The Mechanisms Of Melatonin Mediated-cadmium Stress Tolerance And Detoxification In Tomato (Solanum Lycopersicum) | Posted on:2017-02-16 | Degree:Doctor | Type:Dissertation | Country:China | Candidate:Md. Kamrul Hasan | Full Text:PDF | GTID:1223330485462438 | Subject:Vegetable science | Abstract/Summary: | PDF Full Text Request | Cadmium (Cd) raises a great environmental concern, as it is toxic and as such can heavily impact the ecosystems. Rapid industrialization and urbanization accompanied by the burgeoning world population results in the increased emission of pollutants into the environment, which have not only wreaked the havoc on the availability of natural resources but also caused widespread and grave contamination of essential components of life on the planet. Evidence showed that in most countries agricultural crops account for the large quantity of human Cd ingestion. Therefore, this unprecedented bioaccumulation and biomagnifications of heavy metals (HMs) in the environment have become a dilemma forall the living organisms including plants. Hereat, in planta detoxification of Cd is very important to ensure food safety. In response, plant intrinsically equipped with sophisticated cellular mechanisms for Cd detoxification and tolerance, which include immobilization, exclusion, chelation and compartmentalization of metal ions and the repair of cellular structures. However, plants themselves suffer from phytotoxicity under elevated levels of exposure because of rate-limiting antioxidant and detoxification capacity. Hence, hormonal supplementation under such circumstances may provide additional power to strengthen plant-detoxification system in order to combat the stress generated by xenobiotics.In the recent past melatonin (N-acetyl-5-methoxytryptamine) have emerged as a new paradigm in the category of phytohormons, enhances the tolerance of plants to abiotic stressors including hot or cold temperatures, salinity, drought, over-watering, ultraviolet radiation, and chemical or metal pollutants in water and soils. Accordingly, the present study was carried out to understand the stress responses of tomato plant towards Cd as well as to elucidate the possible protective mechanism of melatonin in Cd stress mitigation through series of experiments. The salient findings are summarized as follows:1. Preliminary studies conducted on Cd, showed its toxic effects at low concentration and hampered the growth of tomato plants. It was also observed that detoxification mechanism responded in a coordinate manner upto 100μM concentration of Cd ions. The biosynthesis and consumptions of the metabolites under Cd stress eventually contributed towards the inherent ability of plants to combat and detoxify the generated stress. Therefore, present study was designed to observe the applicationof GSH and inhibitor of GSH biosynthesis on Cd stresstolerance in tomato plants. By analyzing the growth performance of plants under 100 μM Cd, we found that application of 5 mM GSH is most effective in ameliorating the toxicity of Cd. Cd accumulated in chloroplast/plastid, mitochondria and nucleus in root and leaf cells. Interestingly, GSH prompted the reallocation of Cd in the cells and decreased the Cd content in these organelles; however, inhibition of GSH biosynthesis by buthionine sulfoximine (BSO) had the opposite effects. Similarly, GSH promoted, whereas BSO inhibited accumulation of phytochelatins (PCs), which are involved in chelating and sequestering of Cd. In addition, apart from regulating the Cd sequestration, GSH also modulated the cellular redox status and increased the production of nitric oxide (NO) and S-nitrosothiol (SNO). Corresponding to the changes of redox status, GSH positively regulated the expression of several transcription factors, involved in regulation of stress response genes. In paralell, GSH application enhanced the expression and activity of antioxidant enzymes. Our results suggest that GSH increases the tolerance to Cd stress not only by promoting the chelation and sequestrationof Cd but also by enhancing the antioxidant system through a redox-dependent mechanism.2.The present experiment was designed to optimize the high performance liquid chromatography (HPLC) based method for easy, quickand sensitive detection of melatonin and establish its relation with Cd stress. Three important vegetables; tomato, lettuce and cucumber were