| Biological nitrogen fixation(BNF)is the most crucial way of nitrogen fixation.Legume-rhizobium symbiosis accounts for over 65% of the total nitrogen fixed by biological nitrogen fixation process,which is one of the BNF research focuses.Plant growth promoting rhizobacteria(PGPR)can promote plant growth,enhance systemic resistance,and increase crop yield.Our previous study showed that mono-inoculation of PGPR could significantly promote the growth and stress tolerance of legume plants,such as white clover,alfalfa and soybean.It is still unclear the differences that various genus and species PGPR strains regulate the legume-rhizobia symbiosis efficiency,and how to screen specific PGPR strains to improve the symbiosis efficiency.Alfalfa(Medicago sativa)is known as "the king of herbage",it is a kind of good leguminous forage grass.The nitrogen fixation of alfalfa-rhizobia has become one of the model systems for studying symbiotic nitrogen fixation.In view of this,we focused on the interaction characteristics between various genus and species PGPR strains and rhizobium,and further explored the effects of PGPR strains on the symbiosis efficiency of alfalfa-rhizobium.In this study,firstly,we studied the interaction characteristics between various genus and species PGPR strains(AWH-2R8 and AWH-A11 of Arthrobacter,M30-35 and F1-P7 of Pseudomonas,WM13-24,GB03 of Bacillus)and rhizobium Sinorhizobium meliloti 1021(Sm1021,who is also used as Ensifer meliloti),and further explored the regulation of various genus and species PGPR strains on the alfalfarhizobium symbiosis efficiency.The effects of six PGPR strains on growth,nodulation,nitrogen fixation and rhizosphere bacterial diversity of alfalfa were studied.Pearson correlation analysis was used to investigate the correlation between the PGPR strains and Sm1021 interaction characteristics with the growth,nodulation and nitrogen fixation performance of alfalfa.The main results are as follows:1.Strains M30-35 and F1-P7 neither significantly inhibited the growth of Sm1021 colony nor formed obvious synergistic biofilm with Sm1021.Strains AWH-2R8,AWH-A11,WM13-24,GB03 and Sm1021 could form obvious synergistic biofilm,especially strains AWH-2R8 and WM13-24.The resource overlap rate of strain AWHA11 and Sm1021 was only 36.67%,and the resource overlap rate of other PGPRs and Sm1021 were more than 50%.2.All PGPRs promoted the growth of alfalfa in different degrees,increased the shoot biomass,plant height,branch number,average leaf area,chlorophyll content and net photosynthetic rate of alfalfa.Strains WM13-24 and AWH-2R8 had the best effects,which both significantly increased the shoot biomass,branch number,average leaf area,chlorophyll content and net photosynthetic rate of alfalfa.In addition,strain WM13-24 also significantly increased total nodule number,effective nodule number,and enhanced the nitrogenase activity of alfalfa.The synergism biofilm concentration was significantly positively correlated with nodule number,shoot biomass,plant height,branch number,net photosynthetic rate,chlorophyll content,average leaf area and root biomass of alfalfa.3.Strains AWH-2R8,WM13-24 co-inoculated with Sm1021 had the best promotion effect on alfalfa nodulation.Compared with single Sm1021,co-inoculated AWH-2R8,WM13-24 with Sm1021,alfalfa total nodule number were significantly increased by 47% and 50%,respectively,effective nodule number were significantly increased by 40% and 1.47 times,respectively.4.Strains WM13-24,GB03 co-inoculated with Sm1021 increased the richness of bacterial community.All PGPR strains significantly increased the abundance of Ensifer,Bacillus and Pseudomonas.In conclusion,the effects that co-inoculating various genus and species PGPR strains with Sm1021 on alfalfa-rhizobia symbiosis efficiency are different.The interaction characteristics of PGPR strains and rhizobia can reflect the co-inoculation effect,which can provide a theoretical basis for the research and development of new compound nitrogen-fixing agents. |
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