plant surface microbiology.pdf

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Galdino Andrade
Nutrient limitation is one of the main characteristics of soils (Edwards
1993). Microorganisms develop strategies in this oligotrophic environment to
capture nutrients and survive the environmental stress. Spore formation is
one of the strategies of bacterium survival when the environmental conditions
are unfavourable for growth. The spore can endure stress conditions for
long periods of time.
When inoculated into the soil, the spore number falls, after this initial fall,
the number of cells stabilised due to vegetative cell sporulation. Petras and
Casida (1985) observed an exceptional fall in the spore number during the
first 2 weeks, but the number of viable spores stabilised in the third week.
However, West et al. (1985) reported that spores of the B. thuringiensis var
aizawai HD137 presented low mortality when inoculated into sterilised soil
and persisted with little decrease in the initial number for 135 days.
4 History of Bacillus thuringiensis-Transgenic Plants
At the end of the 1980s, tobacco plants were the first to receive the B.
thuringiensis cry gene using the Agrobacterium transformation system. The
Agrobacterium tumefasciens system was used in the transformation of several
dicotyledon plants. However, the electroporation and particle bombardment
methods are more efficient for the transformation of monocotyledon and
other dicotyledon plants (Peferoen 1997).
The first transgenic plants obtained showed low expression of the complete
gene for Cry protein production. From then onwards, only truncated genes
were introduced which codify the toxic nucleus of the Cry protein, thereby
increasing the expression in several plants such as tobacco (Mazier et al.
1997), sugar cane (Arencibia et al. 1997) and peanuts (Singsit et al. 1997). Bttransgenic
potato, cotton and maize cultivation (Schnepf et al. 1998) began in
1996 and, today, there are more than 20 transgenic plant species of agronomic
interest on the market.
Promoters, such as CaMV 35 s of the cauliflower mosaic virus and ubiquitinine-1
from maize are being used to increase the expression of the cry gene to
the required levels. In addition to this strategy, greater cry gene expression levels
were obtained by altering the sequence of the gene to increase the cytosine
and guanine content and enhance the expression level from 0.02 to 0.5 % of the
plant soluble protein.However,the most pronounced expression level of the cry
gene in plants (3–5 % soluble protein) was obtained with the introduction of the
unmodified gene in chloroplasts, a cell organelle which has the transcription
and translation apparatus similar to that of prokaryotes (McBride et al.1995).
New generations of transgenic plants have been developed to express other
types of cry genes or genes expressing proteins that could have insecticide
action, such as protease inhibitors, lectin, kinases, cholesterol, oxidases,
inhibitors of the a-amylase and Vip proteins (Sanchis 2000).