Sci. Aging Knowl. Environ., 20 November 2002
Plants' capacity to renew spent parts thwarts aging, but our green cohabitants might not elude the process completely
Abstract: Compared with animals, plants seem to defy age. Many trees thrive for thousands of years--or maybe more. Some scientists attribute plants' knack for staying fresh to their segmented body plan and ability to renew spent parts. Yet emerging evidence suggests that plants can't elude feebleness entirely. They apparently suffer from the same wear and tear that animals do, an observation that calls their ultimate resilience into question.
When it comes to long-lived species, the plant kingdom boasts a bumper crop. Trees that survive for 80 years are short-lived compared with others, such as bristlecone pines, which can endure for more than 5 millennia without a hint of getting old (see "Growing Old in Style"). Although aging in most animals appears inevitable, whether plants show gradual breakdown with passing years is open to question.
The evidence for or against aging in perennial plants--those that live for an indefinite number of years--remains sparse, but a handful of researchers, from foresters to evolutionary biologists, are now focusing on the issue. By sorting out how plants do and don't hold up, scientists hope to expand their knowledge about how and why living things deteriorate over time. Insights into how plants resist feebleness might reveal crucial age-defying characteristics, potentially providing humans with some life-extending tricks.
Plants' unique body plans might hold the key to immortality. They are composed of organs that act as independent agents that can age and even die while others are being born, allowing the individual to thrive beyond the life-span of its parts. Although whole-plant aging remains contentious, the life and death of plant parts, such as leaves and stems, is well understood from studies of the popular laboratory plant Arabidopsis thaliana. Some scientists think that plants exploit their modular structure and regenerative skill to overcome life's seemingly unavoidable deterioration and the death sentence it spells for other species.
Support for plants' ability to circumvent the damage that bogs down animals stems mostly from observations made while propagating plants from cuttings. Replanting a branch from a 200-year-old tree can produce a new tree, says Richard Amasino, a plant biochemist at the University of Wisconsin, Madison. "In that sense," he says, "they are basically immortal."
However, emerging research challenges that idea, showing that oxidative damage--a side effect of metabolism for both plants and animals and a possible promoter of aging--mounts over time in plant cells. Injury generated by waste products is a major suspect behind old age in animals (see "The Two Faces of Oxygen"), so the observation that it builds in plants suggests that even the longest lived greenery might decline eventually but at a rate that's so slow, it's difficult to discern.
When it comes to questions of age in perennials, we're not out of the woods yet, says Barbara Bond, a forest biologist at Oregon State University in Corvallis: "There's no consensus at all about aging."
Plants Grow Up
Plants do mature, even if they don't eventually deteriorate. Wesley Hackett, a plant physiologist at the University of California, Davis, has spent his career following the life stages of English ivy, a vine native to Europe that often escapes from North American gardens into forests. Initially, the ivy crawls along the forest floor; once the plant finds a tree, it starts to climb. At some point--perhaps in response to an environmental cue such as sunlight, Hackett says--the ivy changes its form completely, becoming a mature shrub that flowers and bears fruit. In this sense, he says, plants change over time; they transform from seedlings to a juvenile stage and on to adulthood. But once they grow up, it's not clear that they show signs of old age.
Still, some age-related alterations that reach beyond maturity have been observed in trees such as red spruce, according to Michael Greenwood, a forest biologist at the University of Maine, Orono. For example, needles of older trees are thicker and tougher than those of younger trees, he reported last year in Tree Physiology. And the rate at which plants transform the energy from light into sugars through photosynthesis also declines, leading to a growth slowdown as plants age.
Although trees slowly increase in size throughout life, their growth rate slackens. As they expand, plants must work harder to move water and nutrients over longer distances. This difficulty might underlie some of the physiological changes that occur as trees grow older. Because size and age grow in concert, however, scientists find it difficult to tease apart these two properties.
To test whether differences in needle attributes and photosynthesis rates between young and old red spruce result from age-related changes intrinsic to the plant's cells or from overall plant size, Greenwood and colleagues grafted portions of old trees onto smaller, younger plants. Last year they reported the results: After four growing seasons, the swapped branches maintained the characteristics of the donor plant rather than adopting the more youthful habits of the recipient. Growth remained slow and the branches continued to flower, both signs of old age. The findings suggest that the seed of the age difference resides not in the organism's bulk but in the branches' own meristem cells responsible for generating new plant structures.
