Plant Hormones and Senescence
Many hormones play a role in senescence. Some of these are able to initiate the process such as ethylene and Abscisic acids, whilst others such as cytokinin are known to be repressors of the process. Much work has been carried out into the roles that plant hormones play in the regulation of senescence and a summary is given.
Plant Hormones Role in Senescence
1. Abscisic acid
The hormone Abscisic acid is commonly known as ABA and plays a key role in developmental processes such as seed germination and plant growth, it also acts as a key hormone in a plants response to the environment. Abscisic acid is thought to promote the senescence and abscission of leaves, and the levels of the hormone increase dramatically in leaves that are undergoing senescence. Some of the key Abscisic acid biosynthesis genes are known to be upregulated when senescence is occurring; these include NECD and aldehyde oxidase genes. It has been put forward that the plant hormone abscisic acid is able to increase H2O2 levels (either by production or accumulation) in leaves and that this accelerates the senescence process.
2. Ethylene
The hormone Ethylene is well known for its involvement in fruit ripening. It is also involved in the control of senescence in leaves and flowers. The levels of Ethylene increase during senescence. One of the genes that is thought to be a regulator of ethylene induced senescence in Arabidopsis leaves is EDR1.
3. Cytokinins
Cytokinins play a major role in the proliferation of cells, and are also involved in shoot formation and branching. With regards to senescence they are known to delay the onset of senescence. There is a drop in the levels of the cytokinin hormone during senescence. Some of the genes that have been shown to be affected by cytokinins in Arabidopsis include SAG12, AHK3 and IPT. It is thought that one of the reasons that cytokinins affect senescence is through regulation of primary metabolism and through the regulation of sucrose levels.
4. Jasmonic Acid
These hormones have been shown to reduce the photochemical efficiency of Photo system II and the levels of chlorophyll content of detached leaves when applied exogenously. A gene thought to be involved with senescence and Jasmonic acid in Arabidopsis is Coronatine Insensitive 1.
5. Salicyclic Acid
The plant hormone Salicyclic acid is known to play a role in a plants response to pathogens. With regards to senescence it is known to play a role in age dependent leaf senescence, and levels of the hormone increase dramatically by as much as four times in leaves undergoing senescence. In Arabidopis many genes associated with senescence such as SAG12 and PR1a have been shown to be undetectable when salicyclic acid is not present in leaves. Interestingly transcriptome changes that occur through the presence of salicyclic acid are similar to those that occur naturally in age dependence senescence. Salicyclic acid is known to be involved in pathogen related cell death, a similar role occurs in senescence associated cell death via the hys1 gene in Arabidopsis.
6. Auxin
The levels of auxin increase during leaf senescence, and many genes involved in auxin biosynthesis such as tryptophan synthase are upregulated during age dependent senescence. Other genes that may be involved in the regulation of leaf senescence by the plant hormone auxin in Arabidopsis include ARF2, transcript levels of which are increased during leaf senescence.
It can be seen from the above that many plant hormones play a role in leaf senescence, this enables a plant to initiate the process at a time that is most beneficial for its survival and development.
Buchanan-Wollaston et al. (2005). Comparative analysis reveals significant differences in gene expression and signalling pathways between developmental and dark/starvation-induced senescence. plant Journal. 42:567 to 85.
Jing et al. (2002). onset of leaf death identify a regulatory pathway controlling leaf senescence. Plant Journal. 32:51 to 63.
Kim et al. (2006). Cytokinin-mediated control of leaf longevity by AHK3 through phosphorylation of ARR2 in Arabidopsis. Proc. Natl. Acad. Sci. USA :814 to 19.
Lim et al. (2007) Leaf Senescence. Annual Review of Plant Biology. 58: 115 to 136.
van der Graaff et al (2006). analysis of transporters and hormone pathways during developmental and induced leaf senescence. Plant Physiology. 141:776 to 792.