For
centuries, humans have been exploring, researching, and, in some cases,
discovering how to stave off life-threatening diseases, increase life
spans, and obtain immortality. Biologists, doctors, spiritual gurus, and
even explorers have pursued these quests -- one of the most well-known
examples being the legendary search by Ponce de León for the "Fountain
of Youth." Yet the key to longevity may not lie in a miraculous essence
of water, but rather in the structure and function of cells within a
plant -- and not a special, mysterious, rare plant, but one that we may
think of as being quite commonplace, even ordinary: the palm.
As an honors botany student at the University of Leeds, P. Barry
Tomlinson wrote a prize-winning essay during his final year titled, "The
Span of Life." Fifty years later, Tomlinson (now a Distinguished
Professor at The Kampong Garden of the National Tropical Botanical
Garden, Miami, FL) teamed up with graduate student Brett Huggett
(Harvard University, MA) to write a review paper exploring the idea that
palms may be the longest-lived tree, and whether this might be due to
genetic underpinnings. Having retained his essay in his personal files,
Tomlinson found that it provided an excellent literature background for
working on the question of cell longevity in relation to palms.
Together, Tomlinson and Huggett published their review in the December
issue of the American Journal of Botany.
A component of an organism's life span that biologists have been
particularly interested in is whether longevity is genetically
determined and adaptive. For botanists, discovering genetic links to
increasing crop production and the reproductive lifespan of plants,
especially long-lived ones such as trees, would be invaluable.
In their paper, Tomlinson and Huggett emphasize that in many
respects, an organisms' life span, or longevity, is determined by the
period of time in which its cells remain functionally metabolically
active. In this respect, plants and animals differ drastically, and it
has to do with how they are organized -- plants are able to continually
develop new organs and tissues, whereas animals have a fixed body plan
and are not able to regenerate senescing organs. Thus, plants can
potentially live longer than animals.
"The difference in potential cell longevity in plants versus animals
is a significant point," states Tomlinson. "It is important to recognize
that plants, which are so often neglected in modern biological
research, can be informative of basic cell biological features in a way
that impacts human concern at a fundamental level."
The authors focused their review on palm trees because palms have
living cells that may be sustained throughout an individual palm's
lifetime, and thus, they argue, may have some of the longest living
cells in an organism. As a comparison, in most long-lived trees, or
lignophytes, the main part, or trunk, of the tree is almost entirely
composed of dead, woody, xylem tissues, and in a sense is essentially a
supportive skeleton of the tree with only an inner ring of actively
dividing cells. For example, the skeleton of
Pinus longaeva may be up to 3000 years old, but the active living tissues can only live less than a century.
In contrast, the trunks of palms consist of cells that individually
live for a long time, indeed for the entire life of an individual.
Which brings up the question of just how long can a palm tree live?
The authors point out that palm age is difficult to determine, primarily
because palms do not have secondary growth and therefore do not put
down annual or seasonal growth rings that can easily be measured.
However, age can be quite accurately assessed based on rate of leaf
production and/or visible scars on the trunk from fallen leaves.
Accordingly, the authors found that several species of palm have been
estimated to live as long as 100 and even up to 740 years. The important
connection here is that while the "skeleton" of the palm may not be as
old as a pine, the individual cells in its trunk lived, or were
metabolically active, as long as, or longer than those of the pine's.
Most plants, in addition to increasing in height as they age, also
increase in girth, putting down secondary vascular tissue in layers both
on the inner and outer sides of the cambium as they grow. However,
palms do not have secondary growth, and there is no addition of
secondary vascular tissue. Instead, stem tissues are laid down in a
series of interconnected vascular bundles -- thus, not only is the base
of the palm the oldest and the top the youngest, but these tissues from
old to young, from base to top, must also remain active in order to
provide support and transport water and nutrients throughout the tree.
Indeed, the authors illustrate this by reviewing evidence of
sustained primary growth in two types of palms, the coconut and the sago
palm. These species represent the spectrum in tissue organization from
one where cells are relatively uniform and provide both hydraulic and
mechanical functions (the coconut) to one where these functions are
sharply divided with the inner cells functioning mainly for transporting
water and nutrients and the outer ones for mechanical support (the sago
palm). This represents a progression in specialization of the vascular
tissues.
Moreover, there is evidence of continued metabolic activity in
several types of tissues present in the stems of palms, including
vascular tissue, fibers, ground tissue, and starch storage. Since the
vascular tissues in palms are nonrenewable, they must function
indefinitely, and Tomlinson and Huggett point out that sieve tubes and
their companion cells are remarkable examples of cell longevity as they
maintain a long-distance transport function without replacement
throughout the life of the stem, which could be for centuries.
Despite several unique characteristics of palms, including the
ability to sustain metabolically active cells in the absence of
secondary tissues, seemingly indefinitely, unlike conventional trees, in
which metabolically active cells are relatively short-lived, the
authors do not conclude that the extended life span of palms is
genetically determined.
"We are not saying that palms have the secret of eternal youth, and
indeed claim no special chemical features which allows cells in certain
organisms to retain fully differentiated cells with an indefinite
lifespan," states Tomlinson. "Rather, we emphasize the distinctive
developmental features of palm stems compared with those in conventional
trees."
Tomlinson indicates that this reflects the neglect of the teaching of
palm structure in modern biology courses. "This paper raises
incompletely understood aspects of the structure and development of
palms, emphasizing great diversity in these features," he concludes.
"This approach needs elaborating in much greater detail, difficult
though the subject is in terms of conventional approaches to plant
anatomy."
