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Plant Cells of Different Species Can Swap Organelles

Reporter : Irina Robu, PhD

Farmers have used plant grafts to grow fruit trees and grapevines, but plant grafts also occur in nature when closely related plants that touch each other eventually fuse, or when parasitic plants form connections to their hosts. At the graft site, the plants form a kind of scar or callus, that reestablishes the flow of water and nutrients through vascular tissues across the wound and sometimes gives rise to new shoots. Plant geneticists noticed that two plants that grew together, the cells of each plant showed signs of having picked up substantial amounts of DNA from the other one. They knew that horizontal transfer of genes is not uncommon in bacteria, even animals, fungi and plants but in this case, the transferred DNA seems to be the entire intact genome of chloroplasts.

And in order to understand this, researchers at Max Planck Institute of Molecular Plant Physiology, in Dr. Ralph Bock’s laboratory discovered that not only are cell walls sometimes more porous than was thought, but plants seem to have developed a mechanism that enables whole organelles to crawl through the cell wall into adjacent cells.  The genetic transfer between plants was not only interesting, but a challenging puzzle. The fact that the only openings in cell walls were tiny narrow bridges (0.05 microns) that allow adjacent plant cells to exchange proteins and RNA molecules. The chloroplast, typically about 5 microns in diameter looked like it miraculously showed up in the other cell.

Researchers in Dr. Brock’s lab were determined to see what exactly was going on with the callus at graft site. He was able to observe that the cells had openings larger than previously noticed, up to 1.5 microns across. While seeing live cells in the callus, he noticed that the chloroplasts can migrate. Some of the chloroplasts changed into more primitive, more motile proto-plastids that could get as small as 0.2 microns and the proto-plastids crawled along the inside of the cell membrane positions underneath the fresh discovered holes in the cell wall. Budlike protrusions of the cell membranes then protruded into neighboring cells and transported the organelles. As the tissue organization in the graft reestablished itself, the plastids returned to the normal size for chloroplasts. 

Even though the metamorphosis of the chloroplasts is not understood, it seems that carbon starvation can lead to photosynthesis. And how well transferred plastids function in their new host cells depend on the related the two species are. If the genetic If the genetic mismatch with the nuclear DNA is too extreme, the organelles may fail to work and will eventually be lost. But they could thrive in the cells of close relatives.  Whole-organelle migration can help clarify the observation that the chloroplasts from clumps of different species. They hypothesized that plants move chloroplasts between cells routinely in response to injuries or other events. The researchers point out that once a graft callus starts to produce roots, shoots and flowers, it could give rise to a new species or subspecies.

SOURCE

https://www.quantamagazine.org/plant-cells-of-different-species-can-swap-organelles-20210120/

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