Leaf Senescence: Its Molecular Mechanism

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In the July 2001 issue of Trends in Plant Science (5(7):278-82), Dr. Richard Amasino and his associates at the Department of Biochemistry, University of Wisconsin, have discussed the present status of our knowledge about the molecular mechanism underlying the syndrome of senescence and have suggested the future direction of research for a thorough understanding of this process.

In the beginning of the review,  the authors have explained how the leaf senescence begins by the loss of chlorophyll starting at the leaf margins and advancing to the interior of the lamina. Along with the visible signs, there is simultaneous degradation of proteins, lipids, nucleic acids etc. followed by reallocation of nitrogen, phosphorus and metals to younger leaves and to growing seeds.
At this point the authors also describe benefits which may be accomplished by controlling  senescence. For instance, the induced delay in leaf senescence in cut flowers and leaf crops such as lettuce will increase their shelf life. The authors then distinguish between age-dependent senescence and that induced by phytohormone application, shading, temperature and pathogen attack and try to answer the question whether the molecular basis of senescence,  caused by different treatments are the same.

The expression of “Senescence Associated Genes” (SAGs) in response to different senescence-inducing  treatments has been analyzed to determine the extent of overlap between age-dependent leaf senescence and senescence induced by other factors.
Detachments of leaves followed by dark incubation induced the expression of all but one of the genes that are expressed in age-mediated leaf senescence. Compared to this,  in detached leaves left in the light, all age-related genes were activated but the induction was weaker. Many genes induced during leaf senescence have been identified. Some of these gene products are similar to the pathogenesis-related proteins (PR proteins) and as such have been designated  as products of DR (defense related) genes. Recent studies show that in Arabidopsis plants grown in sterile conditions, there is an expression of DR genes indicating that these genes may play a critical role in causing senescence.
To settle the question whether or not salicylic acid, which is induced by DR genes,  is essential to initiate senescence process, transgenic plants in which SA is not accumulated, were tested. The results indicated that in the absence of SA, the five-senescence related genes were expressed dismissing the idea that SA is directly involved in the

aging process
The authors have summarized  the above findings by concluding that there appears to be some overlap between the leaf senescence and pathogen-defense program. A reduction in photosynthesis in older leaves may initiate the senescence process
In soybean and the grass Festuca pratensis, the authors indicate that mutants have been identified that remain green long after wild-type leaves have yellowed. This trait of the mutant results from a block to chlorophyll degradation. In these mutants, other biochemical changes occur indicating that the remaining gene actions leading to senescence do not stop. In some Arabidopsis mutants, in which the leaf remained green compared to the control, the senescence process started several days later.
Because of the uncertain situation regarding the genes controlling senescence, the authors suggest the use of activation tagging in which the vector would consist of DNA insertion elements (T-DNA) with a strong plant promoter or enhancer. This would result in the generation of dominant genes known as gain-of-function mutations. This technique will enable the researchers to discover genes which hasten the process of senescence from those which delay it. Furthermore, reviewers point out that now that the SAGs have been identified, researchers should start to change their focus to identify the function of these genes through the application of techniques such as reverse genetics. Compared to the limitation of traditional techniques,  in which  one can study the phenotypical or biochemical characters of a single or rarely double mutants, in the latter technique, the effects of multiple genes involved in the process of senescence can be studied. simultaneously. Such an approach will provide insights as to how senescence is regulated by the combinations of various factors such as metabolite export and different catabolic processes.

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