The Physiological And Molecular Mechanisms Study On Asparagus Tolerance To Salt Stress

Ecological protection and high-quality development in the Yellow River Basin are major national strategies.The efficient development and utilization of saline land in the Yellow River Delta is a top priority.Salt tolerance is a key stress-resistant trait of plants,and research on the mechanism of salt tolerance and the development of new salt-tolerant varieties is the focus of current research.Asparagus is an important crop with strong salt tolerance and had high economic,ecological and medicinal value,however,few studies on the physiological and molecular mechanism of salt tolerance of asparagus are available.Therefore,it is of great significance to study the physiological and molecular mechanism in salt tolerance of asparagus in order to promote asparagus cultivation in saline-alkali land of the Yellow River Delta.In this study,22 asparagus cultivars were treated with different concentrations of NaCl at germination stage and seedling stage,respectively.The salt tolerance of different cultivars was initially analyzed based on different physiological factors such as germination rate,seedling relative growth and other indicators.The salt tolerant cultivar Apollo and salt susceptible cultivar Jinglulu 1 were treated with NaCl concentration of 0(CK)and 0.7%at seedling stage.A comparison of the growth and development,the ion content,osmotic regulator content and antioxidant enzyme activity,as well as the hormone content and gene expression of the two cultivars was carried out in order to understand the physiological and molecular mechanism for salt tolerance of asparagus.The main results are as follows:1.Evaluating the salt tolerance of asparagus:The maximum coefficient of variation of relative germination rate at the germination stage was 0.7%～0.9%NaCl,and the maximum coefficient of variation of relative growth amount at the seedling stage was 0.7%NaCl.Therefore,the 22 asparagus germplasm were treated with 0.7%NaCl,and evaluated further using the relative germination rate,relative germination index,relative root length,relative plant height,relative root fresh weight,relative shoot fresh weight,relative root dry weight,relative root fresh weight,relative chlorophyll content and other indexes.ZF 315,Grande,Apollo were evaluated as salt tolerant cultivars.Atlas,JL 1,Grace,SF were considered as salt susceptible cultivars.The Apollo and Jinglvlu 1 were selected for further characterization and subsequent experiments.2.Growth of asparagus cultivars with different salt tolerance under salt stress:Salt stress inhibited the growth and development of shoot of the two cultivars resulting in decreased plant height,shoot fresh weight and dry weight,whereas the salt susceptible cultivar showed more decrease.Salt stress inhibited the root growth of salt susceptible cultivar,and inhibited the shoot growth of both cultivars more than the root growth.Appropriate salt concentration can promote the growth and development of root system of salt tolerant cultivar.3.Changes of osmotic regulatory substances in different salt tolerant asparagus cultivars under Salt Stress:Salt stress significantly increased the proline content in roots and shoots of the two asparagus cultivars,and the increase of proline content in salt susceptible cultivar was greater than that in salt tolerant cultivar.With the extension of salt stress time,soluble sugar and soluble protein contents in Apollo roots and shoots increased gradually.The content of soluble sugar and proteins of JL 1 increased initially and then decreased in root system,and continued to decrease in the shoot as well.These results suggested that salt tolerant cultivar could enhance osmotic regulation ability and improve their salt tolerance capacity by increasing soluble sugar and protein.Noticeably,the osmotic regulation ability of root was stronger than that of shoot.4.Changes of antioxidant enzyme activity and MDA content of asparagus cultivars with different salt tolerance under salt stress:Both cultivars showed significant increase in SOD,POD and CAT activities under salt stress.With the extension of salt stress time,the activities of these antioxidant enzymes in roots and shoots of Apollo increased gradually.The activities of three antioxidant enzymes in roots of JL 1 showed a distinct trend of first increasing and then decreasing,and the activities of antioxidant enzymes in shoots of JL 1 gradually decreased.At the same time point,MDA content in root and shoot of the two cultivars increased gradually,and the content increase range of MDA in salt susceptible cultivar were greater than that in salt tolerant cultivar.Based on these findings,we speculate that salt tolerant cultivar showed better antioxidant protection,with a stronger antioxidant capacity in roots than shoots.5.Changes of K~+and Na~+ion contents of asparagus cultivars with different salt tolerance under salt stress:Under salt stress,K~+content and K~+/Na~+in roots,stems and leaves decreased,while Na~+content increased significantly.K~+/Na~+in roots was higher than that in stems and leaves.With the extension of salt stress time,K~+content in JL 1 decreased more than that of Apollo.The increase rate of Na~+and decrease of K~+/Na~+was decreased in Apollo,while the increase rate of Na~+and decrease of K~+/Na~+was decreased in JL 1,and the change rate was greater than that of Apollo.6.Changes of hormone content in asparagus cultivars with different salt tolerance under Salt Stress:Under salt stress,the ABA content of both cultivars increased significantly.In Apollo,the IAA content was increased in roots and decreased in shoots.However,the IAA content of JL1 in both roots and shoot was decreased.These results suggested that salt susceptible cultivar could respond to salt stress by increasing ABA content,while salt tolerant cultivar could increase IAA content and enhance adaptation to salt stress.7.Transcriptome analysis of asparagus roots with different salt tolerance under salt stress:Transcriptome analysis of the two cultivars was carried out by RNA-Seq.Under 0.7%NaCl treatment salt stress,the differentially expressed genes(DEGs)of asparagus roots are mainly enriched in carbon metabolism,amino acid metabolism,secondary biological metabolism and other pathways,which are important metabolic pathways in response to salt stress.There are 112up-regulated and 364 down-regulated differentially expressed genes that are unique to Apollo,and 61 up-regulated and 33 down-regulated DEGs were unique to JL 1.These DEGs are involved in osmotic regulation substance synthesis,protein transport and degradation.The expression of genes related to antioxidant,hormone signal transduction and transcription factor was different between the two cultivars.