Last Friday, I was a guest on the radio show Science Friday with photographer Rachel Sussman. We talked about her new book, The Oldest Living Things in the World, for which I wrote the introduction. You can listen here. (And you can read my whole introduction to Sussman’s book here.)
I talked on the show about some of the ideas that scientists are exploring about why some species live for a long time and some don’t. What’s intriguing me a lot right now is how old some plants can get. There are a few animal species can reach a very ripe age, such as clams that live for five hundred years. But Sussman’s book is dominated by plants–by ancient trees and shrubs and such–that can live for many thousands of years.
Scientists can’t offer a simple, straightforward answer to why plants can get so much older than animals. But they have gathered a lot of intriguing evidence that may lead them to one. For one thing, the biology of aging is different in some important respects in animals and plants, as Howard Thomas of Aberswyth Aberystwyth University in Scotland Wales explained last year in the journal New Phytologist.
As we animals get older, things go wrong. For example, as our cells divide, their DNA sometimes mutates. This can cause the cells to malfunction or even turn cancerous. This burden of mutations only gets greater the older we get. We can try to fix this damage–repairing DNA, killing off defective cells, and so on–but that takes a lot of energy, energy that animals could otherwise use for other purposes, like reproducing.
Plants don’t seem to have to deal with these challenges. Trees that are 4700 years old don’t have more mutations in their cells than much younger plants. It’s possible that they lack those mutations because a kind of evolutionary struggle taking in the tissues of old plants. If some cells suffer mutations, other cells that are in better shape will take over and continue to grow healthy tissue.
Thomas also suggests that plants aren’t trapped in a trade-off between repairing their cells and growing like animals are. That’s because animals have to eat their food, while plants manufacture theirs from the sun and the air. They’ve got a lot more energy to work with.
The very bodies of plants may also give them an opportunity to grow very old, while ours do not. An animal is made up of two kinds of cells: somatic cells that make up most of the bodies, and a small collection germ cells (sperm or eggs) that can give rise to a new animal. That division gets established early in the development of an animal embryo and never changes. Plants don’t have such a stark division between somatic and germ cells. As they grow, they add new modules, each of which may produce germ cells (hence, a cherry tree is covered in blossoms, rather than just one blossom). Some of those modules may get stressed and even die, but the other modules can survive and continue to grow.
Recently a team of scientists from Ghent University in Belgium pointed out in Trends in Cell Biology that plants are different from animals in another respect: their stem cells.
Stem cells, which grow in both animals and plants, have the potential to grow into new tissue. In animals, they can maintain a healthy, young body. If they stop rejuvenating muscle, skin, and other tissue, an animal becomes old. (See my post this week for more details).
Plants have stem cells, too, which are concentrated where the plants are putting on new growth, such as their stems and root tips. But they also have what you might call stem cells for stem cells. Known as quiescent cells, they form a tiny patch in the middle of a cluster of stem cells. They grow very slowly, and each time a quiescent cell splits in two, one of the new cells becomes a true stem cell. That new stem cell divides rapidly into still more stem cells, which in turn can develop int a root or a leaf or some other part of a plant. But the other cell from that original division is yet another quiescent cell, which remains behind in reserve.
Quiescent cells appear to be vital to plants. If scientists remove all the quiescent cells from a root, for example, some of the stem cells in the root will turn into new quiescent cells. It’s possible, the Belgian scientists write, that they are also crucial to the ability of plants to keep rejuvenating for a long time. They can create a supply of stem cells for millennia.
After thousands of years, in other words, a bristlecone pine may still be young at heart.