Because plants do not possess a proper germline in the way that animals do, undesirable mutations that occur in the soma can be passed on to gametes. In the field of plant genetics, it has generally been assumed that the large number of somatic cell divisions separating zygote from gamete formation in long-lived plants should result in many mutations. However, two recent studies [one in tomato, the other in thale cress (Arabidopsis)] showed that surprisingly few cell divisions separate apical stem cells from axillary stem cells in annual plants –– challenging this view.
To test this concept further, authors [see attached article and editorial] generated and analysed the full genome sequence of two terminal branches of a 234-year-old oak tree (on campus of the University of Lausanne in Switzerland) and found very few fixed somatic single-nucleotide variants (SNVs) –– whose sequential appearance in the tree could reliably be traced back along nested sectors of younger branches. Authors discovered this unexpected stability after sequencing the genome in different branches of the tree. [It should be noted that their work is posted as a bioRxiv preprint, and has not yet been peer-reviewed.] Their data are consistent, however, with a growing body of evidence that plants are able to shield their stem cells from mutations. This behavior in plants may be valuable for sustaining their health over a lifespan that can reach hundreds of years.
Each time an animal cell divides, mutations can arise because of errors made while copying the genome. Animals shield their reproductive cells from these mutations by isolating them (i.e. in testis, ovary) early during development. These cells (called the germ line) then follow a different developmental path, and typically have a low rate of cell division. But plants have cell walls and do not have a dedicated germ line: the cluster of stem cells that gives rise to the reproductive parts of flowers also generates plant stems and leaves. Because of this –– scientists thought the stem cells would accumulate many mutations, and that the DNA of newer branches at the top of a long-lived tree would be remarkably different from that of the lower branches close to the root. But authors found that the number of mutations was much lower than would be expected –– based on calculations of the number of cell divisions that had occurred between the lower branch and the much higher one. A clearer picture of plant development should help breeders as they increasingly focus on long-lived, perennial plants.
Preprint at bioRxiv http://dx.doi.org/10.1101/149203 (2017) [full-length article]
plus Nature 22 Jun 2o17; 546: 460–461 [editorial]