| Silicon(Si)can significantly alleviate salt stress in many kinds of plant species,however,the mechanisms,especially the molecular mechanisms,are still veiled.In this study,we used a combined approach including rice mutants defective in Si uptake,iso-osmotic stress treatments,SEM-EDX,quantitative PCR,high-throughput sequencing et al.to investigate the mechanisms behind Si-induced salt stress resistance in rice with the focuses on 1)alleviative effects on salt stress of different levels Si addition in rice,2)discrimination of Si-induced alleviation of ionic toxicity from osmotic stress,3)Na uptake and translocation,4)K uptake and translocation,and5)root endophyte bacteria assembly.The major results are presented as follows:(1)The effects of different levels of Si addition(0,0.5 and 1.5 m M)on the growth,photosynthesis system,Na/K homeostasis and root morphological traits in rice under salt stress(0,50 and 100 m M Na Cl)were investigated using hydroponic experiment.The results showed that 100 m M Na Cl caused more severe growth inhibition,oxidative damage and Na/K imbalance in rice than 50 m M Na Cl.Exogenous Si addition alleviated salt stress while better effect was observed with the addition of Si at 1.5 m M Si than at 0.5 m M Si.Moreover,we found that the addition of Si at 1.5 m M Si regulated the activities of SOD,CAT,APX and POD,and the expression of Os SOS1,Os NHX1,Os NHX3 and Os NHX5,thereby alleviating salt stress-induced oxidative damage and maintaining Na/K balance in rice.(2)To discriminate the role of Si in alleviating ionic stress from osmotic components of salt stress,rice seedlings were grown hydroponically in iso-osmotic stress conditions developed from either Na Cl or polyethylene glycol(PEG).The effects of Si on the growth of shoot and root were compared using Principal Coordinate Analysis(PCo A).Generally,Si significantly promoted shoot and root growth in rice exposed to both Na Cl and PEG.PCo A showed that the Si-induced distance change under Na Cl treatment was larger than that under PEG treatment in shoot,but was at an equal level under both Na Cl and PEG treatments in root.Under salt stress,Si decreased Na concentration and Na/K ratio in shoot but not in root.However,Si decreased net Na uptake and increased root Na accumulation content.The osmotic potential was increased in shoot but decreased in root by Si addition.Si decreased soluble sugar and proline concentrations in shoot but increased soluble sugar and soluble protein concentrations in root.Besides,Si promoted shoot transpiration rate and root morphological traits.In summary,Si ameliorated ionic toxicity by decreasing Na uptake and increasing root Na reservation.Si alleviate osmotic constraint by regulating root morphological traits and root osmotic potential but not aquaporin genes expression for water uptake,and promoting transpiration force but not the osmotic force in shoot for root-to-shoot water transport.(3)We investigated the roles of OsLsi1 and OsLsi2 in Si-induced reduction of bypass flow and its resultant alleviation of salt stress by using lsi1 and lsi2 mutants(defective in OsLsi1 and OsLsi2,respectively)and their wild types(WTs).Under salt stress,Si promoted plant growth and decreased root-to-shoot Na translocation in WTs,but not in mutants.Simultaneously,quantitative estimation and fluorescent visualization of trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic(PTS,apoplastic tracer)showed Si reduced bypass flow in WTs,but not in mutants.Energy-dispersive X-ray microanalysis(EDX)shows that Si was deposited at root endodermis in WTs,but not in mutants.Moreover,results obtained from root split experiment using lsi1 WT showed that down-regulated expression of Si transport genes(OsLsi1 and OsLsi2)in root accelerated Si deposition at root endodermis.In summary,our results reveal that Si deposition at root endodermis and its resultant reduction of Na bypass flow is determined by OsLsi1 and OsLsi2 and regulated by the expression of OsLsi1 and OsLsi2.(4)Through comparing the effects of Si on K uptake and translocation in lsi2 and lsi2 and their WTs,we investigated the mechanisms of Si-induced K status promotion in rice under salt stress.Generally,Si promoted K accumulation,shoot K distribution and xylem sap K concentration in lsi1 WT and lsi2 WT but not in lsi1 and lsi2 mutants.NMT showed that Si increased K uptake rate in lsi1 WT and lsi2 WT but not in lsi1 and lsi2 mutants.Genes expression analysis indicated that Si addition promoted the expression of genes responsible for K uptake(Os AKT1 and Os HAK1)and xylem loading(Os SKOR).In conclusion,Si can promote K uptake and root-to-shoot translocation by regulating the expression of related genes,thereby alleviating salt stress-induced K deficiency.(5)We investigated the effects of Si on root endophytes assembly in rice under salt stress by using high-throughput sequencing and root split experiment.Sequencing analysis showed that Si can regulate root endophyte assembly in rice under salt stress which could contribute to Si-induced salt stress resistance.The root split experiment showed that the regulative effects of Si on endophyte assembly were controlled by shoot.In addition,Si promoted the performance of photosynthesis system and decreased osmolytes accumulation in shoot of rice under salt stress.In rice root,Si addition increased osmolytes accumulation.Our results indicate that Si can regulate energy balance between shoot and root in rice under salt stress which determines the assembly of root endophyte bacteria and contributes to the adaption to salt stress in rice.In conclusion,our results show Si can improve salt stress resistance in rice by alleviating ionic and osmotic stresses while the alleviation effects and underlying mechanisms differ between shoot and root.Si can ameliorate ionic toxicity and promote Na/K balance in rice under salt stress by multiple strategies including Na bypass flow reduction,K uptake and translocation regulation,and membrane stabilization.In addition,we build the relationships among Si,endophytes and abiotic stress resistance. |
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