assessment of the release of transgenic crops: spread of herbicide resistance
genes from wheat and foxtail millet to weedy species
The need for herbicide resistant wheat in developing countries is immense.
If biotechnologically derived resistant varieties are released, a major
biosafety issue that needs to be investigated, is the ability of any local
weeds to acquire resistance genes from wheat, creating a situation which
would jeopardise herbicide use. Although crosses of wheat with wild relatives
are reputed to be unlikely and to produce sterile hybrids, a recent report
described fertile spontaneous hybrids between wheat and Aegilops cylindrica,
a pernicious weed found in the United States. Wheat has three different
genomes, A, B, and D. Only D is common with Aegilops cylindrica,
but other wild relatives have genomes with partial homologies to A, B
or D, which can make gene flow easier. However, nothing is known about
the behaviour of hybrid descendants in the field. In contrast, the experimental
study of the frequency of resistance genes in weed field populations can
be studied with foxtail millet, which is as self-fertile a cereal as wheat.
Descendants of hybrids between millet and wild relatives have already
been described by botanists and three independent herbicide resistances
have been bred using classical techniques. The main objective of this
project is to provide scientific information on the possibility and probability
of transgene escape from biotechnologically-derived, herbicide-resistant
wheat, to wild plants, via spontaneous interspecific hybridisation. This
includes studies in different regions where different wild relatives are
indigenous (including China, France, Israel, Spain and Switzerland), studies
on different wheat transformants, and different modes of inheritance of
transgenes that could be less prone to transferring the gene. The impact
of the transfer of herbicide resistance genes into a wild population is
also addressed using millet as a model for cereals.
Approach and methodology
Four key approaches are being undertaken. The first involves the development
of genetically different types of herbicide-resistant wheat from a large
number of transformations and their characterisation using introgression-substitution
lines and DNA tools to identify on which genome the resistance gene was
located. Herbicide resistance is both a very desired trait in agronomy
and a good marker to detect gene escape and exert a clear-cut selection
The second approach involves the identification of grasses that can breed
with wheat in specified ecological conditions and countries. This includes
field study of flowering time and weed characteristics of wheat-related
grass species, the estimate of the possibility of the intercrossing of
wheat and the wild relatives by hand-crossing and embryo rescue techniques,
and spontaneous interspecific hybridisations in the greenhouse and the
garden. The rate of spontaneous mutations in cell culture of wild plants
will be compared to gene escape from transgenic wheat.
The third approach involves a comparison of the growth, developmental
fitness and natural reproduction of the different hybrids to those of
the parents. This is followed by monitoring the introgression between
wheat and wild relatives by means of chromosome identifying tools and
DNA markers among hybrid progeny and further generations.
The fourth approach examines the collection of data on the spread of resistance
genes after a large scale release of a resistant crop and the behaviour
of resistance genes when inherited in different ways and under different
regimes of selection pressure. This applies to herbicide-resistant millet
used as an advanced model because resistant varieties are already available.
Main findings and outcome
Several experimental approaches and laboratory techniques have been established
including the development of wheat lines which are resistant to an inexpensive,
environmentally safe herbicide, and a quick test for screening and evaluating
Dalapon resistance. The first interspecific hybrids between wheat and
one of its wild relatives have also been produced. Wild relatives of wheat,
and conditions leading to increased risks in fields, have been identified.
Other techniques which have been established are a suitable and quick
test to identify on which wheat genome (A, B or D) the transgene is located,
cell culture conditions for several wild grass species, and the identification
of molecular and cytogenetic markers specific for millet and wild relatives.
This project will help provide genetic engineering recommendations to
wheat breeders to prevent uncontrolled gene escape by hybridisation between
wheat and related grass weeds in the field. It will determine the likelihood
of the spread of resistance genes into a weed population, the biosafety
implications of such movements, and the extent (if any) of negative impact
on farming activity and on the environment. It aims to propose guidelines
about actual gene flow and its management, providing more information
on the risk/benefit balance, and minimising the risks as far as is possible.
Gressel J., "Molecular biology of weed control".
Transgenic Research, 9, 2000, pp. 355-382.
J., "Potential failsafe mechanisms against the spread and
introgression of transgenic hypervirulent biocontrol fungi".
TRENDS in Biotechnology, 19, 2001, pp. 149-154.
February 1999 - January 2002
The Weizmann Institute of Science
Instituto Nacional de Investigación y Tecnología Agraria
Hebei Academy of Agricultural and Forestry Sciences
Chinese Academy of Agricultural Sciences
University of Bern (CH)