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 Roger 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.