Effects And Mechanism Of Suaeda Salsa Root-Associated Microorganisms On Maize Growth And Resistance Under Salt Stress

Soil salinization is a continuing threat to agriculture and ecology worldwide,which is predicted to aggravate with climate change.High salt environment caused damage to plants through osmotic stress,ion stress and oxidative stress,which severely restricted crop growth and reduced crop yield and quality.The multifunctional microbiome carried by plant rhizosphere and endophyte is known as the “second genome” of plants,which not only contributes to plant growth and development,but also participates in plant immune response,and becomes an important “helper” in alleviating plant stress.The endophytic and rhizosphere microorganisms of halophytes are thought to contribute to host adaptation to high salinity environments,but the effect of root-associated microorganisms of halophytes on the salinity tolerance of heterologous crops has been less studied,and the function mechanism after inoculation is currently unclear.In this study,91 salt-tolerant microorganisms were isolated from the root and rhizosphere soil of Suaeda salsa.A bacterium Providencia vermicola BR68 and a fungus kiliense Sarocladium FS18 were selected to interact with maize seedlings under salt stress and the following five treatments were set: control;Na Cl treatment(salt stress without inoculation);BR68 treatment(salt stress+BR68);FS18 treatment(salt stress+FS18)and MIX treatment(salt stress+BR68+FS18),to evaluate the effect of inoculated strains on maize seedlings and soil physicochemical properties under salt stress.Analyze the response of maize rhizosphere microbial communities and the relationship between inoculated strains,soil physicochemical properties,maize physiologies and growth by amplicon sequencing and bioinformation analysis,so as to reveal the mechanism of inoculated strains to improve maize salt tolerance.The main results were as follows:(1)Both bacterium BR68 and fungus FS18 improve the growth of maize seedlings under salt stress.FS18 inoculation alone increased the biomass of maize seedlings under salt stress most significantly,and root length and plant height increased by 24.2% and 27.6% compared with Na Cl treatment,respectively,and the roots were more developed after inoculation.In addition,the net photosynthetic rate in BR68,FS18 and MIX treatment increased by 6.9%,13.6% and 6.3% compared with Na Cl treatment,respectively,the dry weight also increased significantly,indicating that the accumulation of organic matter in maize seedlings was improved under salt stress.(2)Inoculation of two strains alleviated the damage of maize seedlings under salt stress.The inoculant strain significantly improved the antioxidant oxidase activity of maize seedlings.Compared with Na Cl treatment,SOD activity of maize seedlings under BR68 treatment increased by 11%,and APX activity under FS18 treatment increased by 33.2%.In addition,the activities of catalase(CAT)and peroxidase(POD)in BR68,FS18 and MIX treatments were significantly increased(P < 0.05),while the production rate of reactive oxygen species was significantly decreased(P < 0.05),indicating that the increase of antioxidant enzyme activity of inoculated strains could help to remove reactive oxygen species in leaves of maize seedlings.In addition,compared with Na Cl treatment,the content of free proline in BR68 and FS18 maize seedlings was decreased by 65.13% and 71.32%,respectively,the accumulation of MDA was also decreased,and the root activity was significantly increased(P < 0.05).(3)Inoculated strains significantly improved soil enzyme activities and partially affected maize rhizosphere microbial communities.The activities of catalase,sucrose and FDA hydrolase in BR68 treatment were increased by 41.9%,22.0% and 12.8%,respectively,compared with Na Cl treatment.The sucrase of FS18 treated soil was 34.9% higher than that of the control,and the activities of acid phosphatase and dehydrogenase were also significantly increased(P < 0.05).In addition,inoculated strains BR68 and FS18 had no significant effect on bacterial community structure,while inoculated strain FS18 alone significantly changed fungal community structure(P < 0.05).The relative abundance of Proteobacteria and ascomycetes increased in BR68 and MIX treatments,while the relative abundance of verrucobacteria and chytridomycetes increased in FS18 treatments.(4)Inoculation of salt-tolerant microorganisms was significantly associated with the abundance of specific genera.Oligoflexus was significantly enriched in FS18 and MIX treatments,and showed a significant positive correlation with FS18.In addition,Oligoflexus showed almost the same correlation trend with physiological and soil physicochemical indexes of maize and with FS18 and this genus may be used as a potentially beneficial genus to improve maize salt tolerance with inoculated strains.The abundances of Reyranella and Gemmata were significantly down-regulated in BR68 treatment and negatively correlated with strain BR68.Abundances of these two genera were negatively correlated with antioxidant enzyme activity,but positively correlated with reactive oxygen species and malondialdehyde content.(5)The two salt-tolerant microorganisms had different mechanisms to improve maize growth under salt stress.The inoculating strain BR68 indirectly affected bacterial community by directly affecting BR Specific biomarkers.Bacterial community directly contributed to soil properties,and then affected maize physiologies.Finally,maize growth was improved.The inoculation strain FS18 directly affected soil properties,and then affected maize physiologies of.Both maize physiologies and soil properties affected maize growth.In summary,the isolation of preferred salt-tolerant microorganisms from the roots and rhizosphere soil of halophytes(Suaeda salsa)can effectively assist maize to resist salt stress,The bacteria BR68 and fungus FS18 affect the maize physiologies through direct or indirect effects on soil physicochemical properties and/or rhizosphere microbial communities,respectively,thereby reducing the damage of salt stress to maize and improving the growth and resistance of maize under salt stress.In this study,the function mechanism of inoculated strain was comprehensively analyzed from the aspects of plant physiologies,soil properties and rhizosphere microbial communities.The established relationship between salt-tolerant microorganisms,soil and plants could provide new ideas and information for improving crop cultivation strategies in saline-alkali land.