Study On Function Of Spermidine Synthase Gene(SlSPDS2) In Tomato Seedlings Under Saline-alkali And Cold Stress

Tomato（Solanum lycopersicum L.）as an important horticultural crop,is an indispensable food on the table.In recent years,due to the abuse of fertilizer in agricultural planting,coupled with the instability of air temperature during the overwintering and early spring cropping,tomato facility cultivation is easily affected by low temperature and saline-alkali stress,especially at the seedling stage.Polyamines（PAs）are organic compounds containing two or more amino groups that exist widely in various organisms.They can participate in the process of plant resistance to stress through direct or indirect action.Spermidine synthase gene（SPDS）is an important gene in the process of polyamine anabolic metabolism,but its role in tomato seedling resistance to salinity and low temperature stress is rarely studied.Therefore,in this study,SlSPDS2 overexpression and silenced materials were constructed against the background of’Ailsa Craig’tomato（AC）,and AC wild-type tomato seedlings,SlSPDS2 overexpression and silenced tomato seedlings were used as experimental materials to explore the effects of SlSPDS2 on tomato’s resistance to salt-alkali and low temperature stress.Thus,it provides a theoretical basis for the study of tomato resistance varieties.The main research results are as follows:1.There are 5 spermidine synthase homologous genes in tomato,among which SlSPDS2 has the highest expression level in all parts of tomato,suggesting that it may play an important role in the growth and development of tomato.Bioinformatics analysis of SlSPDS2 showed that SlSPDS2 gene was located at the front of tomato chromosome 5,and its promoter region had a large number of cis-acting elements related to plant growth and development,cell metabolism,hormone regulation,response to stress and photosynthesis.The amino acid number of SlSPDS2 protein was 342,the relative molecular weight was 37678.90,the isoelectric point was 4.81,the extinction coefficient was 35910,the half-life was 30,and the instability coefficient was 56.18.SlSPDS2 protein mainly played a role in spermidine synthesis.Among the established phylogenetic trees,the St SPDS protein in potato,which is also a Solanaceae crop,has the highest similarity to the SlSPDS2 protein sequence of tomato.2.SlSPDS2 played an active role in enhancing the saline-alkali stress resistance of tomato seedlings.Overexpression of SlSPDS2 could improve the transcription levels of polyamine synthesisation-related genes（Sl ADC,Sl ODC,Sl SPMS,Sl SAMDC）in tomato seedlings under saline-alkali stress,and produce more free Putrescine（Put）,Spermidine（Spd）and Spermine（Spm）.Overexpression of SlSPDS2 increased the transcription level of the genes encoding antioxidant enzymes（Sl SOD,Sl POD and Sl CAT）,and increased the activities of peroxidase（POD）,superoxide dismutase（SOD）and catalase（CAT）.Reduce malondialdehyde（MDA）content and ROS((O₂^?-and H₂O₂)accumulation in tomato leaves.Overexpression of SlSPDS2 increased Na+and K+channel protein coding genes（Sl NHX1,Sl NHX2,Sl NHX3,Sl HKT1,Sl SOS1 and Sl HKT1;2）,decreased the content of Na⁺and Na⁺/K⁺in the above-ground and underground parts of tomato seedlings,increased the content of K⁺,decreased the relative electrical conductivity（REC）,increased the relative water content of tomato leaves（RWC）,and improved the saline-alkali stress resistance of tomato seedlings.Under saline-alkali stress,the above indexes showed opposite rule in silted SlSPDS2tomato seedlings,and the tolerance of tomato seedlings to saline-alkali stress was decreased.3.SlSPDS2 plays an active role in the resistance of tomato seedlings to low temperature stress.Overexpression of SlSPDS2 improved the resistance of tomato seedlings to cold stress.Overexpression of SlSPDS2 could improve the transcription level of polyamine synthese-related genes in tomato seedlings under cold stress,increase the contents of free state Put,Spd and Spm,improve the transcription levels of Sl ICS and Sl PAL in tomato leaves,produce more salicylic acid（SA）content,and reduce the degradation of chlorophyll a,chlorophyll b and carotenoid.The MDA content and ROS accumulation in tomato leaves were decreased,the transcription levels of Sl SOD,Sl POD and Sl CAT were increased,the activities of POD,SOD and CAT were increased,the contents of proline and soluble sugar in tomato leaves were increased,REC was decreased,RWC in tomato leaves was increased,and the resistance to cold stress of tomato seedlings was improved.Under cold stress,the above indexes showed the opposite pattern in silent SlSPDS2 tomato seedlings,and the tolerance of tomato seedlings to cold stress was decreased.In conclusion,SlSPDS2 plays an important and positive role in the resistance of tomato seedlings to saline-alkali and cold stress.