Plant senescence
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Plant senescence is the study of aging in plants. It is a heavily studied subject just as it is in the other kingdoms of life. Plants, just like other forms of organisms seem to have both unintended and programmed aging. Leaf senescence is the cause of autumn leaf color in deciduous trees.
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[edit] Unprogrammed senescence - Plant Aging
Although there are differences between plants and other organisms, biological senescence in plants has similarities such as free radical damage and telomere shortening that apply as in other organisms.
[edit] Programmed senescence
Programmed Senscence seems to be heavily influenced by plant hormones. The hormones Abscisic Acid and Ethylene are accepted by most scientists as the main culprits, but at least one source, believes Gibberellins and Brassinosteroids are equally to blame.
[edit] Annual Versus Perennial Benefits - Theory
Some plants have evolved into annuals, dying off at the end of each season and leaving seeds for the next, whereas closely related plants in the same family have evolved to live as perennials. This may be a programmed "strategy" for the plants.
The benefit of an annual strategy may be genetic diversity, as one set of genes does continue year after year, but a new mix is produced each year. Secondly, being annual may allow the plants a better survival strategy, since the plant can put most of its accumulated energy and resources into seed production rather than saving some for the plant to overwinter, which would limit seed production.
Conversely, the perennial strategy may sometimes be the more effective survival strategy, because the plant has a head start every spring with growing points, roots, and stored energy that have survived through the winter. In trees for example, the structure can be built on year after year so that the tree and root structure can become larger, stronger, and capable of producing more fruit and seed than the year before, out-competing other plants for light, water, nutrients, and space.
[edit] Plant Self Pruning - Theory
There is a speculative hypothesis on how and why a plant induces part of itself to die off. The theory holds that leaves and roots are routinely pruned off during the growing season whether they are annual or perennial. This is done mainly to mature leaves and roots and this is done for two reasons, either both the leaves and roots that are pruned are no longer efficient enough nutrient acquisition-wise or that energy and resources are needed in another part of the plant because that part of the plant is faltering in its resource acquisition.
- Poor Productivity Reasons for Plant Self Pruning - The plant rarely prunes young dividing meristematic cells, but if a fully grown mature cell is no longer acquiring nutrients that it should acquire, then it is pruned.
- Shoot Efficiency Self Pruning Reasons - For instance, presumably a mature shoot cell must on average produce enough sugar, and acquire enough oxygen and carbon dioxide to support both it and a similar sized root cell. Actually since plants are obviously interested in growing it is arguable, that the "directive" of the average shoot cell, is to "show a profit" and produce or acquire more than enough sugar and gases than is necessary to support both it and a similar sized root cell. If this "profit" isn't shown, the shoot cell is killed off and resources are redistributed to "promising" other young shoots or leaves in the hope that they will be more productive.
- Root Efficiency Self Pruning Reasons - Similarly a mature root cell must acquire on average, more than enough minerals and water needed to support both it and a similar sized shoot cell that does not acquire water and minerals. If this doesn't happen, the root is killed off and resources sent to new young root candidates.
- Shortage/ Need Based Reason for Plant Self Pruning - this is the other side of efficiency problems.
- Shoot Shortages - If a shoot is not getting enough root derived minerals and water, the idea is that it will kill part of itself off, and send the resources to the root to make more roots.
- Root Shortages - the idea here is that if the root is not getting enough shoot derived sugar and gases it will kill part of itself off and send resources the shoot, to allow more shoot growth
Clearly this is an oversimplification; in that it is arguable that some shoot and root cells serve other functions than to acquire nutrients. In these cases, whether they are pruned or not would be "calculated" by the plant using some other criteria. Also it is arguable that for example mature nutrient acquiring shoot cells would have to acquire more than enough shoot nutrients to support both it and its share of both shoot and root cells that don't acquire sugar and gases whether they are of a structural, reproductive, immature, or just plain of a root nature.
The idea that a plant doesn't impose efficiency demands on immature cells is that most immature cells are part of so called dormant buds in plants. These are kept small and non-dividing until the plant needs them. They are found in buds for instance in the base of every leaf. This bud normally does not grow unless the leaf is lost, excised, or if apical dominance is lifted. They give the plant lots of choices and flexibility as to where to grow from. Also it's clear that immature cells, although they can perform some of their intended ultimate role whether structural or nutrient acquisition while they grow, in the short run, are not expected to "produce" at the level of a mature cell.
[edit] Hormonal Induction of Senescence - Theory
There is not a lot of theory on how plants induce themselves to senesce, although it is pretty widely accepted that some of it is done hormonally. Plant scientists generally concentrate on Ethylene and abscisic acid as culprits in senescence, but neglect gibberellin and brassinosteroid which inhibits root growth if not causing actual root pruning. This is perhaps because roots are below the ground and thus harder to study.
- Shoot Pruning - it is now known that Ethylene induces the shedding of leaves much more than abscisic acid. ABA originally received its name because it was discovered to have a role in leaf abscission. Its role is now seen to be minor and only occurring in special cases.
- Hormonal Shoot Pruning Theory - a new simple theory says that ethylene induces senescence in leaves due to a run away positive feedback mechanism. What supposedly happens is that ethylene is released by mostly mature leaves under water and or mineral shortages. The ethylene acts in mature leaf cells however, by pushing out minerals, water, sugar, gases and even the growth hormones auxin and cytokinin (and possibly salicylic acid in addition). This causes even more ethylene to be made until the leave is drained of all nutrients.
- Root Pruning - the concept that plants prune the roots in the same kind of way as they abscise leaves, is not a well discussed topic among plant scientists, although the phenomena undoubtedly exists. If gibberellin and brassinosteroid are known to inhibit root growth it takes just a little imagination to assume they perform the same role as Ethylene does in the shoot, that is to prune the roots too.
- Hormonal Root Pruning Theory - in the new theory just like Ethylene, GA/BA are seen both to be induced by sugar and gas shortages in the roots, and to push sugar and gases, as well as minerals, water and the growth hormones out of the root cell causing a positive feedback loop resulting the emptying and death of the root cell.
- Parallels to cell division - the theory perhaps even more controversially, asserts that just as both auxin and cytokinin seem to be needed before a plant cell divides, in the same way perhaps ethylene and GA/BA are needed before a cell would senesce.
- Discussion of the complete mechanism - what really may occur is that because ethylene pushes out all nutrients out of the shoot cell including sugar and gases, eventually this causes a shortage of these nutrients in the organ (the shoot) which is supposed to procure them. This shortage leads to GA/BA synthesis in the rapidly declining shoot cell, and this simply adds fuel to the fire.
- A Role for Abscisic Acid - finally a question may be to ask, what is the role of Abscisic Acid, the hormone which was first thought to be the primary mover in this department? According to author of the theory we have been discussing, ABA is induced when plant cells are encountering stress other than that of a nutrient shortage kind. In this case a senescing cell experiencing a drain of nutrients may experience a strain which causes it to produce ABA. Indeed it may not be that ethylene and GA/BA alone are needed for programmed cell senescence, but that all three are needed.
Similarly, auxin and cytokinin may not be enough for plant cell division alone, but a proposed compliment of ABA, SA, may be needed in addition.
