| Selenium(Se)is a ubiquitous element with average levels in soil ranges 0.1-2 mg kg-1.It(Se)is prerequisite metalloid for humans and animals due to its richness in nutritional quality.The lower doses of Se are considered beneficial for plants growth with additional ability to mitigate heavy metals stress.But,Se is phytotoxic at excessive amount which is still debatable especially in higher plants.Oilseed rape(B.napus L.)is well-renowned for edible oil production and has great ability to withstand heavy metals/metalloids toxicity up to a certain limit.Therefore,B.napus is a potential candidate to exploit against the dual nature of Se.Currently,many researchers focus on the use of plant growth regulators to alleviate heavy metals/metalloids phytotoxicity.Based on the above facts,present study was designed to assess the dual behavior of Se and B.napus tolerance against Se toxicity by using melatonin(MT)as growth hormone.For this,a series of experiments were carried out and their results provide the following findings.(1)A lab based study was performed to examine the potential impacts of selenium(Se)as sodium selenite(0,25,50,and 100?M)on physio-chemical,cell ultra-structural and genomic alterations in 10 days-old seedlings(hydroponic)of four cultivars of B.napus(cvs.'Zheda 619','Zheda 622','ZS 758',and'ZY 50').Results showed that excessive Se(?)(especially 100?M)significantly reduced the plant growth parameters,pigment contents,water-soluble protein levels,and overproduced the H2O2 and MDA contents.Also,a significant increase in antioxidant enzyme activities and transcript levels of superoxide dismutase(SOD),peroxidase(POD),ascorbate peroxidase(APX),glutathione reductase(GR),except catalase(CAT)and levels of reduced glutathione(GSH),except oxidized glutathione(GSSG)in roots,were noticed in both leaves and roots.Excessive Se(?)induced the ultrastructural damages in leaf mesophyll and root tip cells.Overall,less significant phytotoxic effects were observed mainly at 50?M Se(?).Se(?)-supplementation at 25?M displayed marginal beneficial effect by enhancing plant growth,pigment contents,protein levels and restrict the overproduction of H2O2 and MDA levels.A marginal increase/decrease in ROS-detoxifying enzymes(except CAT activity)and elevated GSH and GSSG levels were noticed.The accumulation of Se(?)was much higher in roots as compared to leaves.This accumulation was maximum in'Zheda 622'and minimum in'ZS 758',,followed by'Zheda 619'and'ZY 50'.Overall findings showed that'Zheda 622'was the most sensitive and'ZS 758' as most tolerant to Se(?)toxicity.(2)The dual effects of Se(?)(0,25,50,and 100?M)were broadly investigated on the physio-biochemical,anatomical and molecular alterations in 28 days-grown(greenhouse field conditions)seedlings of different Brassiccanapus L.cultivars(cvs.'Zheda 619','Zheda 622','ZY 50',and 'ZS 758').Findings revealed that Se-supplementation markedly boosted the plant growth and biomasses by improving mineral uptake,water-soluble protein,sugar,photosynthetic efficiency regarding the pigments and gas exchange parameters.Higher Se-levels impaired the photosynthetic efficiency,deplete nutrients-uptake,osmotic stress by proline accumulation and higher Se-accumulation in roots led to growth and biomass reduction.Se-supplementation minimized the accumulation of ROS(hydrogen peroxide,superoxide radical),malondialdehyde and methylglyoxal(MG)levels by activating the enzymes engaged in AsA-GSH cycle and ROS-MG detoxification.But,elevated-Se impaired the oxidative metabolism by desynchronizing the antioxidants as revealed by decreasing levels of ascorbic acid,activities and expression levels of catalase,glutathione reductase,and dehydroascorbate reductase.Up-regulation of secondary metabolites genes(PAL,PPO)revealed the role of Se in regulating transcriptional networks involved in oxidative stress.The damages in leaf and root ultra-structures disclosed the Se-phytotoxicity.Together,outcomes uncovered the protective mechanism of Se(till 25?M)by reinforcing the plant morphology,photosynthesis,osmo-protection,redox balance,enzyme activities for ROS-MG detoxification by reducing ROS and MG components.Excessive-Se prompt phytotoxicity by impairing above mentioned parameters,especially at 100?M Se.Among all B.napus cultivars,'Zheda 622' was discovered as highly-susceptible and 'ZS 758' showed greatest-tolerance against Se stress.(3)This study evaluated the defensive role of exogenous melatonin(MT)(0,50,and 100?M)against Se(?)(0,50,100,and 200?M)stress in 28 days-grown seedlings(field conditions)of Brassica napus via physiological traits,thiol biosynthesis,and antioxidant system.Results showed a dose-dependent inhibition in plant growth and biomass accumulation,impairment in pigment contents,net photosynthetic rate(Pn)and photochemical efficiency of PSII(Fv/Fm)was noted under Se(?)stress,which was effectively alleviated by the addition of MT in the solution.Moreover,exogenously applied MT lowered the oxidative damages and lipid peroxidation,and protected cellular membrane from the negative impacts of Se-toxicity by lowering the Se-induced accumulation of reactive oxygen species.Melatonin also alleviated the osmotic stress by restoring the leaf water and sugar levels.Higher levels of ROS-detoxifying enzymes(SOD,APX,GR,CAT,proline,free amino acids,thiol components(GSH,GSSG,NPTs,PCs and Cyst)and their metabolic enzymes(Y-ECS,GST and PCS)under MT + Se treatment as compared with Se alone stress,suggested the greater potential of MT in scavenging Se-induce oxidative damages.Melatonin induced accumulation of chelating agents in roots suggested its greater potential to detoxify Se in roots and,thus lowered Se-translocation into leaves.Overall findings suggested that MT-induces improvement in osmoprotection,antioxidant defense,and biosynthesis of thiolic ligands,ultimately helps in Se-detoxification/sequestration through enhancing B.napus tolerance against Se(?)stress.(4)In the current study,we employed RNA sequencing analyses to get insights into the B.napus response against Se stress and tolerance mechanisms involved by the exogenous application of melatonin(MT).Digital gene expression(DGE)and transcriptome profile analysis(TPA)techniques were employed to uncover the mechanisms involved in Se stress and MT-induced mitigation in two B.napus cultivars.Data regarding differently expressed genes(DEGs)showed that Se significantly reduced the number of DEGs in comparison with control plants(more DEGs down-regulation in'Zheda 622')and MT enhanced the number of DEGs than Se alone treatments.Among the gene ontology(GO)terms,the stress responsive light stimulus,response to the karrikin,transmembrane receptor protein tyrosine kinase signaling pathways,wax biosynthesis process and fatty acid biosynthesis were prominent mainly in biological process and cellular components.KEGG pathways were inhibited by the Se-exposure as observed in fatty acid metabolism,porphyrin and chlorophyll metabolism in'Zheda 622',and photosynthesis,photosynthesis-antenna biosynthesis,glyoxylate and dicaroxylate metabolism in'ZS 758'.MT improved or recovered the above mentioned attributes under Se-stress conditions with enhanced tolerance in'ZS 758'than'Zheda 622'. |
PDF Full Text Request