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 mini-chromosome 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 co-workers 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.




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