| Air pollution has been a problem for a long term for urban and agriculture areas but recently it has become a social problem due to industrial and economic development.Acid rain(AR)formed from two major air pollutant namely sulfur dioxide(SO2)and nitrogen oxide(NOx).Acid rain raises a great environmental concern,as it is toxic and as such can heavily impact on the ecosystems.Nowadays,it become one of the serious environmental problem worldwide and considered a major abiotic stress to agriculture.Evidence showed that acid rain had the antagonistic effect in plant especially on growth,photosynthetic activities,antioxidant activities and molecular changes.In response,plant intrinsically equipped with sophisticated cellular mechanisms for acid rain detoxification and tolerance,which include immobilization and the repair of cellular structures.However,plants themselves suffer from phytotoxicity under elevated levels of acid rain exposure because of rate-limiting antioxidant and detoxification capacity.Hence,hormonal supplementation under such circumstances may provide additional power to strengthen plantdetoxification system in order to combat the stress generated by xenobiotics.Melatonin(N-acetyl-5-methoxytryptamine)is a multifunctional signaling molecule,ubiquitously distributed in different parts of plants and responsible for stimulating several physiological responses to adverse environmental conditions.In the recent past,melatonin 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.Therefore,the present study was carried out to understand the stress responses of tomato plant towards acid rain as well as to elucidate the possible protective mechanism of melatonin in acid rain stress mitigation through series of experiments.The salient findings are summarized as follows:1.Preliminary studies conducted on acid rain,to better understand the mechanistic basis of plant stress responses,we sought to identify and functionally analyze the photosynthesis activities,different key enzymatic and non-enzymatic antioxidant compounds under simulated acid rain(SAR)stress.This study explored the effects of two p H levels of SAR on photosynthesis and activity of different enzymatic and non-enzymatic key antioxidant compounds comparing with control in two different tomato cultivars.The results indicates that the increasing levels of acidity of SAR,decreased significantly the plant growth,chlorophyll,carotenoids,soluble protein and soluble sugar contents in leaves of both tomato cultivars.The accumulation of different enzymatic antioxidant key compounds was the maximum at p H 3.5 and degraded at p H 2.5 of SAR treatment for both tomato cultivars.In contrast,the growth of hydrogen peroxide(H2O2),malondialdehyde(MDA)and proline content were increased by SAR treatment which depends on the level of p H value of SAR.In addition,marked increase in phenolic,flavonoid and reducing antioxidant activity were found at p H 3.5 followed by p H 2.5 of SAR and control in both tomato cultivars.The findings of the study suggested that the tomato seedlings produced more reactive oxygen species(ROS)scavenging enzymatic and non-enzymatic antioxidant compounds to SAR stress at 3.5 p H level.Meanwhile,the inhibition of growth as well as photosynthesis of tomato seedlings and the severity of oxidative damage were found at p H 2.5 of SAR.2.The current experiment was design to better understand the mechanism of melatonin in simulated acid rain stress responses and we sought to analyze plant growth,ultrastructure of leaf,photosynthesis activity,oxidative and membrane damage,and different antioxidant compounds.This study investigated the possible role of melatonin on different physiological responses involving reactive oxygen species(ROS)metabolism in tomato plants under simulated acid rain(SAR)stress.SAR-stress caused the inhibition of growth,damaged the grana lamella of the chloroplast,photosynthesis,and increased accumulation of ROS and lipid peroxidation in tomato plants.The results showed that plants under SAR-stressed had increased both enzymatic and nonenzymatic antioxidant substances compared with control plants to cope the detrimental effect of SAR-stress.But such increased in antioxidant activities were incapable to inhibit the destructive effect of SAR-stress.Meanwhile,melatonin treatment increased SAR stress tolerance by repairing the grana lamella of the chloroplast,improving photosynthesis and antioxidant activities compared with those in SAR-stressed plant,but these possible effects of melatonin depends on concentration.Moreover,the study suggests that 100 ?M melatonin treatment improve the SARstress tolerance by increasing photosynthesis and ROS scavenging antioxidant activities in tomato plants.3.We have also studied the effects of melatonin in tomato fruits against acid rain stress.To well understand the impact of exogenous melatonin in the enhancement of tomato fruit production as well as nutritional values under simulated acid rain condition,we sought to observe in different nutritional compounds,antioxidant enzymes and nonenzymes,and membrane damage in tomato fruits.This observation aims to identify the influence of melatonin treatment under simulated acid rain(SAR)condition on fruit qualities,phenolics,flavonoids,and carotenoids concentration in fruits,and yield of tomatoes.The study results showed that the fruits of SAR-stressed plants had higher quality traits and antioxidant bioactive compounds by increasing antioxidant activities against SAR-induced oxidative stress compared with fruits of control plants.Nonetheless,these improvement of antioxidant activities in fruits under SAR-condition stayed unable to prevent the reduction of the yield.However,SAR-stressed plants treated by melatonin exhibited upgradation on the fruit quality traits,antioxidant compounds and yield attributes through accelerating oxidant-scavenging antioxidant actions in fruits compared with fruits of SAR-stressed plants.Meanwhile,our result suggest that exogenous melatonin plays an important role in improvement of bioactive compounds and yield traits in tomato fruits through regulating antioxidant system.4.In present study,we compared the effect of melatonin treatment to control and SAR-stressed tomato plant by transcriptome analysis to better understand how exogenous melatonin regulates the differential gene expression,plant secondary metabolites related gene and transcriptional factor in responses to simulated acid rain.This observation aims to expose the possible role of exogenous melatonin on differential expressed genes(DEGs),biosynthesis of secondary metabolites related genes and transcript family responses pattern against acid rain stress in tomato.The results of transcriptome profile through RNA-sequence analysis identified 1228,1120 and 1537 DEGs in control plant(Ctr)vs simulated acid rain stressed plant(P25)comparison,control plant vs melatonin treatment in simulated acid rain stressed plant(P25M)and P25 vs P25 M,respectively.Among them,152 DEGs were commonly expressed in all comparison.DEGs in all comparison were more involved in biosynthesis of secondary metabolites,metabolic pathways,plant pathogen interaction and plant hormone signal transduction.Moreover,151,137 and 191 transcript factor encoding genes were identified in Ctr vs P25,Ctr vs P25 M and P25 vs P25 M,respectively where ERF,WRKY,MYB and b ZIP families' protein accounted more number.The results indicated that exogenous melatonin is closely associated to acid rain stress moderator which might be involved in alteration of DEGs,biosynthesis of plant secondary metabolites and transcriptional factor encoding genes expression.Taken together;the results of the study suggested that melatonin could be a promising strategy for enhanced plant adaptation or tolerance by regulating growth,physiological and molecular activities in tomato under acid rain stress. |
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