The Physiological And Molecular Mechanisms Of Exogenous Silicon Alleviating CA-induced Autotoxicity Stress In Cucumber (Cucumis Sativus L).

Autotoxicity is the main factor of the continuous cropping obstacles of greenhouse cucumbers,which caused in the growth retardation,yield and quality decrease of greenhouse cucumbers.Improving the resistance of cucumber to autotoxic stress has been widely studied and concerned.In recent years,the application of exogenous substances to improve crop resistance has been widely reported.Many studies have shown that silicon（Si）,as the second largest element in the earth’s crust,plays an important role in plant resistance to adversity stress,but the mechanism of enhancing crop resistance to autotoxic stress is not yet clear.In this experiment,cinnamic acid（CA）was used to simulate the autotoxicity stress of cucumber,and the effects of exogenous Si（Na Si O₃·9H₂O）on cucumber seedling growth,root morphogenesis,carbon and nitrogen metabolism,As A-GSH cycle under moderate CA stress were studied by hydroponics,Water metabolism,ion absorption and transcriptome,the physiological and molecular mechanisms of Si alleviating the CA stress in cucumber seedlings were preliminarily discussed,providing a theoretical basis for overcoming the continuous cropping obstacles of greenhouse cucumber.The main results are as follows:1、CA（0.8 mM）stress can significantly inhibit the growth of the shoots and roots of cucumber seedlings,as well as the growth of plant height,stem diameter,leaf area,and root morphological development.1.0 m M exogenous Si can significantly improve the plant height,stem diameter,leaf area,and dry and fresh weight of seedlings under CA stress,promote the seedling roots morpholog,and significantly increase the total root length,average root diameter,and total root volume.The total root surface area,root and branch number,and indicated that the appropriate concentration of exogenous Si can alleviate the inhibitory effect of CA stress on the growth of cucumber seedlings.2、Under CA stress,exogenous Si significantly increased the contents of photosynthetic pigment and starch,and the activities of Rubisco,GAPDH,FBPase,FBA and TK in Calvin cycle;At the same time,the improvement of Fv/Fm,Y（II）,q P and ETR enhanced the light catching efficiency and light energy conversion efficiency of antenna pigment,and alleviated the damage of CA to the photosynthetic system;The activity of key enzymes of sucrose metabolism,such as SPS,SS,AI and NI,was significantly increased,and the accumulation of fructose,glucose and sucrose in cucumber leaves and roots was significantly reduced,maintaining the normal carbon metabolism process in the plant.In addition,exogenous Si significantly increased the activities of nitrate reductase（NR）,nitrite reductase（NIR）,glutamine synthetase（GS）,glutamic acid synthase（GOGAT）and glutamate dehydrogenase（GDH）in leaves and roots of cucumber seedlings under CA stress by 20.78%-108.90%.3、Exogenous Si significantly inhibited the accumulation of H₂O₂ and O₂^·-in leaves and roots of cucumber seedlings under CA stress,and enhanced the activities of APX,MDHAR,AAO,GR,DHAR and GST in ASA GSH key enzyme activity（enzyme system）,increase the content of As A,GSH,As A/DHA and GSH/GSSG（non enzyme system）,decreased the contents of DHA and GSSG,enhanced the antioxidant capacity of cucumber plants,and maintained a relatively stable REDOX cell environment,so as to relieve the oxidative damage of CA stress on cucumber seedlings.4、Exogenous Si significantly improved the water content of cucumber seedling leaves,and increased the relative water content（RWC）,free water content（FWC）and leaf water potential of cucumber leaves under CA stress.The absorption capacity of mineral elements was improved to different degrees,and the accumulation of large amount（N,P,K）,medium amount（Ca,Mg）and trace elements（Fe,Mn,Zn）in leaves and roots was increased.5、High throughput sequencing was used to analyze the transcriptome of leaves and roots of cucumber seedlings under CA stress at 8 h and 10 d.With the extension of treatment time,the number of differential genes in cucumber leaves increased,but the number of differential genes between CK and CA+Si decreased significantly from 8h to 10d,and the alleviation effect was obvious.With the extension of treatment time,the number of differential genes in cucumber roots decreased significantly,and the main differential genes changed from down-regulate to up-regulate.The results showed that there were 6,017 differentially expressed genes in leaves and 20,195 differentially expressed genes in roots;The GO items enriched by differential genes in leaves were mainly"photosynthesis,light response","metabolite precursor and energy generation","transmembrane transport"and"detoxification".The main KEGG items of gene enrichment were"secondary metabolite biosynthesis","phenylpropanoid biosynthesis","indole alkaloid biosynthesis"and"plant hormone signal transduction";The GO items of the enrichment genes in the root system include"metabolic process","growth regulation","transmembrane transport","MAPK cascade positive regulation".The KEGG items of gene enrichment were mainly"glutathione metabolism","peroxidase","phenylpropanoid biosynthesis","amino acid metabolism"and"plant hormone signal transduction".Analysis of differentially expressed genes in GO and KEGG showed that,exogenous silicon may regulate the resistance of cucumber seedlings to cinnamic acid autotoxic stress by regulating the expression of auxin,ABC transporter,peroxidase,amino acid and ion transporter.