By Susan Milius
Plants of different species can swap chloroplasts, the little cellular factories that capture energy from sunlight, when stems graft together. The surprising discovery may explain why evolutionary histories based on chloroplasts sometimes disagree with those based on other sources of DNA.
“If you had asked me before I did this work, I would have said, ‘This isn’t happening,’” says plant geneticist Pal Maliga of Rutgers University in Piscataway, N.J.
Chloroplasts contain their own genetic material, which is typically passed to offspring as mother plants form seed. Now it appears that two plants of different species can exchange chloroplast DNA nonreproductively, by swapping the whole cellular organs through a graft, Maliga’s team and an independent group in Germany report online January 30 in the Proceedings of the National Academy of Sciences.
“It’s genetic engineering done by Mother Nature,” declares geneticist Ralph Bock of the Max Planck Institute for Molecular Plant Physiology in Potsdam-Golm, who led the team in Germany. Plenty of grafts fuse plant to plant in nature, he says, and traveling chloroplasts now offer one explanation for chloroplast-based evolutionary family trees that don’t agree with trees based on DNA from other cell structures.
Chloroplast swapping by grafts may also help researchers engineer better plants, Maliga says. Plants would need to be closely related enough for their tissues to fuse, but grafting could simplify such tasks as introducing specially efficient chloroplasts into lineages of plants, like potatoes, that can lose desirable trait mixes during the genetic shake-up of producing seeds.
It has been hard to imagine plants swapping whole cellular organs, Maliga says, because unlike animals, plants encase each cell with a rigid wall. Connections do string together plant cells, but these living links, called plasmodesmata, typically carry smaller stuff, such as protein molecules.
In 2009, Bock reported seeing genetic material moving through grafts between tobacco plants of the same species. To see if chloroplast DNA would make a bigger jump between species, he and his colleagues set up a series of grafts using various combinations of the cultivated tobacco, another herbaceous tobacco species and a third tobacco that grows into a tree. One member of a graft pair carried genes in its cell nucleus to resist one kind of antibiotic, and the other graft partner carried chloroplast genes that resist a different antibiotic. To detect cells that had borrowed a chloroplast, researchers dosed graft tissue with both antibiotics and looked for cells that still flourished.
Maliga took much the same approach, but grafted cultivated tobacco with yet another tobacco species. The result that really surprised him, he says, was that all the genetic material of a chloroplast seemed to move to the new home instead of just fragments infiltrating the graft partner’s resident organs. He does not detect any genetic mashups of chloroplasts from the two partners, and he argues that grafts can set whole chloroplasts on the move.
If grafts somehow allow inter-species travel for chloroplasts, researchers can wonder about other cell organs. Mitochondria, which convert food to stored energy, also have DNA useful for creating plant family trees. “It would be really interesting to find out if mitochondrial movement also occurs,” says Pamela Soltis, an evolutionary geneticist at the Florida Museum of Natural History in Gainesville.
