Transgenic
Bt technology
-C Kameswara Rao Foundation
for Biotechnology Awareness and Education, Bangalore, India
pbtkrao@gmail.com
Transgenic technology, involving a wide range of
pesticidal genes from the bacterium bacillus thuringiensis (Bt),
dominates the scenario of agricultural biotechnology. At the same time,
Bt technology is also the most focused target of vehement anti-tech
activism.
While the terms Bt cotton, Bt corn, Bt potato, etc., are familiar, the
level of understanding of what the technology actually means, what it
can and what it cannot do, is very poor. A variety
of issues such as the biology of bacillus thuringiensis, its proteins,
use of Bt as a biopesticide, transgenic Bt crops, benefits and
limitations of the technology and biosecurity, are important components
of public awareness.
Bacillus thuringiensis
Bt is a rod shaped, gram-positive, soil bacterium, discovered in
1901. Bt is among the most thoroughly studied bacterial
species of agricultural importance, its diverse aspects having been
researched for over a century. The book
‘Bacillus thuringiensis: Biology, Ecology and
Safety’ (T.R. Glare and M. O’Callaghan, 2000, John
Wiley) refers to over 8,000 research publications by over 10,000
biologists, in over 60 years, and deals with most of the issues raised
against the use of Bt. Ignorance of this and other subsequent
publications on Bt or a deliberate indifference to them, led to a much
exploited misunderstanding of Bt technology.
Concept of Bt
The term Bt now refers to not a single simple species entity, but to a
large group of subspecies and varieties, based on over 60,000 isolates,
collected from all over the world. There are more
than 80 serologically characterized (using specific antibodies) types
of Bt.
The controversy about distinguishing Bacillus thuringiensis from the
related pathogenic Bacillus cereus and Bacillus anthracis was
adequately addressed (de Maagd, Bravo & Crickmore, July
2005). When types of Bt can be identified
serologically, a microbiologist can certainly distinguish the three
species.
Bt in nature
Bt is a universally occurring soil bacterium, isolated from several
thousand soil samples from 80 different countries. It commonly occurs
also on the aerial parts of plants such as leaves and on even washed
fruits and vegetables we consume. It may be present in water, possibly
as a wash off from the soil and plant surfaces. Bt
may be transported in the atmosphere, as inferred from its presence
deep in the polar ice cap.
Bt grows and competes, but poorly in soil. Bt or
its proteins may persist for about 100 days in soils, for 24 hr in
running water and for 12 days in stagnant water bodies. Bt
seems to require an association with plants and insects to perpetuate
for longer periods in nature.
Bt as a biopesticide
Bt produces a wide range of insecticidal proteins that have been in use
in pest control since 1938. There are about a 100 biopesticides
exclusively based on Bt and over 90 per cent of commercial
biopesticides, used even in organic farming, contain
Bt.
Bt proteins and their
encoding genes
Bt produces a large number of proteins that are toxic to specific
insect groups under specific conditions. Bt also
produces a) several enzymes, b) some compounds that lyse erythrocytes,
and c) some that are enterotoxic to vertebrates. Bt toxins are produced
either within the bacterial cell (endotoxins), or on the cell surface
(exotoxins).
More than 170 toxin-encoding genes have been isolated form Bt
collections. Among the endotoxins, the insecticidal crystalline
proteins, called the delta-endotoxins, are significant in Bt
technology. The crystalline proteins are described
para-sporal, as they are co-produced and co-exist along with spores
(the means of bacterial propagation), in the bacterial cells.
When the bacterial cell lyses to release the spores, the crystalline
proteins are also routinely released into the soil.
The names of the genes that encode the crystalline proteins are
prefixed with ‘Cry’, as for example Cry1Ab, Cry1Ac,
Cry9c, etc., and the proteins that are encoded by these genes are
‘Cry’ proteins. The
non-crystalline endotoxins are prefixed with ‘Cyt’.
Pest specificity of Bt
toxins
Bt proteins are per se not toxic. To function as toxins Bt proteins
require a specific set of biochemical and biological parameters which
are available for different Bt proteins only in specific insect groups,
which makes Bt toxins insect group specific. Cry1Ac and Cry2Ab control
the cotton bollworm, Cry1Ab controls corn borer, Cry3Ab controls
Colarado potato beetle and Cry3Bb controls corn rootworm. The Bt genes
that are incorporated into different crops are specific to moths and
butterflies (Lepidoptera, having wings covered by
scales).
Pre-requisites for
pesticidal activity of Bt proteins
The following conditions are essential for an effective insecticidal
activity of the Bt proteins:
- The pest must take a few bites of the plant tissue; Bt
transgenics are not effective against sucking pests (Homoptera, with
wings without scales), as they do not ingest plant tissue.
- An alkaline environment (pH 9.5 and above) in the gut of
the insect pest is essential for the Cry proteins to dissolve in the
gut fluids and to be converted into an active molecule to function as
an insecticidal compound (mammalian stomachs are highly acidic).
- The pest specificity of different Bt toxins depends upon
the presence of appropriate receptors, in the lining of the mid-gut
(brush border) of the pest, which are absent form some pests, as
evidenced by different Bt proteins being toxic to specific pest
species. The toxin binds to the receptors and causes
disturbance in the integrity of the gut wall, leading to leakage of the
contents, followed by starvation and death of the
pest.
Fundamentally, the alkaline gut environment and the presence of an
appropriate toxin binding receptor are crucial for insecticidal
activity of Bt proteins. Basing on such requirements, the genes that
encode pest specific toxins are chosen for developing different
transgenic crops.