However, the presence of physical age-related changes doesn't necessarily imply that plants deteriorate and ultimately perish as many animals do, Greenwood says. Long-lived plants seem to keep on ticking until something knocks them out or they collapse under the weight of their hulking bodies, he adds. Even annuals and biennials--plants that die after 1 or 2 years--succumb rapidly after reproducing in response to a programmed set of hormonal cues rather than waiting for time to wear them down. The idea that plants age in a degradative sense--gradually weakening with each passing year--remains unproven.
Built to Last
There's no doubt that many plants can keep going for hundreds if not thousands of years--records that tower over those for most animals. Part of plants' secret might lie in their construction.
Unlike the human body, which functions as fully a integrated system, a plant's body suffers little from the loss of one of its parts, such as a single leaf or flower. The components act so independently that some scientists consider a plant to be a collection of separate parts, each with their own birth and death. Plants' body plan might allow the individual--a hodgepodge of parts--to survive indefinitely even as the green luster of its organs fades to brown.
"Even in enormously old plants, like bristlecone pines, [no original parts] are left alive after 4000 years," says Michael Reid, an environmental horticulturist at the University of California, Davis. Portions of the tree die, and these dead parts continue to hold up the rest of the organism. "The living plant tissue is actually relatively young" and distinct from the original tree, Reid says. "In that way, plants are different from animals." Even though people regenerate some cells, most of a human's functioning parts were born at the same time as the individual who's composed from them.
Plants owe their remarkable capacity for renewal to a healthy supply of all-purpose meristem cells with the blueprints to produce any plant structure--or even an entirely new organism. When plants' components falter, meristem cells pump out replacements. Whether the stem cells of adult animals maintain the kind of flexibility that plant meristem cells show remains a matter of intense debate. Although some studies suggest that certain stem cells can be coaxed into producing a variety of organs, others have failed to replicate the finding. And even if scientists can create the multitalented cells in the laboratory, their existence in the bodies of normal adult animals is less certain. Animals, it seems, won't match plants' potential for renewal without a lot of prodding.
The White Cedar Paradox
Evidence that plant structure combats aging comes from species that take the segmented lifestyle to the extreme. Some variants of typically short-lived plants are breaking records, supporting the link between long life and a modular body plan.
More than a decade ago, Doug Larson, a botanist at the University of Guelph in Ontario, Canada, made a surprising discovery while studying white cedars growing in a harsh cliff environment. In forests, the cedars typically die when they're 80 years old. Larson expected that those on the cliff face would behave similarly. Instead, he reported last year in Experimental Gerontology that some were still going strong after 1890 years.
He and his colleagues took a closer look, tracing a path in the bark from a dead portion of the crown back to the trunk, and noticed that the trees appeared to die in sections. To determine the pattern of death, they sliced a few trees lengthwise and examined the sections in turn. They found that 2.5 years after a root died, a section of bark followed, suggesting that the cedars were constructed in an unusual way.
In most plants, water sucked up by one root spreads throughout, and the death of a single root doesn't have a noticeable impact on the whole organism. On the other hand, the demise of a substantial number of roots can kill the entire plant. The coordinated death of roots and small regions of the bark suggested that the cedars consist of many "quasi-independent" sections, Larson says. Their construction might be adaptive in the harsh cliff environment, allowing a portion of the plant to persist even when rockslides destroy many of the roots, he speculates, a plight that would prove fatal for other plant species.
To elucidate the tree's construction, Larson and colleagues mixed dye with water and fed it to particular roots. If the roots operated as a cohesive unit, the dye would color the whole plant. But if each root linked exclusively to particular parts, only those sections would light up. The dye didn't penetrate the whole tree but stuck to discrete portions of the plant, the researchers reported last year.
"Roots are not supposed to do what our cedars' roots did," Larson says. "What's unusual in cedar is [that] when part of the root system dies, only that part of the stem and crown is affected, while the rest of the tree doesn't even know there has been a problem." Therefore, each partition of the plant's body acts as a kind of insurance on the life of the whole plant, gracing it with as many lives as it has modules. Similar root segregation might underlie the persistence of other trees, including the tenacious bristlecone pine, Larson suspects.
Slow Down and Live It Up
The cliff-dwelling white cedars might owe their staying power to other talents as well, Larson says. Not only do the trees endure longer in the harsh, dry slope environment than they do in the marshy forest, but they also live at a much slower pace. The cliff plants are, in essence, surviving on a modest diet of scarce resources. Experiments examining the link between limited nutrients and life extension in plants have yet to be performed, but scientists have noted other examples of plants living longer in lean times.
