Depending on the species, different external and internal factors determine the rate of foliar senescence, a physiologically regulated process. The death of the leaves is preceded by the degradation of chlorophyll, the loss of molecular integrity, the transfer of nutrients through the phloem, changes in phytohormones (auxins, cytokinins, abscissic acid, ethylene and jasmonic acid), by the disintegration of cell walls in the petiole and, finally, from cell death [240,241].

To determine how senescent processes are affected by melatonin, Arnao and Hernandez-Ruiz [242] segmented barley leaves (Hordeum vulgare L.) and placed them in Petri dishes containing a suitable medium; therefore, the leaf segments were treated with melatonin or not. Judging from the loss of chlorophyll, the leaves treated with melatonin exhibited a delayed senescence. The effect of melatonin is concentration dependent (most effective concentration was 1 mM). The mechanisms by which melatonin has differed for senescent changes in barley leaves has not been determined, but may have been a consequence of its activity. scavenging free radicals or secondary to an inhibition of some associated senescence genes [242]. In a series of in-depth studies, Wang and collaborators [243-245] have clarified the role of melatonin in preventing leaf senescence. When the apple leaves were kept in the dark to cause faster aging, those treated with 10mM melatonin lost chlorophyll more slowly (see figure) and maintained the maximum efficiency potential of photosystem II [243]. Moreover, melatonin inhibits gene expression for the key enzyme that degrades chlorophyll (feide to oxygenase) and also inhibited the gene 12 associated with senescence, both of which contributed to delayed aging in melatonin-treated leaves. It was hypothesized that the factors determining the observed changes were the toxic derivatives of oxygen since melatonin suppressed the accumulation of H2OR2 in the leaves; therefore, melatonin's ability to determine the rate of senescence is a consequence of its antioxidant activity.

A study in vivo, in which the apple plants were grown in a soil integrated regularly with melatonin, changes that responded to delayed aging were evident [244]. The authors compared a range of metabolic endpoints in control leaves and melatonin-treated plants. It has been shown that melatonin has delayed the degradation of proteins, has significantly maintained a greater activity of Photosystem II considering the levels of chlorophyll conserved, as well as of the three photosynthetic final products (sorbitol, sucrose and starch). Melatonin-treated leaves also had a better concentration of nitrogen, total soluble proteins and Rubisco protein concentrations. This study clearly reveals the large number of positive metabolic actions of melatonin in plants, which could contribute to its ability to delay senescence. Wang et al. [245] added an additional layer of information in their studies on the role of melatonin in apple leaf aging by performing a proteomic analysis of leaves in the natural aging phase compared to those that age more slowly due to melatonin treatment. . A GO analysis of Blast2GO showed that hundreds of the proteins altered by melatonin were located mainly in the plastids. In general, melatonin downregulates proteins that are typically found upregolate during the senescence process. This is certainly the most complete study that correlates the action of melatonin with protein metabolism in any plant and the results contribute to providing information on the mechanisms by which melatonin delays aging in plants.

A detailed analysis of some of the genes involved and the measurement of melatonin levels associated with Arabidopsis development have recently been published [246]. Furthermore, this study defined the role that applied exogenous melatonin has on the senescence of rosette leaves. During plant development, and particularly in the later stages (40-60 day old plants), endogenous melatonin levels increase rapidly from 0.5ng / g of FW per day 30 to 2.0ng / g of FW per day 60. When 60-day-old Arabidopsis plants were treated with additional exogenous melatonin, leaf senescence was delayed as indicated by conserved chlorophyll levels. Furthermore, when the plants were supplemented with melatonin, the level of expression of the IAA17 gene (auxin resistant 3 (axr3) indole-3-acetic acid inducible 17) were significantly downregulated. Down-regulation of IAA17 by melatonin can cause a drop in the expression of senescence gene 4 and senescence-associated gene 12, which affect the aging of Arabidopsis, which may be part of the pathway of signaling with which melatonin modulates the aging of the plant [245].

Abstract of the thesis "Melatonin in the plant world (Phytomelatonin): therapeutic properties and future prospects" by Dr. Giorgio Guerrini

For further information, please refer to the bibliography page.