The Regulation of Leaf Senescence
Leaf senescence is an age related process that allows the movement of nutrients from areas where they are no longer required to areas where they are needed such as in reproductive development. The process itself is ordered and allows the cell to keep alive for the longest possible time; this enable the maximum salvage of nutrients. There are many genes thought to be involved in senescence, and over 800 genes have been identified by microarray technology in Arabidopsis. The senescence process is intricate, allowing for the controlled termination of leaf cells, tissues and organs and thus requires to be finely regulated. Although an age dependent process senescence is influenced by many internal and external factors such as plant hormones and shading response.
Factors Involved in the Regulation of Leaf Senescence
In addition to a plants developmental age other stimuli are involved in the regulation of the onset of senescence:
1. External Factors
a. Nutrients
b. Temperature
c. Drought
d. Pathogen interaction
e. Shade
f. Ultraviolet light and ozone damage
2. Internal Factors
a. Hormones (Abscisic acid, Auxin, Cytokinins, Ethylene, Jasmonic acid, Salicyclic acid).
b. Reproductive factors
Another process that is involved in the age dependent onset of senescence of leaves is that of metabolic rate.
These stimuli act alongside developmental age to form a complex and finely controlled senescence regulatory network. Ultimately these factors lead to the onset of senescence. The senescence processes include:
1. Macromolecule degradation
2. Detoxification
3. Defence
4. Salvage of Nutrients
5. Translocation of Nutrients
6. Cell death
a. Nucleotide cleavage
b. Plasma disintegration
c. Membrane digestion
Regulation of Senescence by Sugar
It is thought that one of the processes that is involved in the control and response to senescence regulatory networks is that of sugar status. A high level of sugar in plants has been shown to both decrease the levels of photosynthetic activity and to initiate the senescence process.
Regulation of Leaf Senescence by Hormones and Other Factors
The senescence of leaves is an evolutionary advantageous process for plants. Although it may be thought that cell death in disadvantageous in reality it allows the plant to transfer nutrients from areas no longer required to those undergoing cell development, such as those involved in reproductive cell production. An example of leaf senescence is the decay of leaves on trees in the autumn, where nutrients are removed from the leaf and translocated to other parts of the plants.
Senescence can be thought of as the last process of cell development and is thus age dependent; additionally other factors both environmental, such as shade and pathogen response, and endogenous such as hormonal stimuli are known to be involved in senescence regulatory pathways. This section of plant biology advice takes a look into the role that hormones play in leaf senescence regulation.
Hormones Involved in Leaf Senescence Regulatory Networks
Many hormones have been shown to be involved in the regulation of leaf senescence, these include cytokinins, abscisic acid, auxin, ethylene, jasmonic acid and salicyclic acid. Of especial interest is the senescence pathway regulated by the plant hormone salicyclic acid, as this shows a great similarity to that of which is seen in natural leaf senescence. It is thought that these phytohormones are able to regulate the initiation of senescence via the regulation of stress responsive genes.
Non-Hormone Rregulation of Leaf Senescence
In addition to hormones, many other factors such as age dependent changes in sugar levels are known to have an impact upon the senescence process. There are many environmental aspects that affect the regulation of senescence such as analysis of light quality and temperature. This strongly suggests that genes involved in these processes may have an impact upon the control of senescence.
Additionally many genes have been identified as being differentially regulated in microarray during the senescence process. These include NAC proteins, one of which NAP has been shown to delay leaf senescence in Arabidopsis when mutated. Many genes involved in lipid metabolism such as the acyl hydrolase gene are also though to affect senescence.
More than 800 genes have been shown to be associated with senescence in Arabidopis, it is thought that a better understanding of how these genes are regulated, and of the proteins that they encode will further knowledge of the leaf senescence process.
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