Mechanisms Of Silicon On Enhancing Salt Stress And Potassium Deficiency Tolerance In Sorghum Seedlings

With global warming and sustained population growth,the world including China is facing severe food security pressures.In the practical agricultural production,the major hindrance for the achievement of crop yield potential is the limitations of environmental stress.Therefore,it is important to explore effective ways to improve crop stress resistance.Silicon(Si),as a beneficial element,has also been widely reported to improve plant various stress resistances.In agricultural production,the application of Si fertilizer could be used as anefficient way to comprehensively improve various stress resistance.Therefore,it is of great significance to understand the mechanisms of Si on improving plant stress resistance for the improvement of crops broad spectrum stress resistance and the rational utilization of Si fertilizer.Previous studies on the mechanism of Si enhancing plant stress resistance mostly focus on the physical barrier effects of Si deposited in plants and the influence of Si on plant antioxidant capacity.In recent years,some evidence suggests that Si can not only function as physical barrier,but also as a bioactive molecule to actively participate in the plant response to various stress.The objective of this study is to investigate the mechanism of Si-enhanced sorghum resistance to salt stress and potassium(K)deficiency.The main results are as follows:Hoped toclear the effects of Si on polyamines metabolism and signal under salt stress and K deficiency and clarify the effects and mechanism of Si-enhanced sorghum resistance to salt stress and K deficiency by mediating polyamines metabolism;and toclear the effects of Si on plant water status under K deficiencyand clarify the effects and mechanism of Si-enhanced sorghum resistance to K deficiency by improving the plant-water status.(1)Si improves the salt tolerance in sorghum by mediating the polyamines and ethylene synthesisPolyamines,as plant growth regulating substances,are widely implicated in the regulation of plant growth,development,senescence,and resistance to stress conditions.There is a positive relationship between PA levels and salt tolerance;polyamines can enhance the plant salt tolerance by maintaining the K+/Na+balance of plant tissue under salt stress throughadjusting the ion channels.In the previous study,we found that silicon can increase the expression of S-adenosylmethionine decarboxylase(SAMDC),a key gene for polyamines synthesis,and increase the accumulation of polyamines under the salt stress in sorghum.Therefore,this study was designed to clear the effects of Si on polyamines metabolism and signal under salt stress clarify the effects and mechanism of Si-enhanced sorghum resistance to salt stress and by mediating polyamines metabolism.Here,the the growth,concentrations of Na+ and K+,the levels of polyamines and ethylene,and the expression of polyamine and ethylene synthesis genes of sorghum seedlings were analyzed 1,3,and 7 days after treated with(NaCl:100 mM)combined with or without Si(H2SiO3: 1 mM).Thegrowth,ion concentrationsof seedlings exposed to polyamine and polyamine synthesis inhibitor(DCHA)combined with salt and Siwere also measured.The results showed that the growth was reduced and Na+ accumulationsin root,stem and leaves was increased by salt stress,but Si application significantly alleviated the growth inhibition and the Na+ accumulation.Simultaneously,both free and bound putrescine(Put),spermidine(Spd)and spermine(Spm)concentrations were increased and ethylene precursor 1-aminocyclopropane-1-carboxylic acid(ACC)concentration was decreased by exogenous Si under salt stress.Several key PA synthesis genes arginine decarboxylase gene(ADC),N-carbamoylputrescine amidohydrolase(CAP)and SAMDCwere up-regulated by Si under salt stress.To further confirm the role of PA in Si-mediated salt tolerance,seedlings were exposed to Spdor DCHA combined with salt and Si.Exogenous Spd showed similar effects as Si under salt stress,whereas exogenous DCHA eliminated Si-enhanced salt tolerance and the beneficial effect of Si in decreasing Na+ accumulation.These results indicate that polyamines and ACC are involved in Si-induced salt tolerance in sorghum and Si improves the salt tolerance in sorghum by mediating the polyamines and ethylene synthesis.(2)Si improves the potassium(K)deficiency tolerance by alleviating K-deficiency-induced leaf chlorosis and necrosis through mediatingpolyamine synthesis and metabolismK is an essential macronutrient for plants;K deficiency seriously affects the metabolic processes of plants and leads to crop yields reduction.Si can significantly improve plant tolerance to K deficiency,but the underlying mechanismsare not clear.The accumulation of polyamines,especially the Put is a universal plant response to K deficiency.Therefore,this study was designed to clarify the effects and mechanism of Si-enhanced sorghum resistance to K deficiency by mediating polyamines metabolism.Here,the effects of Si application(H2SiO3: 1 mM)on sorghum growth,leaf chlorosis symptoms,K+ concentration,polyamine levels,amineoxidase activities,Put synthesis gene expression levels and antioxidant enzyme activities were measured under K-deficient(KCl: 0.05 mM)conditions.The results showed that Si application significantly alleviated the growth inhibition induced by K-deficient stress.K deficiency induced leaf chlorosis and reduction in several leaf chlorosis-related metrics,including photosynthesis,photosystem II photochemistry efficiency,chlorophyll content and chlorophyll a/b ratio;all of these changes were moderated by Si application.Si application did not influence the K+ concentration in leaves under both K-sufficient or K-deficient conditions,indicating that Si had little effect on the uptake and distribution of K+ and Si’s protective effectagainst K-deficiency-induced leaf