plant surface microbiology.pdf

7 The Functional Groups of Micro-organisms and Biotech Products 125
5 Persistence of the Protein Crystal in the Soil
The release of the use of transgenic plants with cry genes for agricultural pest
control in the 1990s has raised a lot of controversy and concern in the scientific
community. It is common sense that four factors must be very carefully
assessed: (1) the potential of selection for insects resistant to the Cry proteins,
(2) the persistence of the crystal released by the root exudates and lysis in the
soil, (3) the non-selectivity towards other non-pathogenic insects, (4) the
impact of the bio-insecticide crystal protein on the functional groups of soil
microorganisms.
Several studies on the persistence of the B. thuringiensis toxin released by
transgenic plants into the soil have shown that the toxin degradation is relatively
quick during the first 45 days, and less than 25 % of the initial bio-activity
is maintained after 120 days (Palm et al. 1994, 1996; Sims and Holden 1996;
Sims and Ream 1997). On the other hand, Tapp et al. (1995a) showed that part
of the insecticide activity of B. thuringiensis may be maintained because of
the rapid adsorption and binding of the toxin in the soil clay minerals. Other
authors reported that a substantial proportion of the Cry1Ab toxin released in
the Bt-maize roots exudates could be detected and maintained their bioinsecticide
activity for 234 days after release into the soil. This indicates that
the protein crystal is very stable in the soil and is protected from microbial
action due to adsorption by the soil clay (Saxena et al. 1999). Experimental
results on adsorption of the protein to soil particles (Saxena and Stotzky
2000) indicated that the cry1Ab gene coded toxin released by Bt-maize root
exudates in sandy soil supplemented with montmorillonite and caullinite
bound preferentially to clay minerals. This confirmed that adsorption to clay
minerals is one of the main factors in the permanence and activity maintenance
of the bio-insecticide crystals in the soil.
Crystals were detected by the ELISA method (enzyme-linked immunosorbent
assay). The permanence of the toxin was also determined in other soil
types, with predominance of those with low organic matter content. Palm et
al. (1994, 1996) observed a fall in the purified toxin concentration in the soil in
the first 14 days after inoculation, with stabilisation after this period. In the
case of transgenic plant-produced toxin, the fall in the soil concentration
occurred for about 10 days and then remained stable. However, toxin production
was continuous throughout the plant lifecycle with a consequent accumulation
of bio-insecticide crystals in the soil. The lowest rate of recuperation
after extraction was obtained in soil with a high quantity of organic matter,
indicating that much of this protein may also be adsorbed by the soil organic
matter.
Saxena and Stotzky (2000) observed that the B. thuringiensis toxin
expressed in Bt-transgenic maize was released into the rhizosphere through
exudates and lysates and that much of the released crystal remained active
for several months. Although Bt-transgenic plants produce and accumulate