treated with two different doses (25 uM and 100μM) of Cd. After Cd exposure, the melatonin content were determined in leaves and roots of control and treated vegetables with time course by using optimized method of HPLC with fluorescence detector. The optimized method is sensitive and highly reproducible, as it shows the elution of melatonin at 18.33 mins.The total run time including washing of columnwas 42 mins significantly shorterwith detection limit uoto 25 fmol per 10μL injection. The expression patterns of melatonin biosynthesis and Cd uptake and translocation in plant corresponding to each other. The results revealed that the melatonin content in both the leaves and roots increased due to Cd stress and inclination was time and stress dependent.3. We have also studied the effects of melatonin in tomato plants against Cd stress. The roots of tomato plants were subjected to 100 uM Cd and the leaves were simultaneously treated with different melatonin concentrations. Results explored that optimal dose of melatonin could effectively ameliorate Cd-induced phytotoxicity in tomato. The contents of Cd and melatonin were increased under Cd stress. However, such increase in endogenous melatonin was incapable to reverse the detrimental effects of Cd. Meanwhile, supplementation with melatonin conferred Cd tolerance as evident by plant biomass and photosynthesis. In addition to notable increase in antioxidant enzymes activity, melatonin-induced Cd stress mitigation was closely associated with enhanced H+-ATPase activity, micronutrient uptake and increased biosynthesis of glutathione and phytochelatins. Although exogenous melatonin had no effect on root Cd content, it significantly reduced leaf Cd content, indicating its role in Cd transport. Analysis of Cd in different subcellular compartments revealed that melatonin increased cell wall and vacuolar fractions of Cd. Our results suggest that melatonin play critical roles in regulating antioxidant capacity, biosynthesis of thiol compounds and vacuolar sequestration of Cd in tomato plants against Cd stress. Such a mechanism may have potential implication in safe food production.4. Melatonin mediated integrated regulation of plant growth and stress tolerance in association with sulfur acquisition and assimilation under Cd challenged environment were studied in this experiments. We have demonstrated thatmelatonin modulated sulfate uptake, reductive assimilation and integration into organically bound sulfur is the central processes of stress tolerance. Results explored that melatonin act as a upstream activator of ATP sulfurylase, APS reductase, SiR and cysteine synthase (CS) [O-acetylserine(thiol)lyase] enzymes, which are committed in the important steps of sulfur assimilation consequences. Additionally serving as a signal melatonin confess sulfur acquisition and assimilation through selective modulation of the genes involved in this pathway in the face of conditions that perturb sulfur homeostasis. In turns of accelerating cysteine flow melatonin promotesy-glutamylcysteine (y-EC), GSH and PCs biosynthesis in response to stress compromisation. By contrast, inhibition of melatonin biosynthesis by knock down of ASMT gene through VIGS had the opposite effects. The increased ranges of PCs biosynthesis by melatonin in Cd stressed plant, presumably leading to the ultimate chelation of cytoxic Cd. In consequences melatonin decreased Cd translocation from root to shoot an important issue of food-safety. Apart from regulating sulfur homeostasis, melatonin as an antioxidant protects cellular macromolecules from damage and structural alteration by stress-induced over ride ROS. In addition to their direct effects, melatonin regulates cellular redoxhomeostasis by promoting 2-Cys Prx. activity of leaf chloroplast in oxygenic conditions through increased metabolic flow of cysteine.Taken together; our results suggested that melatonin could be a promising strategy for enhanced plant adaptation or tolerance by regulating cellular redox balanced and sulfur homeostasis in tissue under challenging environment. | Keywords/Search Tags: | Tomato, Cadmium, Melatonin, Antioxidant enzymes, Adenosine triphosphatase (ATPase), ATP sulfurylase(ATPS), Chlorophyll fluorescence, Cell ultrastructure, DNA damage, Detoxification, Glutathione, High performance liquid chromatography(HPLC) | PDF Full Text Request | Related items |
| |
|