| Global food production is facing enormous challenges due to the destruction of farmland and increasing population pressures around the world.Therefore,it is essential to significantly increase agricultural productivity in decades.Agricultural practices are evolving towards more sustainable and environmentally friendly approaches to this end,which include more and more plant growth-promoting bacterias（PGPB）used as part of mainstream agricultural practice.In this paper,two plant growth promoting bacteria（WF18 and WF32）with good properties were selected by using the method of high-throughput analysis combined with determinating the property of endophyte bacteria isolated from different pea flower and root samples.Combined with the existing laboratory strains YN22,SB3,WS52 and WL24,the effects of different plant growth-promoting bacteria on dwf4 gene expression of Zhongmu No.5 and Saidi No.10 alfalfa seedlings at different stages were detected by Quantitative Real-time PCR.Through analyzing the fresh weight,soluble sugar content,soluble protein content and structure of alfalfa seedlings,the mechanism of seed germination and seedling transformation of alfalfa dealting with PGPB was discussed.The main results are as follows:（1）In this study,87 strains of nitrogen-fixing bacteria were isolated from flower and root samples of pea,and a strain WF18 with the most complete growth-promoting characteristics was selected by measuring the growth-promoting characteristics of 87 strains of nitrogen-fixing bacteria,such as phospho-solubilizing potassium,nitrogenase activity,cellulase and pectinase activity,and indole acetic acid secretion.（2）Through high-throughput sequencing,endophytic bacteria in flower and root samples of pea were found to be very rich in community structure,which was mainly divided into four phyla: Proteobacteria,Firmicutes,Actinobacteria and Bacteroidetes.According to the correlation analysis,Rhizobium is the key group in the endophytic network between pea flower and root.Eighty-seven strains of nitrogen-fixing bacteria were identified by 16 S r DNA bacterial identification technique,and one strain WF32 was selected.（3）WF18 and WF32 strains significantly promoted the growth of Zhongmu no.5and Saidi 10 seedlings.They significantly increased the fresh weight of seedlings at 1,3 and 5 days,respectively.The promoting growth showed by YN22,SB3,WS52 and WL24 strains on alfalfa seedlings were unstable and dispalyed a dynamic change.（4）PGPB did not promote or inhibit the content of soluble sugar in alfalfa seedlings at different development stages,This result was possible due to the dynamic change of release and utilization during the development of alfalfa seedlings.Therefore,PGPB regulated different content of soluble sugar to promote the growth of alfalfa seedlings.（5）YN22 could significantly increase the soluble protein content in Zhongmu No.5 and Saidi No.10 alfalfa seedlings at 1 day,and significantly increase the soluble protein content of Saidi No.10 alfalfa seedlings at 5 days.This speeds up the breakdown of stored proteins in the seeds.WF18 and WF32 could respectively promote the accumulation of metabolic protein in alfalfa seedlings at 3 days,enhance the metabolic response of seedlings at this time,and promote the growth of seedlings.（6）WF18 and SB3 significantly promoted dwf4 gene expression in Zhongmu No.5 alfalfa seedlings at 1 day,WF18 and WF32 significantly promoted dwf4 gene expression in Saidi No.10 alfalfa seedlings at 1 day,which increased brassinosteroid level in seedlings and promoted seedling growth and development.Based on the above results,this study has shown that PGPB can increase the seedling fresh weight,adjust the soluble sugar content of seedling,accelerate the decomposition of storage protein and metabolic protein accumulation.Finally,PGPB achieve the purpose of regulating the alfalfa seedling implementation and transformating alfalfa seed germination.This research proposed and verified the hypothesis that PGPB could promote seed germination and seedling growth through regulating the expression of dwf4 gene from alfalfa seedling. |
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