More than a decade ago, plant geneticists noticed something peculiar when they looked at grafted plants. Where two plants grew together, the cells of each plant showed signs of having picked up substantial amounts of DNA from the other one. In itself, that wasn’t unprecedented, because horizontal transfers of genes are not uncommon in bacteria and even in animals, fungi and plants. But in this case, the transferred DNA seemed to be the entire intact genomes of chloroplasts. This posed a conundrum, because plant cells seal themselves inside a protective cell wall that offers no obvious way for so much DNA to get in.
“The real novelty is that they’ve shown the actual physical organelle is moving, [and] not only from one cell to another,” said Charles Melnyk, a plant biologist who studies grafting at the Swedish University of Agricultural Sciences in Uppsala. “It’s two different plants that are exchanging organelles.”
“This is not what you would expect from a plant cell,” said Pal Maliga, a plant scientist at Rutgers University who has independently found genetic evidence for the transfers of chloroplasts and mitochondria inside grafts. Plant cells are armored with a stiff cell wall, so “my image of a plant cell was the cytoplasm sitting in a cage, and nobody goes anywhere,” Maliga said.