chlorosis was not due to increased leaf K+ concentration.The Put and Spd concentration in K-deficient leaves was 14-and 3 fold of that in control-condition,while the Spm concentration decreased by 38 % compared with control conditions.Si application decreased the K-deficiency-induced Put accumulation.In Si-treated plants,the Put concentration in K-deficient leaveswas 52 % lower than that in Si-untreated K-deficient plants.Siapplication did not change Spd and Spm levels,however.Simultaneously,the expression of Put synthesis gene ADC,CAP and spermine synthase(SPDS)were down-regulated by Si application under K-deficient conditions.In addition,Si application decreased K-deficiency-induced arginineaccumulation.These results indicate that Si reduced the accumulation of Put by inhibiting the synthesis of polyamines under K deficiency conditions.UnderK-deficient conditions,DAO and PAO activities increased by 42% and 35%,respectively,but these increases were eliminated by Siapplication.Meanwhile,Sidecreased K-deficiency-induced H2O2 accumulation,but the K-deficiency-enhanced antioxidant enzyme activities were reduced by Si.Therefore,these results indicate that Si application could reduce K-deficiency-induced Put accumulation by inhibiting Put synthesis and could decrease H2O2 production via polyamine oxidation.Decreased H2O2 accumulation contributes to the alleviation of cell death,thereby alleviating K-deficiency-induced leaf chlorosis and necrosis.Combined with the results of section 1 and 2 in this study,we can conclude that Si could improve the sorghum stress tolerance by mediatingpolyamine metabolism under both salt stress and K deficiency,but the mechanisms are different.The effects of Si on polyamine metabolism were maintaining the plyamine at a level conductive to plant growth,rather than straightforward increasing or decreasing the polyamine content.Under stress condition,when the tissue polyamines content is low(such as salt stress),Si promotes the accumulation of polyamines and enhances its beneficial effects;while the tissue polyamines content is high(such as K deficiency),Si inhibits the polyamines synthesis,and reduces the toxicity of the overaccumulation of polyamines.(3)Si improves the K deficiency tolerance by alleviatingplant-water status through enhancing root water uptakeWater balance is the basis of plant survival,growth and development.K+plays an important role in regulating the plant water balance,and severe K defciency causes tissue dehydration.Recent researches reported that enhanced root hydraulic conductance by aquaporin regulation is responsible forSi-alleviated plant-water status under drought and salt stresses.Therefore,Simay alleviate the plant growth inhibition by maintaining plant water balance under K deficiency.To verify this assumption,the effect of Si application(H2SiO3: 1 mM)on plant growth,water status,whole-plant hydraulic conductance,root hydraulic conductance,K+ concentration in xylem sap,xylem sap osmotic potential,and the expression of aquaporin and K+ uptake and transport related genes in sorghum seedlings underK deficiency were analyzed.The results showed that K deficiency decreased the leaf relative water content and water potential,inhibited the photosynthetic rate and the biomass accumulation.But Si alleviated these harms,indicating that Si improved the plant water status under K deficiency.Si application did not influence the K+ concentration in eithershoots or roots under control or K-defciency conditions,indicating that Si alleviatingthe K defciency was not due to a direct increase in K+ absorption.Si significantly enhanced the sorghum transpiration rate,whole-plant hydraulic conductance and root hydraulic conductance under K deficiency,suggesting that Si improved the root water uptake capacity under K deficiency.Further research found that 29% of Si-alleviated transpiration was eliminated by HgCl2 treatment.And the expression of plasma membrane aquaporinsPIPswere up-regulated by Si.These results suggested that the beneficial effect of silicon on root water uptake under K deficiency is partly due to the Si-enhances aquaporin activity.Meanwhile,the K+ concentration in xylem sap was significantly increased and the xylem sap osmotic potential was decreased by Si application.In addition,the expression of Stelar K+ Outward Rectifer(SKOR)was up-regulated,while the transcripts ofHigh Afnity K+ Transporter5(HAK5)were down-regulated by Si application under K-deficient conditions.These results show that Si enhanced the root water uptake by mediating K+ accumulation in xylem and hence increasing the osmotic gradient between the external solution and the xylem sap.Therefore,Si enhanced the water uptake by mediating K+ accumulation in xylem and improving the aquaporin activity,thereby alleviating the plant-water status under the K-deficient condition.Combined with the results of section 2 and 3 in this study,we can conclude that on one hand in root,Si could alleviate the K deficiency-induced growth inhibition by enhancing root water uptake and moderating the plant water balance;on the othe hand in leaves,Si could improve the plant K deficiency tolerance by alleviating K deficiency-induced leaf chlorosis and necrosis through reducing the Put accumulation.This study clarified the mechanisms of Si enhancing sorghum tolerance to K deficiency from aspects of polyamine metabolism and water balance.It will contribute to the efficient utilization of silicon and K fertilizer in agricultural production.This study clarified the mechanisms of Si improving sorghum resistance to salt stress and K deficiencyand cleared that Si could enhance sorghum stress tolerance through active roles other thanthe physical barrier of depositedSi and the improving plant antioxidant capacity.This study will provide great reference value for improving the broad spectrum stress resistance of plant and the rational and effective utilization of silicon fertilizer.