Chloroplastic biosynthesis of melatonin and its involvement in protection of plants from salt stress

Abstract

Within the chloroplasts reactive oxygen species (ROS) are generated during photosynthesis and stressful conditions. Excessive ROS damages chloroplasts and reduces photosynthesis if not properly detoxified. In this current study, we document that chloroplasts produce melatonin, a recentlydiscovered plant antioxidant molecule. When N-acetylserotonin, a substrate for melatonin synthesis, was fed to purified chloroplasts, they produced melatonin in a dose-response manner. To further confirm this function of chloroplasts, the terminal enzyme for melatonin synthesis, N-acetylserotonin-O-methyltransferase (ASMT), was cloned from apple rootstock, Malus zumi. The in vivo fluorescence observations and Western blots confirmed MzASMT9 was localized in the chloroplasts. A study of enzyme kinetics revealed that the K m and V max of the purified recombinant MzASMT9 protein for melatonin synthesis were 500 μM and 12 pmol/min·mg protein, respectively. Arabidopsis ectopicallyexpressing MzASMT9 possessed improved melatonin level. Importantly, the MzASMT9 gene was found to be upregulated by high light intensity and salt stress. Increased melatonin due to the highlyexpressed MzASMT9 resulted in Arabidopsis lines with enhanced salt tolerance than wild type plants, as indicated by reduced ROS, lowered lipid peroxidation and enhanced photosynthesis. These findings have agricultural applications for the genetic enhancement of melatonin-enriched plants for increasing crop production under a variety of unfavorable environmental conditions.Chloroplasts are critically important plant cellular organelles with the major function of performing photosynthesis. The chlorophyll pigments in chloroplasts capture energy from photons, with this energy being used to synthesize organic molecules from carbon dioxide via the Calvin cycle. Large quantities of reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated during photosynthesis, especially under the stressful conditions such as exposure to excessive light intensity, heat, salt, cold, drought and environmental pollutants 1-4 . ROS and RNS, if not properly detoxified, damages chloroplasts and reduces their photosynthetic efficiency, ultimately killing the cell. This damage is referred to as oxidative stress. During evolution, plants developed an array of mechanisms to protect themselves against oxidative stress 5 . One mechanism is the production of antioxidants; which include ascorbic acid, carotenoids, tocopherol, glutathione and polyphenols 6 . Interestingly another antioxidant, melatonin, initially thought to be exclusively an animal hormone, was identified in plants in 1995 7,8 . Since then melatonin has been detected in many different species of plants and plant products [9][10][11][12][13] . The significance of melatonin in plants is related to its potent free radical scavenging and antioxidant capacity [14][15][16][17] . In contrast to several other antioxidants, melatonin can enter every sub-cellular compartment due to its amphiphilic nature 16 . In addition, melaton...

Apple tree flowering is mediated by low level of melatonin under the regulation of seasonal light signal

“…Considering that melatonin is induced by both dark and high intensity of light in plants, which is different from animals, it is possible that melatonin functions as a chemical message of both light quality and quantity to regulate flowering. Different from animals, in apple leaves high intensity of light promoted melatonin production, which surpassed its production during darkness in summer . As we have proved, that chloroplasts are the major places for melatonin synthesis.…”

Melatonin Improves Waterlogging Tolerance of Malus baccata (Linn.) Borkh. Seedlings by Maintaining Aerobic Respiration, Photosynthesis and ROS Migration

“…It seemed that melatonin absorbed from outside mainly resulted in melatonin increase. In addition, for the MbASMT9, a melatonin synthetic rate-limiting enzyme, which was located in chloroplasts (Kang et al, 2013; Zheng et al, 2017), its upregulated RNA expression did not result in elevated protein level. In contrast, its protein level reduced after melatonin application.…”

Melatonin induction and its role in high light stress tolerance in Arabidopsis thaliana

“…Similarly, more melatonin was shown to be produced in water hyacinth grown under high sunlight than in plants grown under low artificial light, suggestive of melatonin induction upon HL exposure. Supporting these results, melatonin levels were highly enhanced in apple leaves under HL intensity (summer) compared to weak light intensity (early fall), although the direct role of melatonin against HL stress was not studied . An additional study indicated that melatonin treatment dissipated excess light energy by accelerating nonphotochemical quenching, resulting in tolerance against chilling stress in tomato seedlings …”