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Figure 1: the model plant Arabidopsis thaliana. The total height is approximately 30 cm
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Ageing is almost a universal phenomenon in living organisms. In higher plants it is most obviously manifested by the golden autumn colours, which is a dramatic visualisation of the senescence of leaves. The leaves of perennial plants senesce in a seasonal manner to survive harsh winters or severe droughts. Annual plants undergo leaf senescence mainly during their reproductive stage. Leaf senescence is highly predictable and essential for plant survival. It is a programmed, active process that enables the plant to use the nutrients from photosynthetic tissue for the development seeds or for growth in the next season.
The process of leaf senescence is being studied in the model plant Arabidopsis thaliana (Figure 1). Although not an economically important plant, the common garden weed Arabidopsis (Common name: 'thale cress'; 'zandraket' in Dutch) has developed into the dicotyledonous model plant during the past 20 years. Its complete genome sequence is available and it has a short life cycle of 8 to 10 weeks. It can grow to about 50 cm in height and requires as little as 1 cm3 of soil. The possibility to cross and self-fertilise this plant, further smoothed the progress of making this plant such an important model plant species.
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Figure 2: The rise and fall of an Arabidopsis leaf. Leaves of increasing age are displayed. After four weeks of growth the senescence process starts, usually from the tip of the leaf (fourth leaf). 5 days later the process is completed (sixth leaf).
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In Arabidopsis, leaf senescence is controlled by age in a relatively predictable manner (Figure 2). The lifespan of an Arabidopsis leaf is around 30 days, depending on the growth conditions. Each individual leaf has a similar lifespan and therefore, leaves that develop later in life, will senesce later. During the final stage of seed development, the whole plant senesces, presumably to provide nutrients for the developing seeds. In additon to age, plant hormones and environmental cues can modulate the progression of leaf senescence. Of particular interest is the volatile plant hormone ethylene. Ethylene will cause rapid senescence of older leaves. However, the effect of ethylene is strictly dependent on developmental signal(s), and the hormone does not affect the young leaves. Therefore, ethylene is not believed to regulate senescence itself, but it exaggerates senescence once the leaves are primed to senesce. Thus, both age- and ethylene-dependent signals are perceived and integrated to control the senescence process.
We are mainly interested in understanding the regulation of leaf
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Figure 3. Model for the regulation of leaf senescence in Arabidopsis. Senescence strictly depends on age-related developmental signals and environmental cues. Each individual leaf goes through three developmental stages.
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senescence. Therefore, we have isolated mutants that are specifically affected in the developmental signals that influence senescence. The detailed analysis of the wild type plants and the senescence mutants showed that each leaf autonomously goes through three developmental stages (Figure 3). Young leaves do not respond to stress-signals with leaf senescence. Shortly before the leaf reaches its mature size it changes its developmental program such that it can respond to environmental changes with rapid senescence of the leaves. However, under favourable circumstances the leaf will remain green and photosynthetically active. In the final developmental phase, the leaf will undergo senescence, independent of environmental cues. The switches in the developmental phases strictly depend on leaf age. The isolated mutants go through the individual phases; however, the timing is altered as a result of an altered leaf-age perception.
The isolated mutants have a defect in the switch from one to the other developmental phase, as shown in Figure 3. Two fundamentally different types of mutants are currently being studied. In the first group, the developmental senescence program is initiated earlier than in the wild type (Figure 4A.) These mutants may have a shorter 'Never senescence' and/or 'Ethylene-dependent senescence' phase. The second group contains mutants that challenge the ageing process, so called Methuselah mutants (Figure 4B). For example, the Methuselah mutant may remain much longer in the 'never senescence' phase than the wild type and therefore it doesn't respond to adverse conditions with the senescence of the leaves.
We have identified several of the genes that are involved in these developmental switches and the molecular analysis of these genes showed that the ageing genes play a role in important housekeeping aspects throughout plant development. This is
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Figure 4. The effect of ethylene on wild type Arabidopsis and ageing mutants. 4A: A wild type and a mutant Arabidopsis plant were grown for 21 days in air, followed by 3 days in air supplemented with ethylene, and photographed. The ageining mutant responds with rapid senescence of most of the leaves, while the wild type is largely unaffected.
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similar to what has been found for genes involved in the regulation of ageing of animals and yeast. Further study may help us in understanding the ageing process in animals as well and may ultimately allow us to control senescence and, therefore, improve crop characteristics such as shelf life and productivity.
Students projects: 3, 4
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4B: Wild type plants can respond to adverse environmental challenges with rapid senescence of the older leaves. The Methuselah mutant remains fully green.
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| Last modified: | December 14, 2011 15:27 |
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