassays with insects susceptible to the bio-insecticide action of the crystal.
According to the results obtained by these authors, the bioassay allows the
detection of smaller quantities of the protein (around 0.5 ng/ml in the diet)
compared to the ELISA test (50.0 ng/g of soil). Sims et al. (1997), working with
bioassays on B. thuringiensis var kurstaki transgenic cotton toxin inactivation
in soils, observed that the toxin mean life in the soil ranges from 15 to 32 days,
and less than 25 % of the initial activity remains after 120 days.
6 Effect of Bacillus thuringiensis and Its Bio-insecticide
Protein on Functional Soil Microorganism Assemblage
Plant roots and their surfaces constitute dynamic habitats densely colonised
by soil-borne microbiota. The high microbial activity in these habitats is due
to a flow of organic substances from the photosynthetic parts of the plants to
the roots (Olsson and Person 1999). This flow consists of low molecular
weight organic substances (e.g. sugars, fatty acids and amino acids), as well as
more complex substances (e.g. starch, cellulose and proteins). The chemical
composition of this organic matter (the rhizodeposition) varies among plant
species and growth stages, and is affected by plant growth conditions (Curl
and Truelove 1986). The functional groups of microorganisms of nitrogen,
phosphorus and carbon cycling are important to the maintenance of nutrient
turnover. These microorganisms interact with the plant roots, supply nutrients
and participate actively in plant nutrition and growth (Andrade 1999).
Mycorrhizal fungi are ubiquitous soil inhabitants and form a symbiotic
relationship with the roots of most terrestrial plants. When arbuscular mycorrhizae
(AM) form, there are significant changes in the plant and root physiology.
Photosynthetic rates increase and the nutritional status of the host tissues
changes and thus, the quality and quantity of root exudates (Linderman
1992). Altered exudation induces changes in the composition of microbial
communities in the rhizosphere soil (Andrade et al. 1997) that may influence