| The Three Gorges Project(TGP)is the largest water conservancy project and attract the eyes of the whole world.Due to the construction of the Three Gorges Dam(TGD),nearly 30m-vertical height difference of the riparian zone were formed on the both sides in reservoir area.The riparian zone changed the original habitats of the plants,and caused serious damage to the plants in reservoir area.As we all know that plants under the waterlogging environmentsare usually in a state of hypoxia.The hypoxia environment destroyed the metabolic balance of reactive oxygen species(ROS)and induced the accumulation of ROS,which could cause oxidative damage of plant cells and eventually result in plant death.Myricaria laxiflora is an endangered evergreen shrub,which is the only species in the genus Myricaria distributed in the low-altitude riverbank within the water-level fluctuation zone in reservoir area of the Yangtze River Valley.Previous studies showed that M.laxiflora is highly tolerant to waterlogging environment,and a species for conservation of water and soil in riparian zone.Endophytic fungi are a group of important biological resources in plants,and form a stable mutualistic symbioses with their host plants in the process of long-term coevolution.That is,endophytic fungi obtain the environment and nutrients from the host plants,meanwhile provide a wealth of metabolites beneficial to the host.These metabolites can enhance the ablity of the host plant-resistance to biotic and abiotic stresses by resisting the exogenous stress directly or participating in the process of the regulation of certain resistance gene expression indirectly.The investgation on plant flooding stress showed that plants and their endophytic fungi can produce several enzymes and a network of low molecular mass antioxidants,which can scavenge the redundant ROS in the body system,so as to keep the metabolic balance of plant active oxygen in the body system and protect plants from the damage of ROS.In this context,we selected M.laxiflora and its endophytic fungi as the research object in this study,(1)separated and indentified the phytochemicals of the plants and(2)measured the antioxidant capacity of the fermentation broth of 46 endophytic fungi,respectively.In this study,we assessed the relationship between the chemical composition and the plant-resistance to waterlogging stress by analysis the phytochemical composition and the antioxidant capcity of the fermentation broth of endophytic fungi.Meanwhile,the microbial resources with antioxidant activity obtained by screening,established basic foundation for further research on the interaction between the endophytic fungi and the host plants.The phytochemical investigation of M.laxiflora was conducted,which led to the yield of nine compounds(1-9).Their structures were unambiguously elucidated by analysis of one-dimensional(1D)and two-dimensional(2D)nuclear magnetic resonance(NMR),high resolution-electron spray ionization-mass spectrum(HR-ESI-MS)spectroscopic data and X-ray diffraction as well as by comparison with literature data,including seven triterpenoids(1-7),one sterol(8)and one phenolic acid compound(9).Among them,(5R,8R,9R,10 R,13S,17 S,18S)25,28-dihydroxy-D-friedoolean-14-en-3-one(2)was a new oleanane type triterpenoid.Specially,all of these compounds(1-9)were firsly isolated from M.laxiflora,and compounds 1 and 6 were firstly reported from the genus Myricaria.Previous studies indicated that many of the triterpenoid,sterol and phenolic compounds have the capacity of antioxidant activity.According to our literature survey,we assumed that these compounds(1-9)may assist or participate in the process of eliminating the redundant ROS,so as to help the survival of M.laxiflora under waterlogging stress.The antioxidant activity of fermentation broth of 46 endophytic fungal strains isolated from the stems of M.laxiflora was investigated in this study.After preliminary screening test,10 strains with better total antioxidant capacity(? 15 U/mL)were screened,and then a further screening test on the fermentation broth extracts of 10 strains were determined.At last,the strain MS19-3 exhibited the strongest total antioxidant activity by a comprehensive evaluation.The total antioxidant capacity of the fermentation broth extracts of MS19-3 could reach 27.47±1.03 U/mL,which was up to 22.42% of that of Vitamin C(Vc)in the same concentration.Hence,we analysed the content of total polyphenols and flavonoids of the fermentation broth extracts of MS19-3,their scavenging activity on DPPH,hydroxyl radical and reducing power,respectively.It was shown that the total phenolic and flavonoid content could reach up to 60.02±0.63 mg/g and 42.78±0.79 mg/g,respectively.The crude extract exhibited a certain screening efficiency on DPPH,hydroxyl radical,and a better reducing power.In addition,the strain MS19-3 was identified as Aspergillus clavatus based on the morphological characteristics and the internal transcribed spacers(ITS)sequence(ITS1-5.8S-ITS4)by molecular biology method.These findings demonstrated that the chemical compositions of plants and the metabolites of the endophytic fungi may assist or participate in the process of eliminating ROS in a certain degree,directly or indirectly,and keep the balance of plant active oxygen metabolism in the body system,so as to help the survival of M.laxiflora under waterlogging stress.Additionally,the obtained microbial resources with moderate antioxidant activity,established basic foundation for further research on the interaction between the endophytic fungi and the host plants,and which may provide a guidance for enhancing the antioxidation of the plants,improve the ecological environment in riparian zone of TGR,as well as provide a new approach to obtain the antioxidants. |
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