Contents - Faperta

418 Biotechnological Approaches for Pest Management and Ecological Sustainability
transformation have been recorded within a population of P. stutzeri. During infection of
its host, the plant pathogen, R. solanacearum naturally develops a state of competence.
Agrobacterium tumefaciens and P. fl uorescens (Migula) can undergo natural transformation
in soil microcosms. A survey of a collection of rhizobacteria associated with potato indicated
that the kind and diversity of bacteria capable of taking up DNA is much larger than
previously known. The number of transformants decreases proportionally with the size of
the DNA. Experiments following the fate of DNA in tobacco leaves subjected to different
decaying conditions (grinding, enzymatic attack, etc.) have shown a rapid decrease of high
molecular weight DNA in the plant and of transformation frequencies within a few days.
The antibiotic gene used as a marker to select for gene transfer may lead to resistance in
pathogens infecting human beings. However, the general scientifi c view is that the risk of
compromising the therapeutic value of antibiotics is almost negligible. Most genetically
engineered plants contain a gene for antibiotic resistance as an easily identifi able marker.
Hypothetically, antibiotic resistance genes may move from a crop into bacteria in the
environment. Since bacteria readily exchange antibiotic resistance genes, the antibiotic
resistance genes also have the potential to move into disease-causing bacteria. Gene transfer
from plants to microorganisms is possible in laboratory studies (Gebhard and Samalla,
1998), and possibly has happened during evolution (Doolittle, 1999). The probability
of movement of genes from plants to human pathogens (antibiotics) is negligible. Under
laboratory conditions, plasmid transfer between B. thuringiensis subsp. tenebrionis and
B. thuringiensis subsp. kurstaki HD 1 (resistant to streptomycin) strains occurs at a frequency
of 10 2 (Thomas et al., 1997). However, no plasmid transfer has been observed in soil release
experiments, and in insects on leaf discs.
Gene Flow, Selection Pressure, and Enhanced Fitness of Herbivores
Gene Flow and Selection Pressure from Herbivores
Given the absence of information on the identity, level of susceptibility, and ecological
roles of insects exploiting specifi c wild relatives of Bt transgenic crops, efforts should be
made to assess possible consequences of arthropod mortality on resistant wild relatives
(Letourneau, Robinson, and Hagen, 2003). The Bt transgenic B. napus and B. rapa B. napus
hybrids are lethal to larvae of diamondback moth, Plutella xylostella (L.) (Mason et al.,
2003). No measurable plant fi tness advantage (reproductive dry weight) was observed for
Bt B. napus and Bt transgenic B. rapa B. napus hybrid populations at low insect pressure
(one larva per leaf). Establishment of the Bt trait in wild B. rapa populations may increase
its competitive advantage under high insect pressure, although credible scientifi c basis to
confi rm the associated risk is lacking.
One of the hazards in gene transfer from the transgenic plants to the wild relatives is the
possibility of reduced selection pressure (biological control) on the wild relatives from the
pest. If the target pest does not play a signifi cant role in population regulation of the wild
hosts, the gene transfer will not constitute any hazard. The build up of resistance in the
wild relatives can also act as a component of pest management for the target pest if the wild
relative acts as an alternate host, and helps in carryover of the pest population from one
season to another. Resistance to insects and diseases can also make the plant more persistent
in the wild environment, and confer similar advantages on the wild relatives. The chances