These observations suggest a connection between plants and animals: Calorie restriction prolongs life in a number of animal species, including worms, mice (see "Dieting Dwarves Live It Up"), and even primates (see "Monkey in the Middle"). Although the mechanism underlying the life-extending effect of skimpy portions remains contentious (see "Running Lean and Mean" and "High-Octane Endurance--Yeast in the Metabolic Fast Lane Live Longer"), some researchers have speculated that the explanation lies in decreased vandalism by reactive oxygen species (ROS), a byproduct of metabolism; much work has linked this type of damage to aging.
Like animals, plants accumulate ROS through the breakdown of food. Furthermore, photosynthesis spurs buildup of additional noxious oxidative waste. But plants have "more mechanisms to protect against the damage than animals do," says Russell Jones, a molecular biologist at the University of California, Berkeley. For example, they produce suites of antioxidants, including vitamin C, which many animals must acquire from plants. As a consequence, oxidative injury might not reach problematic proportions in plants, Jones says.
However, new work suggests that plants, like animals, might suffer from an increase in the cellular nicks that ROS inflict as years pass. Sergi Munn�-Bosch, a plant biologist at the University of Barcelona, Spain, noticed that when a Mediterranean shrub, Cistus clusii, experienced drought, it languished prematurely compared with individuals that had enjoyed moister conditions. From previous work, he knew that oxidative damage accrues in dry periods.
The connection between stress and signs of age led Munn�-Bosch to probe whether oxidative damage might also correlate with the normal aging process in plants. To test for such a link, he measured the accumulation of a chemical called malondialdehyde--produced as a result of oxidative stress--in the young leaves of 1-, 3- and 7-year-old plants. The effects of oxidative stress mount progressively as the plant ages, he reported earlier this year in Planta, particularly in chloroplasts, the sites of photosynthesis in plant cells. "The findings suggest that the behavior of animals and plants is not so different," Munn�-Bosch says.
An Evolving Story
Few researchers have monitored any plant, not even Arabidopsis, closely enough to track births and deaths under natural conditions, because they live so much longer than fruit flies or nematodes, says Deborah Roach, an evolutionary biologist at Duke University in Durham, North Carolina. Many examples of extreme longevity exist among plants, but long life alone doesn't necessarily imply a lack of aging: The process might unfold so slowly that it's hard to spot, she says. Despite the modest evidence at present, Roach finds reason to suspect that plants do deteriorate.
To test for traces of aging in plants, she tested whether the ribwort exhibits increases in mortality with age under natural conditions. She cultivated 10,000 seedlings in 1 year and another 10,000 the next. Because the plants grew side by side in a single environment, the death rate explained by accidents should have been equivalent between the two groups. Therefore, Roach could ascribe differences in overall mortality rate between younger and older plants to age.
In the third and fourth years, the mortality rate was higher in the more senior plants than in the youngsters, she reported last year in Experimental Gerontology. The result supports the idea that the plantains aged, growing more susceptible to environmental stress with time.
Although Roach found that plantains weaken over time, she remains open to the possibility that some species remain strong. "It might turn out [that] some plants really don't age," she says. In that case, identifying the characteristics that provide the immortality ticket is a key goal for research, she says. Dissecting the crucial differences between species that defy decrepitude and those that succumb to it should shed light on the aging process.
Are Plants Alone?
Whether immortal or not, species such as the bristlecone pine and white cedar challenge the view that aging is inevitable, says gerontologist Caleb Finch of the Ethel Percy Andrus Gerontology Center at the University of Southern California in Los Angeles. But plants might not be unique after all, he adds. Recent findings suggest that some turtles escape aging, he says, and new evidence indicates that bowhead whales might live for more than 200 years.
"We don't know if any animals live for 5000 years," Finch says, "but I would not dismiss the possibility that some live that long. Our survey of great longevity is so incomplete, all we can say is that new examples keep stretching the limits."
Organisms are composed of the same fundamental building blocks--cells--assembled in different ways, he points out. Yet species vary by a millionfold in terms of life-span. The basis for that disparity remains mysterious, he adds, and a lot can be learned from plants. Once the essential elements that afford the endurance of our leafy friends come to light, perhaps people can find ways to become evergreen.
November 20, 2002
Kendall Morgan is a science writer in southeastern Idaho. She hopes that if she listens hard enough, she'll hear the secret to long life whistling through the trees.
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Citation: K. Morgan, Perennially Young? Science's SAGE KE (20 November 2002), http://sageke.sciencemag.org/cgi/content/full/sageke;2002/46/ns9
Science of Aging Knowledge Environment. ISSN 1539-6150