| Nitraria sibirica Pall.is a typical halophyte in China and an afforestation tree species for improving saline-alkalized soil.As a new exogenous substance,?-aminobutyric acid(GABA)can effectively alleviate the damage caused by biotic and abiotic stress to plants.In this study,salt-tolerant seedlings of N.sibiria were used as experimental materials,different concentrations of GABA,were sprayed to screen the best concentration of GABA for alleviating N.sibiria under200 mmol·L-1 NaCl stress,and the relationship between polyamine,proline metabolisms and endogenous GABA metabolism in N.sibiria under exogenous GABA and NaCl stress was studied,so as to provide theoretical basis for exogenous GABA to alleviate plant salt stress injury and improve plant salt tolerance.The main results are as follows:(1)Exogenous spraying of 15 mmol·L-1 GABA could significantly weaken the inhibitory effect of NaCl treatment on dry and fresh weight and plant height of shoots and roots of N.sibiria seedlings.Under NaCl stress,MDA,relative electrical conductivity,soluble sugar and proline content of N.sibiria seedlings increased significantly,while relative water content,leaf photosynthetic pigment and soluble protein content decreased significantly,which inhibited the growth of N.sibiria seedlings.Exogenous GABA treatment could make the relative water content and photosynthetic pigment of leaves of N.sibiria decreased at first and then increased with the extension of treatment time,MDA and relative conductivity decreased significantly,the content of soluble sugar increased at first and then decreased,the content of soluble p rotein and proline increased continuously with the extension of treatment time,and the relief effect was the best after7 days of treatment.(2)NaCl stress could increase the activities of polyamine synthase ADC,ODC and SAMDC and polyamine degradase DAO,PAO and significantly up-regulate the transcriptional levels of ADC,ODC,SAMDC and PAO.After 7-14 d treatment,the expression of genes encoding synthase was higher than that of genes degradation enzyme,and the content of polyamine was maintained at a high level.NaCl stress increased the content of free polyamines Put,Spd and Spm in the leaves of N.sibiria seedlings and promoted the rapid transformation of Put to Spd and Spm in N.sibiria seedlings to improve the tolerance of seedlings.Exogenous GAB A is involved in regulating the metabolism of polyamines in N.sibiria seedlings under NaCl stress.Exogenous GABA can increase the activities of ADC,ODC and SAMDC in N.sibiria seedlings,inhibit the activities of DAO and PAO,further enhance the ability of polyamine synthesis,and then increase the content of polyamines.The peak value of(Spd+Spm)/Put was also earlier than that of salt treatment.Exogenous GABA treatment of 14 d increased the activities of DAO and PAO,enhanced the ability of polyamine degradation to GABA,and increased salt tolerance of N.sibiria seedlings,and the increase of endogenous GABA content had negative feedback regulation on polyamine degradation.(3)NaCl stress could increase the activities of proline synthase P5CS and?-OAT in N.sibiria seedlings.With the extension of stress time,the accumulation of Pro was positively correlated with the activities of P5CS and?-OAT.Exogenous GABA is involved in the regulation of proline metabolism in N.sibiria seedlings under NaCl stress.Exogenous GABA treatment further up-regulated the activity of P5CS,inhibited the activities of?-OAT and Pro DH,and significantly up-regulated the transcription level of Pro DH under non-salt stress and salt stress.Exogenous GABA promoted the accumulation of Pro in N.sibiria leaves under salt stress by activating Pro synthesis pathway and inhibiting degradation pathway.It can also increase the content of Pro and Put under NaCl stress,promote the transformation of Pro and other substances and maint ain dynamic balance during salt stress,so as to enhance the salt tolerance of N.sibiria seedlings.(4)Exogenous GABA can form positive feedback regulation of GABA in plants,which can increase the resistance of N.sibiria seedlings.NaCl stress could increase the content of endogenous GABA in leaves of N.sibiria,decrease and then increase the activity of GAD,increase the activity of DAO and PAO,up-regulate the activity of GABA-T and its transcriptional level,and promote the accumulation of endogenous GABA in leaves of N.sibiria.Exogenous GABA spraying significantly promoted the accumulation of endogenous GABA under normal and stress conditions,and further up-regulated the activity of GAD and its transcription level in leaves after 3 and 7 d of treatment.The activity of 14 d GABA-T increased significantly and promoted the degradation of GABA.Exogenous GABA could maintain the high salt stress adaptability of N.sibiria,and spraying for 7-14 d had the best mitigation effect on N.sibiria. |
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