| MadSci Network: Botany |
The telomere biological clock hypothesis states that telomere length is reduced every time a cell divides and eventually gets too short for normal cell division. In some tissues, the enzyme telomerase can repair the telomere and prevent cell aging. Asexual reproduction is very widespread in the plant kingdom and often a very important method of reproduction for a plant species. Therefore, it may be that, unlike animals, plants normally have telomerase in all their meristems, which is where cell division occurs. Some plants are like people in the sense that they have a programmed life cycle of limited duration. Annual plants live one growing season, and biennials live two. Perennial plants, especially nonwoody perennials, often seem to have their life cycle ended not by their programming but by one or more environmental factor(s), such as drought, flood, wind storm, ice storm, cold temperature, pollution, disease, pest, mineral deficiency, competition from other plants, etc. Perennial plants are designed to produce new organs, such as leaves, stems, roots, and flowers, every year. People are designed very differently. We have one set of organs, such as heart, lungs, eyes and brain, and parts such as arms and legs, for life. We can repair them to some extent but not make new ones. If we lose an arm, we cannot regenerate it. However, plants can replace lost branches. Each year, deciduous trees replace all their leaves, add a new layer of conducting tissue throughout their roots and stems, grow some new roots and stems, and usually make new reproductive organs (flowers or cones). The first and second websites, dated March 2001, cast doubt on telomeres and telomerase playing as key a role in plant aging as in animal aging. Annual Arabidopsis plants were altered to eliminate telomerase and followed through ten generations. Detrimental effects appeared in the sixth generation but the plants kept living and their cells kept dividing. They concluded that plants are fundamentally different than animals because plant cells can tolerate loss of telomeres and animal cells cannot. Much research remains to be done because Arabidopsis is just one plant species and an annual species at that. Some plants, such as redwoods and bristlecone pine, may live thousands of years, so their lifespans are much greater than for animals. Theoretically, a plant clone can be asexually propagated forever. Certain asexually-propagated plant clones have been in cultivation for at least hundreds of years. A male quaking aspen clone, called Pando, is thought to be as much as a million years old. It covers 48 hectares in Utah and consists of 47,000 individual trunks connected by a single root system. It spreads by root suckering. There is currently no way to tell the true age of plant clones that are efficient at asexually propagating themselves. Even individual clones of asexually reproducing nonwoody plants, such as dandelions, ferns, or mosses, may have originated thousands or millions of years ago. In many cases, rooting of cuttings actually results in a more juvenile plant than the parent because juvenile parts of the plant root more easily. Shoots at the base of a tree are the most juvenile. Unlike mature plants, juvenile plants cannot produce sex organs. Juvenile plants sometimes have a different appearance than mature plants. For example, juvenile English ivy (Hedera helix) is a vine with lobed leaves. Mature English ivy is a shrub with unlobed leaves. Some grasses are perennial but many are annual, such as wheat, corn, rice, oats, crabgrass, foxtails, annual bluegrass, barnyardgrass and annual ryegrass. Annual grasses use mainly sexual reproduction. Perennial grasses can use both sexual and asexual reproduction. Many perennial grasses spread via stolons or rhizomes. Rhizomes are below ground horizontal stems. Stolons are aboveground horizontal stems also called runners. Some grasses, such as Kentucky bluegrass also clone themselves using asexual seeds, a process called apomixis. Dandelions are also apomictic. There is tremendous interest in developing apomictic varieties of seed-propagated crop plants, such as grains and vegetables, because apomixis simplifies hybrid seed production. References Arabidopsis can survive the loss of an enzyme that prevents aging A&M researchers hopeful a plant will lead to treatment of cancer, aging Ancient Bristlecone Pine Re: Will successive plant clones show genetic degradation? Re: What are some good things and bad things about plant cloning? What is apomixis? Kentucky bluegrass apomixis Scientists Announce a Breakthrough in Research on "Asexual" Maize With its key role in plant maturation, a newfound gene could yield a novel class of genetically modified crops Mitton, J.B. and Grant, M.C. 1996. Genetic variation and natural history of quaking aspen. BioScience 46: 25-31. Pando clone of quaking aspen
Try the links in the MadSci Library for more information on Botany.