The Role Of Plant Secondary Metabolism In The Resistance To Oxidative Stresses And Organic Pollutants And Regulation Mechanism

Greenhouse horticulture is an improtant integrative part of modern agriculture in China.The growth and development of crops are dependent on the suitable environment conditions.However,the productivity and quality of crops are always negatively affected by abiotic stresses(such as chill,high temperature,drought,and salinity) and biotic stresses(pathogen infection,insect attack) due to the relatively poor basic facility and management.Secondary metabolism is a consequence of the interaction between plants and environments in the long process of plant evolution. Secondary metabolites play important roles for plants to improve defensive and competitive abilities as well as correspond to the environments.Therefore,it is of great importance to investigate the synthetic way of secondary metabolism and its response to environment stresses and explore the effective strategy to enhance the tolerance of plants and improve the productivity,quality and safety of agricultural produces.This study plays theoretical and practical roles in the development of sustainable agriculture.To begin with,wild type and pal1 pal2 mutants of Arabidopsis thaliana were used as the material to investigate the effects of PAL1 and PAL2 on secondary metabolism and oxidative stresses.And then cucumber(Cucumis sativus L.) was used as the material to study the effect of 24-epibrassinolide(EBR) on secondary metabolism and analyze physiological and molecular mechanism of EBR on tolerance to toxicities of organic pollutants.The main results are as follows:1.Tolerance to oxidative stresses in pal1 pal2 mutants decreased due to the inhibition of the phenylpropanoid pathway.First the lack of PAL1 and PAL2 resulted in effective changes in plant morphology and biomass,and these effects were associated with the declines of phenylalanine ammonia-lyase(PAL) activity,phenols, flavonoids,lignin and DPPH activity.Moreover,the activities of CAD,G-POD, CA-POD,CGA-POD and PPO were inhibited in pal1 pal2 mutant leaves.Since oxidative stresses induced reductions of chlorophyll fluorescence parameters in pal1 pal2 mutants,it was found that PAL1 and PAL2 played important roles in protecting Arabidopsis from paraquat stress and UV-B stress.The results demonstrated that PAL may be directly involved in plant stress responses through regulating secondary metabolism.2.EBR had a positive effect on the induction of secondary metabolism in cucumber leaves and roots.Levels of phenols,flavonoids,lignin,DPPH activity and enzyme activities related to secondary metabolism were elevated in a short time and reduced at 2 d and returned to basal levels gradually after EBR treatment.We selected the timing of the most remarkable changes in cucumber secondary metabolism, meanwhile,it was found that EBR played the similar roles in regulating the contents of cinnamic acid,ρ-coumaric acid,caffeic acid and ferulic acid.The change of gene expression was closely associated with enzyme activities related to secondary metabolism.In addition,EBR-induced changes were earlier in treated leaves than in roots significantly.These results provided evidence that BRs may be involved in promoting the whole plant secondary metabolism by a novel long-distance signal transmitted from shoots to roots.3.EBR protected cucumber from toxicities of organic pollutants through accelerating the whole metabolism.First of all,cucumber seedlings were supplied with three kinds of typical organic pollutants with different concentrations.It was shown that plant shoot and root growth rate decreased accompanied by increasing concentrations,moreover,we selected experimental concentrations of 2,4,6-trichlorophenol(TCP,5μM),chlorpyrifos(CPF,10μM) and oxytetracycline (OTC,50μM) respectively.EBR at different concentrations had a positive effect on tolerance to toxicities of organic pollutants and 0.1μM EBR was most effective.In addition to its roles in promoting glutathione metabolism and secondary metabolism, EBR application alleviated the increase of MDA and regulated the redox ratio of GSH/GSSG after treatment with TCP,CPF or OTC.EBR induced enzyme activities (such as SOD,CAT,APX,GST,GR,PAL,PPO,and G-POD) and gene expression. The results suggested that BRs may be promising,environmentally friendly,natural substances suitable for wide application to reduce the risks of human and environment exposure to organic pollutants.