How is Leaf Senescence Assayed?

Leaf senescence is advantageous to plants as it allows nutrients to be passed from an area where they are no longer required to areas where nutrients are required, such as in the development of seeds. The process of senescence is age related and coordinated by both internal and external stimuli. The senescence process consists of a programmed cell death the cell to keep functioning until the last possible moment, and therefore allowing the maximum amount of nutrients to be removed possible. There are many molecular and genetic processes that are involved in the senescence process and these have both similarities and differences with other programmed cell processes. To further understand the processes behind senescence many assays have been developed.

Assaying Leaf Senescence

When molecularly and genetically assaying the differences between senescence and other cell processes it is important to consider two main points.

1. Many parameters must be assayed when looking at leaf senescence as many processes are known to be involved.
2. Only single leaves can be measured, as senescence is age related; different leaves on the same plant will be of different ages.

Understanding senescence requires the use of many biological markers, these include:

1. Leaf Yellowing, a sign of mesophyll cell senescence
2. Photochemical efficiency
3. Ion leakage
4. Chlorophyll content
5. Protein levels
6. Expression of known senescence stimulated genes
7. Expression of senescence stimulated enzymes

Some common methods (linked to the above) used in measuring senescence include assaying the activation of RNAase and peroxidase; measuring decrease in chlorophyll levels; measuring ion linkage from membranes; measuring the expression level of the genes CAB2, RBC and SAG. The changes in gene expression levels that occur during senescence can be measured by Northern blotting, qRT-PCR or by use of microarrays. These methods help senescence to be better understood both at the genetic and molecular levels.

The Analysis of Leaf Senescence

As can be read on other parts of the site, the senescence process is programmed and enables the retention of a plants nutrients. It is an age dependent process that is under the influence of external and internal processes, and allows transfer of nutrients from areas where they are no longer required to areas of cell development. Many things can be used to assay senescence biologically including measuring the activity of genes, proteins and membrane ions.

Many mutants have been identified that are involved in senescence in Arabidopsis; this enables genes to be identified that play a role in senescence processes. To identify candidates, mutant screens were developed that identified plants that had both delayed and early senescence (reverse and forward genetics). Another method to identify genes involved in senescence is to measure the expression levels of genes that are undergoing leaf senescence by the use of molecular techniques such as RT-PCR, microarrays and Northern Blotting. Although both of these methods lead to identification of genes involved in senescence, they also identify many false candidates, as genes may be upregulated as a secondary process following senescence or involved in roles such as homeostasis.

Genes Identified in Senescence

The use of microarray technology in the study of senescence processes has identified over 800 candidate genes in Arabidopsis. Additionally DNA microarrays have been used to look at leaf senescence in the tree Aspen. When looking at developmental processes it is interesting to look at the genes that may be involved in underlying these processes such as transcription factors and those that encode signal transduction regulators. Many of the genes that have been identified as differentially regulated during leaf senescence have mutants available from T-DNA insertion lines such as those available from SALK and GABBI. This has allowed the effect of mutations of differentially regulated genes during senescence to be further analysed and better understood.

Buchanan-Wollaston et al. (2005). Comparative transcriptome analysis reveals significant differences in gene expression and signalling pathways between developmental and dark/starvation-induced senescence in Arabidopsis. Plant Journal. 42:567 to 585.
Chen et al. (2002). Expression profile matrix of Arabidopsis transcription factor genes suggests their putative functions in response to environmental stresses. Plant Cell 14:559 to 574.
Lim et al. (2007) Leaf Senescence. Annual Review of Plant Biology. 58: 115 to 136.
Oh et al. (1997). Identification of three genetic loci controlling leaf senescence in Arabidopsis. Plant Journal. 12:527 to 535.

©2005-2015 Plant Biology Advice - Dean Ravenscroft