Effects Of ?-aminobutyric Acid On Physiological Metabolism And SlGADs Gene Expression Of Tomato Grafted Seedlings Under Salt Stress

Tomato is one of the most widely cultivated vegetable varieties at home and abroad,and most of them are moderately salt-sensitive crops.Soil salinization seriously inhibits the growth of tomato seedlings and the improvement of quality.Therefore,it is of great practical significance to study the use of appropriate agronomic techniques to reduce tomato salt stress and clarify its metabolic mechanism.Firstly,in the preliminary experiment,the suitable environmental temperature and pH value for exogenous y-aminobutyric acid(GABA)to induce tomato to respond to salt stress were selected.And under these environmental conditions,RS grafted seedlings was formed by split grafting using salt-sensitive variety'Zhongza 9'(S)as scion and salt-tolerant variety 'OZ-006'(R)as rootstock;the other two grafted materials were the scion self-grafted(SS)and rootstock self-grafted(RR)seedlings.The effects of exogenous GABA on physiological metabolism of three grafted seedlings under salt stress were studied.The tomato seedlings of 'Zhongza 9' were used as test materials to detect the expression levels of four SIGAD genes in GAD that synthesizes GABA in tomato leaves under salt stress induced by exogenous GABA.Clarify the expression pattern of SlGADs under salt stress.The main results are as follows:(1)At different ambient temperatures(15,20,25,30?)and the pH(pH4.5,6.5,8.5),selecting the appropriate temperature and pH value of exogenous GABA-induced tomato in response to salt stress.It is determined that the effect of reducing salt stress is best under the conditions of ambient temperature 20? and nutrient solution pH6.5.(2)Three kinds of grafted seedlings(SS,RS and RR)under 175 mmol·L-1 NaCl stress were treated by exogenous application of 5 mmol·L-1 GABA at an ambient temperature of 20? and a nutrient solution pH6.5.This research was conducted to detect the growth,Na+accumulation,amino acid content and active oxygen metabolism of grafted seedlings were measured to determine the mechanism of exogenous GABA to improve the salt tolerance of the three grafted seedlings(SS,RS and RR).The study showed that exogenous GABA can inhibit Na+ transport to leaves in grafted seedlings,alleviate Na+ toxicity and promote the growth of tomato seedlings under salt stress.However,induced by rootstocks and exogenous GABA,the accumulation of Na+ in seedlings was the highest in SS,followed by RS,and the RR increased the least;Exogenous GABA induced a significant increase in the total amino acid content in the leaves of the three grafted seedlings under salt stress,among them,exogenous GABA mainly induced a significant increase in the content of Pro,Glu,Ser,GABA,Asp,and Ala,Pro changes are the most significant,the highest amino acid content in RR,followed by RS,and the lowest content in SS leaves;Exogenous GABA reduces the production rate of O2-and the content of MDA by increasing the activities of antioxidant enzymes(SOD,POD and CAT)in the leaves and roots of grafted tomatoes.However,the increase of antioxidant enzymes was greatest in RR leaves and roots,followed by RS,and the increase of SS antioxidant enzyme activity was the smallest,which resulted in the accumulation of active oxygen between seedlings SS>RS>RR.However,the effect of GABA on alleviating the salt damage of seedlings was different between SS,RS and RR.The increase of GABA on SS salt tolerance was greater than that of RS and RR.(3)SlGAD1?SlGAD2?SlGAD3 and SlGAD4 genes were successfully obtained using the homologous sequence cloning technology,the amino acid sequence homology of the four SlGAD genes reached 85.82%,SlGAD1,SlGAD2,SlGAD3,and SlGAD4 genes contain an ORF of 1509 bp,1512 bp,1455 bp,and 1482 bp,encoding 502,503,484,and 403 amino acids,respectively.Phylogenetic tree analysis showed that both SlGAD1 and SlGAD2 had the closest relationship with potato,while SIGAD3 had the closest relationship with SIGAD4.Under 175 mmol·L-1 NaCl stress,exogenous addition of 5 mmol·L-1 GABA can induce significantly up-regulated expression of SlGAD1-3 gene and GAD enzyme activity,and significantly down-regulated expression of SlGAD4 gene,The expression level of SlGAD1 gene maintains a high expression level throughout the treatment process,indicating that SIGAD1 is the most sensitive SlGADs gene for exogenous GABA-induced tomato salt tolerance enhancement in tomato.In summary,exogenous GABA at an ambient temperature of 20? and a nutrient solution of pH 6.5 alleviates the damage to grafted seedlings caused by salt stress by inducing tomato salt-tolerant rootstocks to reduce Na+ absorption and upward transport,increase amino acid levels and antioxidant enzyme activities.However,the effect of GABA on reducing salt damage of SS seedlings with weak salt tolerance is more obvious than that of RS and RR with stronger salt tolerance.It is proved that the effect of GABA is related to the tomato seedlings self-tolerance.The increase of RS and RR seedlings salt tolerance was the interaction result of salt-tolerant rootstock and GABA application,while the improvement of SS salt tolerance was mainly induced by GABA application.And SlGAD1 gene plays an important role in exogenous GABA improving tomato salt tolerance.The experimental results clarified the molecular mechanism of exogenous GABA to improve the salt tolerance of grafted tomatoes and the expression of SIGADs gene,which provided theoretical and practical basis for improving the salt tolerance of facility tomatoes.