GENETICALLY ENGINEERED MUSTARD IN
2. GE TECHNOLOGY TO PRODUCE HYBRIDS
C Kameswara Rao
Foundation for Biotechnology Awareness and Education,
For reasons inherent in the
reproductive biology, the highly inbred Brassicas are
difficult to hybridize, using conventional breeding techniques. The hybrids suffer from low vigour making them unsuitable for commercial
cultivation. Vigour can be greatly enhanced in the progeny from crosses of genetically distinct
parents, to outperform the parental lines. This phenomenon called ‘hybrid vigour’ has
been a great boon in plant breeding.
NATURALLY OCCURRING MALE STERILITY IN CROP PLANTS
For a time, plant breeders tried to
use male sterility, the absence of functional pollen while the female gametes
were normal, to produce hybrids in highly inbred crops.
Naturally occurring gene controlled
male sterility occurs only sporadically among crop plants, as they were always
selected for high levels of fertility. Nevertheless, natural male sterile plants were exploited in hybrid seed
production in such crops as cotton, tomato, sunflower, cucurbits, tobacco,
rice, wheat, barely, maize, sorghum and pearl millet.
In hybrid production, several lines
of male sterile (female fertile) plants are planted alternately with one or two
lines of male fertile (pollinator) plants, which also have fertile female
gametes. The undesirable gene
combinations formed from the female gametes of the male fertile plants, have to
be identified and removed. Such a
procedure is difficult to be performed manually even in experimental situations
and impossible in cultivated crop fields.
Decades of research on canola and
mustard led to the identification of very few male sterile lines and imperfect
restorer female lines, making the natural system commercially unviable. Robert Goldberg’s team developed the barnase/barstar genetic system over a decade ago to
overcome this handicap. The objective
is to produce hybrids to exploit hybrid vigour ensuring higher crop performance.
THE BARNASE-BARSTAR GENE
Separate male sterile (MS) and
fertility restorer (RF) lines developed through GE are used to emulate the
natural phenomenon of hybrid vigour. Crosses of the
MS line with the RF line ensure the production of fully fertile hybrids, which
are employed in agricultural production.
The barnase gene, from the bacterium Bacillus amyloliquefaciens, encodes the enzyme barnase (ribonuclease), which is produced at a specific stage early
in the development of the anthers (the pollen bearing parts of the flower) and
in a specific cell layer (called tapetum) of the
anthers. Barnase prevents pollen production, conferring male sterility.
The barstar gene, also from Bacillus amyloliquefaciens, encodes
an enzyme that inhibits barnase. Expression of the barstar gene is also restricted to the anthers.
The hybrid plants derived from crosses
of MS and RF lines are fully fertile, as the expression of the barnase the barstar gene inhibits gene.
Elimination of undesirable hybrids:
The bar gene from the
bacterium Streptomyces hygroscopicus and pat gene from the bacterium Streptomyces viridichromogenes, encode for the enzyme phosphinothricin acetyl transferase,
that detoxifies glufosinate ammonium and confers
tolerance to herbicides with this active ingredient. Genes linked to other herbicides may also be
The herbicide tolerance trait is
used as a selection tool for the barnase-barstar breeding system, to eliminate unwanted hybrid genotypes, by spraying an
appropriate herbicide. This trait also
enables to control weeds in the canola crop, in conjunction with other
The bar and pat genes are good markers useful to detect gene
flow, by spraying an herbicide. Technology now exists to remove herbicide resistance genes at the time
mass production of seed for cultivation purposes, if this was necessary for
political or public concern reasons, though not for scientific reasons.
The nptII gene from the bacterium Escherichia coli confers resistance to the
antibiotics neomycin and kanamycin. Such antibiotic
resistance traits are used as selectable markers in the initial laboratory
stages to screen genetically modified plants. Now non-antibiotic selectable markers are employed.
The barnase/barstar technology:
The whole of the process of male
sterility and selective removal of undesired hybrids from the male fertile
plants, is performed elegantly by the barnase/barstar gene system, in which herbicide resistance
is linked with male fertility, so that the herbicide will kill the male fertile
lines, leaving the seed producing male sterile plants unharmed. The system is used only to develop novel
hybrids and the farmer is provided with highly viable seed of uniform quality
that produces fully (male and female) fertile plants.
The whole set of gene systems used
in canola and mustard are a complicated but an ingenious development in rDNA technology that can confer nuclear male sterility to
self-pollinating plants in a stable manner to produce hybrids using a female
restorer line. It helped to produce
improved hybrids of such crucifer crops as canola and mustard, which is
impossible without this gene system. The barnase/barstar gene system is meant to produce
hybrids, and not herbicide resistant/tolerant crops, as alleged by come
The World’s experience with GE Canola:
Aventis has successfully introduced a GE
canola hybrid, using the barnase/barstar gene system in 1996. Since then, high
yielding GE hybrid canola cultivation has expanded to over a couple of million
hectares in North America and
. Regulatory agencies in such countries as
have approved consumption of
this GE canola. The health and
environmental safety questions about the technology have been settled beyond
any reasonable doubt.
Roundup Ready is a GE canola
resistant to Monsanto’s herbicide Roundup and AgroEvo’s GE canola is resistant to their herbicide
The same barnase/barstar gene system is being used in
produce mustard hybrids.