Senescence of Plant Leaves
The senescence process allows for the termination of cells, tissues, organs or even organisms in a controlled process. Senescence is age dependent and under the control by hormonal, molecular, and genetical processes. Most people are familiar with the senescence process in plants through the colour changes and removal of tree leaves in the autumn, or the decay of leaves and transfer of nutrients to seeds in crop species such as Maize.
The last stage in leaf development is senescence, this involves both leaf decay and a removal of the nutrients that are stored in the leaves to other parts of the plant. Although overall leaf senescence occurs at the organ level but is heavily influenced by cellular and organismal fate. Although it may look like leaf senescence just happens and has little regulation it is a controlled altruistic process that requires changes in gene expression, metabolism and ultimately cell structure.
Changes That Occur During Leaf Senescence
Leaf senescence involves age dependent programmed cell death. One of the major changes that occur during senescence of the leaf involves the breakdown of the chloroplast; this results in a major change in the cell structure. As the chloroplast is rich in protein it is important this source is not lost but utilised by other parts of the plant. In addition to changes in cell structure major metabolic changes occur during leaf cell senescence; the normal processes that occur in the leaf involving carbon assimilation are stopped, and new metabolic processes that involve the catabolism of proteins, chlorophyll, and lipids occurs. This allows the nutrients of the leaf to be broke down and exported out of the leaf and used to further the development of the plant in other areas.
Leaf Senescence is Beneficial to Plants
Although one may think of cell death as being a non beneficial process it is not. It leads to better control of the nutrients that are available to the plant, and allows these to be used at the correct time (such as a transfer of effort to seed development), to give the plant an evolutionary advantage.
The next section on leaf senescence takes a look at the control of leaf senescence and its coordination.
The Coordination and Control of Leaf Senescence
Leaf senescence is a process that is evolutionary advantageous to plants as it allows the transfer of nutrients from no longer required processes to areas where development is occurring, such as seed development. The process is age dependent and involves genetic, molecular and metabolic processes. It is important to the plant that the senescence process is under fine control so that it occurs at a time that is beneficial to the plant and not at one that is disadvantageous.
What Controls Leaf Senescence?
Senescence is an age related process that is ultimately controlled by both internal and exogenous stimuli; the plant is able to integrate this information to analyse the most beneficial time to begin the senescence process. Some of the environmental factors that enable a plant to gather information about its age include abiotic factors such as drought, temperature, light, ozone and availability of nutrients. Biotic processes that lead to senescence include shade and pathogen detection.
The Coordination of Leaf Senescence
There is a locality of cell death in leaf senescence that is dependent upon environmental stress stimuli. This leads to an apparent uneven spread of the senescence program. However when no environmental stress occurs it is seen that the process is coordinated so that senescence starts at the leaf tip and leaf margins, and makes its way towards the leaf base. This allows nutrients to be gathers from the furthest regions of the leaf, and to be passed out of the leaf. If senescence was to first occur at the leaf base then nutrients would not be able to pass from the outer reaches of the leaf to the rest of the plant.
The leaf senescence process is also developmentally coordinated, and is often controlled by the processed that require the nutrients contained in leaves such as seed development. The leaf senescence process may occur at the same time as the mortality of other organs or the whole plant (this occurs in monocarpic plants that only has one round of reproduction), or it may occur in a manner that removes the leaves but does not affect other organs.
A well known control process in senescence is called correlative control; this involves the stimulation of the leaf senescence by signals initiated in reproductive organs. Indeed it has been demonstrated that removing these stimuli in plants such as the pea converts the process of leaf senescence to that of juvenile leaf development.
Not all plants that research into leaf senescence occurs are under correlative control, indeed the monocarpic model organism Arabidopis displays whole plant senescence, but is still useful in the understanding of leaf senescence as its leaf has distinct juvenile, adult, and senescence phases.
Lim et al. (2005). The molecular and genetic control of leaf senescence and longevity in Arabidopsis. . Top. Dev. Biol. 67: 49 to 83.
Lim et al. (2007) Leaf Senescence. Annual Review of Plant Biology. 58: 115 to 136.
Nam HG. 1997. Molecular genetic analysis of leaf senescence. Curr. Opin. Biotech. 8: 200 to 207.