Scientific American
10-23-07
Artificial Chromosome Poised to Pump Up GM Crops with Extra Genes
Easier transfer of multigene "stacks" may help biofuel plants and other crops
reach their potential
By JR Minkel
A new method for creating artificial plant chromosomes may pave the way for
engineering transgenic crops faster, along with more bells and whistles such as
better drought resistance, easier refinement into biofuels or even the ability to
manufacture human medicines.
A team of researchers engineered a "maize mini-chromosome" (MMC) by
stitching together a circular loop of DNA designed to fool a corn cell into treating
it like one of its own chromosomes. As planned, reproducing cells copied the
loop and split the copies among their descendants, according to a report in PLoS
Genetics. When injected into single cells that grew into plants, the minichromosome was passed down to up to 93 percent of the plants' offspring for
three generations.
The method should allow plant engineers to introduce a set or "stack" of
approximately ten genes all at once into a desired plant, says Daphne Preuss,
professor of molecular genetics and cell biology at the University of Chicago and
chief scientific officer and president of Chicago-based Chromatin, Inc., which
bankrolled the research. "When you can build your own chromosome, you raise
the ceiling for what's possible," she says.
The most complex commercial transgenic crops to date are so-called triple
stacks, hybrid corn plants containing three genes that confer herbicide resistance
as well as control of corn borers and rootworms. "What we'd really like," Preuss
says, "is to put in 10 or a dozen or more" genes.
The conventional way of engineering transgenic crops introduces a gene or a
stack of genes at random into the plant's chromosomes, which can disrupt
existing genes or strand the new ones in a part of the genome that prevents them
from switching on. Researchers may therefore have to transform hundreds or
thousands of plants to find the ones that work as desired, says Robert Kemble,
head of crop genetics research at Syngenta Biotechnology, Inc., which makes
transgenic corn, soybeans and other plants. Today's stacks are also limited to
delivering about five genes, he adds.
To engineer a less disruptive, higher capacity gene stacker, Preuss and her coworkers assembled from scratch their own centromere, the spot on the
chromosome that the cell latches onto when its DNA gets copied and divvied up
during cell division, and loaded it with DNA encoding a red fluorescent protein,
along with a second marker, for easy spotting.
"It really has the potential to be the next-generation technology for plant
transformation," Kemble says.
Syngenta announced last week that it had licensed Chromatin's MMC
technology. Monsanto, another company that develops genetically modified
crops, announced a similar partnership in May and has said it intends to engineer
eight-stack plants by decade's end.
Geneticists have only just begun to fiddle with crop plants, says Patrick
Schnable, a professor of maize genetics at Iowa State University in Ames
who has consulted for Chromatin. Humans have spent thousands of years
breeding plants for agriculture, but biofuels need much more work to reach their
full potential, he says.
The U.S. has pledged to double its use of ethanol and other biofuels extracted
from crops such as corn and switchgrass by 2012.
Having a delivery method such as the MMC is a "critical piece" of the
reengineering puzzle, Schnable says, but notes that researchers still have to
figure out how to predict gene combinations that will work.