The Involvement of Programmed Cell Death in Leaf Senescence

The senescence of leaves is advantageous to plants as it allows nutrient resources to be moved from areas where they are no longer required to area where cell development processes are occurring. The leaf senescence process is under stimulus from both internal factors, such as signals from reproductive organs and environmental signals such as shade. The whole process is age dependent, and may occur at the same time as senescence occurs in other organs such is seen in monocarpic plants such as Arabidopis or in a more controlled manner, like is seen when plants such as Maize set seeds, or occurs when trees lose leaves in the autumn.

Genetically Programmed Cell Death and Leaf Senescence

The destruction of cells is triggered by many external and internal stimuli, and is actively controlled by genetic processes. As leaf senescence is controlled genetic mechanisms it stands to reason that by association it is a form of programmed cell death. In leaves the death of cells does not occur all at once but starts in local patches. The process starts with the destruction of mesophyll cells , this is followed by the termination of other cell types.

Differences in Programmed Cell Death in Leaves and Other Processes

Programmed cell death is a common occurrence in plant developmental processes, for example it is used to protect from pathogen attacks and in the formation of tracheary components. However one of the main differences in leaf senescence is that cell death occurs at the whole organ level. Additionally the programmed cell death seen in leaf senescence allows for the distribution of nutrients to other parts of the plant (as opposed to simply destroying cells). This means that overall the mortality program occurs at a much slower pace in senescence than is seen in the processes previously mentioned.

Changes that Occur During the Leaf Senescence Process

The senescence process is beneficial to plants as it allows nutrients to be relocated in the plant to places of need. It is an age dependent process that is under the control of both internal and external stimuli. The senescence process is a coordinated event and can act as either an organ level process, such as the decay of leaves on a tree in autumn, or at a whole plant level such is seen at the end of monocarpic plant lifespans. The senescence of the leaf is controlled by a process known as programmed cell death. This section takes a look at the biochemical and structural changes that occur during leaf senescence.

Leaf Structure Changes During Senescence

One of the main structural changes that occur in leaves during the in senescence process is an ordered change in intracellular organelle disintegration. The first process in leaf senescence involves changes in the structure of the grana located in the chloroplast; this leads to a formation of lipid plastoglobuli. At this stage the mitochondria and nucleus of the leaf plant cells are left alone; this allows for them to function correctly until the final stages of programmed cell death occur.

During the later stages of programmed cell death in senescent leaves vacuolar collapse, condensation of chromatin and the cleaving of DNA occurs; this in turn leads to the death of the cell. In turn this leads to the disintegration of the cell plasma and membranes and the end of cellular homoeostasis.

Biochemical Changes During Senescence

As can be expected the structural changes that occur during leaf senescence are underlaid by biochemical mechanisms.

Initially anabolism is reduced during the senescence process; this results in a lowering of polysomes and ribosomes. This indicates that one of the early process of senescence involves the reduction in the synthesis of proteins, and a lowering of rRNA and tRNA. Chloroplast degeneration lowers the levels of proteins such as Rubisco and CAB.

The next biochemical changes are geared towards the removal of nutrients from the cell. These include hydrolysis and mobilisation of macromolecules. The proteins are broken down to amino acids by peptidases. Enzymes such as phospholipase D are involved in the break down of lipids in leaf senescent cell membranes. Another biochemical change that occurs during the leaf senescence process is a reduction in the amount of nucleic acids such as RNA, which is mediated by RNAase. The fatty acids that are present in the cell are oxidises and used as a source of energy in the break down of the cell.

It is seen that many biochemical and structural changes occur during the programmed termination of cells during leaf senescence.

Comparison Between Plant Cell Death Systems

The use of programmed senescence in plants allows it to control the use of nutrients. The nutrients can be removed from areas where they are no longer required, such as in the leaf, to areas that the plant is developing, such as seeds.

The senescence of leaves is age related and controlled and coordinated by many factors, both internal and external. The senescence process itself involves the programmed death of cells. This occurs in a specific order to enable the recovery of nutrients, and as such it differs from the cell death programs of other processes such as those involved when attacked by pathogens or in the the formation of tracheary components. The next section takes a look into the differences that occur between these cell death processes.

Differences in Cell Death Between Senescent and Non-Senescent Cells

perhaps the most understood process of programmed cell death involves the hypersensitive response in response to pathogens. It therefore seems reasonable that comparing these responses to those that are involved in leaf senescence will help to further the understanding of the processes involved.

In hypersensitive responses to pathogens many of the same genes are differentially regulated as are seen in senescence; these include the cell hypersensitive response cell death marker HIN1, and many genes that encode pathogenesis related proteins. Defence genes such as ELI3 and the metallothionene encoding LSC54 gene are also upregulated in both pathogen responsive and leaf senescent cell death processes. Took all together it seems that many of the processes involved in cell death use shared mechanisms.

So what overlays the differences between the programmed cell death of pathogen hypersensitive response and the slower, functional cell death seen in senescence? Many different genes are expressed in one of the programs but not the other. For example SAG 12 is expressed in senescence only, whilst HSR203J is expressed in pathogen response only. This suggests selectivity in the roles of these genes, and they are likely to be involved in mechanisms specific to senescence or pathogen hypersensitive response cell death respectively. In comparative transcriptome analysis it was found that over 800 genes are heightened in the senescence process, and that only ~330 of them were highly upregulated in starvation cell death, indicating about 500 differing genes. It is thought that these may be involved in distinctive processes in senescence related programmed cell death.

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