| Soil salinization is a serious threat to crop growth,which has become a global problem.With the increase of population and the decrease of cultivated land in China,food security problems exist for a long time.The improvement and utilization of saline alkali land is an important strategy to solve these problems.Coastal saline soil is one of the important types of saline alkali land.The Yellow River Delta saline soil covers an area of 440000 hectares,which has the characteristics of favourable location conditions,concentrated distribution and great agricultural development potential.Rhizobia is a kind of endophytic bacteria that can coexist with legumes.Rhizobia can promote plant growth and help plants adapt to salt environment by improving the ability of nitrogen fixation,dissolving phosphorus and secreting auxin.The aim of this study is to improve the salt tolerance of host plants by constructing legume-rhizobia symbiosis system.The results we obtained are as follows:(1)The diversity and community structure of endophytic bacteria in root nodules of two salt tolerant legumes Sesbania cannabina and Glycine soja were analyzed by high-throughput sequencing.The 16 S r RNA amplicon sequencing results showed that the endophytic bacteria in the root nodules of G.soja,S.cannabina and Glycine max were mainly Proteobacteria,and the rhizobial Ensifer was the dominant group.The species of endophytic bacteria in root nodules of the three legumes are similar,but the community structure is different.E.alkalisoli and E.americanum may be related to salt tolerance of plants.(2)Based on the culture-dependent technology,bacterial rhizobia were isolated from G.soja and analyzed for their salt tolerance and plant growth promotion ability.A total of 87 rhizobial strains were isolated from root and nodule of G.soja plants,including Mitsuaria noduli,Rhizobium leguminosarum,E.adhaerens,Bradyrhizobium japonicum,E.americanus,and 144 non-rhizobial strains,belonging to 4phyla and 17 genera.The dominant genera were Bacillus and Pseudomonas,accounting for 58% and 13%of the total isolates respectively.Five strains with high salt tolerance,16 strains with IAA producing ability,6 strains with 1-aminocyclopropane-1-carboxylicacid(ACC)deaminase producing ability,6 strains with iron carrier producing ability and 16 strains with phosphorus dissolving ability were screened out.(3)We found two strains possessing high salt tolerant and plant growth promotion abilities,and figured out the mechanism of improving salt tolerance of G.soja.The results showed that E.americanus DL3 could promote the growth and nodulation of G.soja,and increase the SOD and CAT activities in leaves under salt stress.Stenotrophomonas lactitubi 127-15 was obtained from non-rhizobial strains.It increased the growth of G.soja under salt stress by improving the proline content and SOD activity of G.soja leaves.(4)Strains DL3 and 127-15 could also improve the salt resistance and growth of G.max.Under salt stress,inoculating strains DL3 and 127-15 could increase the fresh weight and the SOD activity in leaves of G.max plants.The activities of POD and CAT were increased by inoculating DL3 under non salt stress.The results showed that strains DL3 and 127-15 could promote the growth of G.max plant or improve the salt tolerance ability by increasing the activities of antioxidant enzymes.In conclusion,this study found that salt tolerant legumes G.soja nodules have strong nitrogenase activity and high abundance of rhizobia.We also isolated 231 bacterial strains from G.soja nodules and roots.Pot experiments showed that strains DL3 and 127-15 could promote the growth of G.soja and G.max through various ways,and improve the salt tolerance of plants.The above results have important theoretical significance and practical value for the development of soil improvement technology based on microorganisms. |
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