Long-lived humans having nothing on trees. Some forest giants, like the Ginkgo biloba, can live more than 3000 years. Now, in the most comprehensive plant aging study to date, researchers have revealed the molecular mechanisms that allow the ginkgo—and perhaps other trees—to survive so long.
The new study provides the first real genetic evidence for something scientists have long suspected: "The default condition in plants is immortality," says Howard Thomas, a plant biologist from Aberystwyth University who was not involved in the work of Ginkgo biloba Tree.
To make this bold claim, researchers started with thin cores from 34 healthy G. biloba trees in Anlu, in China's Hubei province, and Pizhou, in Jiangsu province. (Excising the cores did not harm the trees.) Examining the growth rings, Li Wang, a plant molecular biologist at Yangzhou University, and colleagues discovered that the ginkgos' growth didn't slow down after hundreds of years—in fact, their growth rates sometimes sped up. What's more, the leaf size, photosynthetic ability, and seed quality of the trees—all indicators of health—didn't differ with age of Ginkgo biloba.
To find out what was happening at a genetic level, the researchers compared gene expression in leaves and the cambium, a thin layer of stem cells between the internal wood and external bark that differentiate into other tissues throughout a tree's life. Because older trees have only a few layers of cambial cells, collecting enough material to work with proved difficult, Wang says. The team sequenced the trees' RNA, examined hormone production, and screened miRNA—molecules that can turn specific genes on and off—in trees ranging from 3 years old to 667 years old.
As expected, the expression of genes associated with senescence, the final and fatal stage of life, increased predictably in dying leaves. But when researchers examined the expression of those same genes in the cambium, they found no difference between young and old tree Ginkgo biloba. This suggests that although organs such as leaves perish, the trees themselves are unlikely to die of old age, they report today in the Proceedings of the National Academy of Sciences.
However, there is evidence that the trees do experience some changes over time. The older trees had lower levels of a growth hormone called indole-3-acetic acid and higher levels of a growth-inhibiting hormone called abscisic acid. Those 200 years or older also saw decreases in gene expression associated with cell division, differentiation, and expansion. This means that cambial stem cells in older trees don't divide into new wood and bark as easily as in younger trees.
Plant biologist Jinxing Lin of Beijing Forestry University and an author of the study, says it's possible that if the division rate of cambial cells continues to decline after thousands of years, tree growth could slow, and ginkgo trees might eventually die of old age of Ginkgo biloba. Most trees, however, appear to die from "accidents" such as pests or droughts.
To see whether the trees become more vulnerable to such stressors as they age, the researchers examined genes related to pathogen resistance and the production of protective antimicrobial compounds called flavonoids. They found no difference in gene expression for trees of different ages, suggesting the trees do not lose their ability to defend against outside stressors. That's a "striking" ability that helps the ginkgos grow healthily for thousands of years, says paper author and molecular biologist Richard Dixon of the University of North Texas, Denton.
Not needing to worry about growing old is "something that for humans is difficult to understand," says plant physiologist Sergi Munné-Bosch of the University of Barcelona, who was not involved in the study of Ginkgo biloba. "Aging is not a problem for this species," he says. "The most important problem that they have to deal with is stress."
The researchers say they will continue to study mutation rates in ginkgo trees and examining the mechanisms behind aging. Meanwhile, Thomas and Munné-Bosch both predict that other scientists may use similar methods to study aging in other trees, ranging from short-lived, "lab rat" poplars to towering, ancient sequoias Ginkgo biloba.
