The most thorough and potentially important investigation linked to the ability of melatonin to influence the yield of crops was conducted by Wei and collaborators [276] who used soy as a model plant. Soybeans represent one of the most important agricultural crops in the world; it is widely used for the production of seed oil, as feed for livestock, for the production of biofuels and as an important source of protein in the human diet [277]. The enormous value of this cultivation is underlined by the fact that world production has increased steadily since 1961 and is expected to reach 372 million tons by 2030. To achieve this goal, the yield of soybean crops must have increased significantly in the land used. For this reason, there is an urgent need to find ways to increase soy production on currently available land [278]. Obviously, it would be of great economic importance to identify any means that can contribute to enlarge the production of soy, or any Agriproduct.
As an aspect of one study aimed at evaluating the action of melatonin on the growth and tolerance to abiotic stress of soybean plants grown from seeds treated with melatonin, Wei et al. [276] also measured the yield of soybeans. For the experiment, the soybeans (Glycine max SuiNong 28 SN28) were initially coated with 500 μL / 100 seeds with a reagent that did not contain melatonin or contained melatonin at concentrations of 50 μM or 100 μM. After the treatment, the seeds were dried at room temperature and then sown in the pre-watered soil. After germination, the plants were grown in a sunlit greenhouse located at 40 ° 22'N and 116 ° 22'E (Beijing, China). The agronomic characteristics that have been recorded have included the number of soy pods per plant, the number of seeds per pod and the weight of 100 seeds. The melatonin concentrations selected for this study were based on observations by Hernandez-Ruiz et al. [253] which suggested that 200 μM melatonin concentrations improve the growth of lupine plants. Wei et al. [276] found that exposure to 50 μM and 500 μM melatonin improved seed germination and plants developed larger leaves; this was statistically verified when the trifoliate leaves of 5-week-old plants treated with melatonin were compared with those of control plants.
A similar stimulating effect of melatonin has been noted on maize and production cucumber by Posmyk and colleagues [229,248,263]. In these studies, rather than coating the seeds with melatonin as described by Wei et al. [276], the authors treated the seeds in a melatonin solution overnight. This treatment resulted in a marked increase in the levels of melatonin in the seeds and, when they sprouted and grown when ripe, the resulting plants carried more product, i.e. corn and cucumbers, than plants grown from seeds treated only with water.
It is extremely important that you study like those of Wei et al. [276] and Posmyk and colleagues [229,248] can be expanded. If the hypothesis that melatonin improves the yield of large-scale crops is verified, this could prove, at least in part, a solution to the problem of producing more product without the use of a larger land. Melatonin is easy to synthesize in pure form and is inexpensive, so its use may prove to be a practical application of this indolamine. The results summarized above may also be of particular interest for another reason. Reports show that the application of exogenous melatonin on seeds, both by coating and by incubation, had an effect on plant growth and cultivation production throughout the life cycle of the plant [229,248,263,276]. Since it would not be so difficult to pre-treat the seeds with melatonin before sowing on a large scale, as described, the application of melatonin to improve agricultural production would probably be feasible. It would also be interesting to determine if the plants germinated and grown from seeds treated with melatonin are more tolerant to stress or if the nutrient composition of the crop is changed.
Considering the importance of exogenous melatonin applied to enrich the yield of crops, the consequences of an upregulation of the endogenous synthesis of melatonin in plants must be evaluated in terms of agricultural production. Considerable progress has been made in recent years and new information has been discovered concerning the biosynthesis pathway of melatonin in plants [279-282].
Although this synthesis route is slightly different from that of animals [239,283,284] (Figure 19), however, tryptophan is the common precursor in all species and the formation of melatonin from serotonin follows the same two-step process in plants as well as in animals [280,281,285]. In plants, chloroplasts can represent an important melatonin production site [286,287]. The genes for enzymes of the synthetic pathway of plant melatonin have been cloned [288,289]. The manipulation of the endogenous synthesis of melatonin in plants that use transgenic technologies is certainly feasible and has already been developed for two species [290]. To date, the genetic modulation of the endogenous melatonin pathway in plants, in order to increase crop production, however, has not yet been completed.
Sun and colleagues [291] tested the effects of melatonin in post-harvest ripening of BMEI cherry tomatoes. The fruits were collected in the green phase of development. After collection, they were placed in one of several melatonin solutions (1, 50, 100 or 500 μM) for 2h. Subsequently, the tomatoes were kept at a temperature of 15 ° C and 80% of relative humidity for 25 days. Exposure to melatonin markedly advanced lycopene levels and color development (Figure 20) and stimulated the expression of several key genes including those of phytoene synthase 1, carotenoid isomerase and aquaporins. In addition, the fruits treated with melatonin showed significantly accelerated softening, a high content of water-soluble pectin and decreased protopectin. These changes were accompanied by an upregulation of cell wall-modifying proteins, including polygalacturonase, pectin isomerase 1, β-galactosidase and expansion 1. Melatonin also influenced the synthesis of ethylene, the perception of ethylene and the signaling ethylene. Ethylene plays an important role during the ripening of tomatoes because of its regulatory role or as it is involved in the synthesis of the carotenoid lycopene, improving the degradation of the cell wall and the conversion of starch into sugars [292]. It is clear from these results that melatonin accelerates the development of the color and flavor of the tomato fruit with its action on the synthesis of ethylene in the post-harvest state. Color and flavor are, of course, the main distinguishable aspects of fruit quality. These results are applicable not only to tomatoes but probably to other horticultural products. Improving product quality reduces product waste.
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.
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