Previous genetic experiments have shown that when plants are grafted together, the whole genome can move between the host and the graft, but it is not clear how the genetic material moves. In a new study, researchers from the Max Planck Institute for plant molecular physiology in Germany have confirmed that whole organelles can move between plant cells and carry their genomes with them. The related research results have been published in the journal Science Advances with the title of “Horizontal genome transfer by cell-to-cell travel of whole organelles”.
Pal Maliga, a plant biologist at Rutgers University (who was not involved in the new study), said that in previous studies, “There is no reasonable explanation for how the whole genome is transferred except into the organelle. Otherwise, you'll see recombination between the incoming genome and the resident genome.” A plastid is a group of small organelles, including chloroplast, mitochondria, and nucleus, which contain genetic material. So people extrapolated that the genome moved through organelles, he explained, “But this paper is a beautiful illustration of what the details are. I am very happy to see that the hypothesis that explained this biological phenomenon at that time actually existed.”
A team led by Ralph Bock of the Max Planck Institute for molecular plant physiology found in a study published in 2009 that grafted tobacco plants can share the entire genome. Horizontal gene transfer - the transfer of genetic material between individuals by some means other than fertilization - has been shown to affect plant evolution at different levels, including the sharing of beneficial genes, but the whole genome movement is less common and less understood. Although initial and subsequent studies have suggested that the transferred DNA is likely to come from the genome contained in the moving organelle, it is uncertain whether the naked DNA has been shared in large fragments.
Alexander Hertle, a postdoctoral researcher at Max Planck Institute of molecular plant physiology, has rich experience in microscopy. He and Bock came together to combine Bock's years of genetic evidence supporting organelle movement with Hertle's imaging expertise. “I'm interested in seeing if what Bock shows in his paper is really true,” says Hertle.
In this new study, Bock and his colleagues used tobacco with an antibiotic resistance gene and a fluorescent reporter gene in the nuclear genome, and another tobacco with a different antibiotic resistance gene and a different fluorescent reporter gene in the chloroplast genome. When they grafted the two tobacco plants together, the resulting plants grew on growth media containing the two antibiotics and produced two fluorescent reporter molecules. These findings are consistent with previous studies, but they are still unable to tell whether genomic information is transferred through organelles or naked DNA.
To make this distinction, the authors dissected the graft junction tissue and imaged the living cells and the fixed cells. This reveals the plastids near the pores in the cell wall and the sites where the cytoplasm of one cell extends to another. These authors also found that plastids are smaller than resident plastids and can pass through intercellular pores. In real-time imaging, these smaller plastids move from one cell to another in an amoeba way. They also confirmed that the moving organelles contained DNA, confirming that they were the source of the genomic transfer they had observed genetically.
Although they can't completely rule out naked DNA moving between cells, “What we see in real-time is that these organelles can move from one cell to another."
Charles Melnyk, a plant biologist at the Swedish University of Agricultural Sciences, who was not involved in the new study, said, “A prominent question is, why does this happen?” He added that the authors had studied the issue in the context of grafting, which is caused by damaging plants. “As long as a plant cell is damaged, it seems to change the characteristics of its cell wall, and then these changes will allow this type of metastasis to occur.”
“Maybe plant cells can do this, even without grafting, but someone has to study it,” Maliga said.
Alexander P. Hertle et al. Horizontal genome transfer by cell-to-cell travel of whole organelles. Science Advances, 2021, doi:10.1126/sciadv.abd8215.