Genome complexity: plants swap organelles, DNA intact (Introduction)

by David Turell , Thursday, January 21, 2021, 04:29 (1402 days ago) @ David Turell

They sneak into tiny pores in cell walls:

https://www.quantamagazine.org/plant-cells-of-different-species-can-swap-organelles-202...

"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.

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“'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.”

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"The mystery persisted until Bock teamed up with his postdoctoral fellow Alexander Hertle, who had expertise in live-cell imaging and microscopy. Hertle was determined to look at what was going on in the callus. Examining thin sections of the graft with electron microscopy, he saw that the cells had openings larger than any previously seen. But even those, which were up to 1.5 microns across, seemed too narrow for the chloroplasts.

"Then, while observing live cells in the callus, Hertle caught images of the chloroplasts in the act of migration. Some of the chloroplasts changed into more primitive, more motile proto-plastids that could get as small as 0.2 microns. As Hertle watched, the proto-plastids crawled along the inside of the cell membrane to positions beneath the newly discovered holes in the cell wall. Budlike protrusions of the cell membranes then bulged into neighboring cells and delivered the organelles. As the tissue organization in the graft reestablished itself, the plastids returned to the normal size for chloroplasts.

“'So there’s definitely holes in the cell wall that would allow the plastids to move through,” Hertle said. The dogma that a plant cell wall is a thick, more or less permanent barrier “basically disappears with this study.”

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"It’s not clear yet how frequently this kind of horizontal genome transfer through organelle migration occurs in nature. Perhaps plants move chloroplasts between cells routinely in response to injuries or other events; no one knows. Bock, Maliga and other researchers were able to document genome transfers only because the differences in the grafted tissues gave away what was happening. But if plants have evolved a mechanism for organelle transfers, then relatively rare natural grafting events may be only one occasion for them.

"Common or not, the phenomenon might have evolutionary or ecological implications. Hertle points out that once a mosaic cell in a graft callus starts to produce roots, shoots and flowers, it could give rise to a new species or subspecies, especially if cell walls open wide enough to admit nuclear genomes."

Comment: I can easily see this as a God-designed method for plant evolution and new species creation.