| Endophyte is a kind of microorganisms living in plants and widely distributed in various organs and tissues of plants.They can help host plants adapt to severe environmental conditions and make them grow normally.Exploring endophyte resources and their biological function diversity under extreme environment are of great practical and theoretical significance for the protection of endangered species and the restoration of degraded ecosystems by microorganism and plant interaction.In this paper,endophytic bacteria from Ammodendron bifolium were used to study their physiological and biochemical characteristics and the host plant growth promoting resistance,and analyze theis mechanisms of action,using ordinary optical microscopy,PCR technology,plate assay,photoelectric colorimetric technique,chromatographic technology together with the seed germination experiment.The results of the study are as follows:1.Forty-five endophytic bacteria were isolated from the root and leaf tissues rather than stem and seed tissues of Ammodendron bifolium,and which belonged to 13 genera of ?-Proteobacteria,Firmicutes and Actinobacteria by 16S rRNA sequence and phylogenetic tree analysis.Among them,?-Proteobacteria and Firmicutes were predominant in population,Bacillus and Pantoea were the dominant genera which accounted for 38%.2.The analysis of growth-promoting characteristics and extracellular hydrolase production of 6 tested strains showed that each strain had at least one growth-promoting property,and presented different growth-promoting ability.In this paper for specific performance:strains AY3 and AG31 had only phosphate solubilizing activity;strain AG33 had nitrogen fixation and phosphate solubilization activity;strain AG43 could solubilize phosphate and produce IAA and ACC deaminase;strain AY9 could solublize phosphate,and produce IAA,ethylene,ACC deaminase,amylase and cellulase;strain AG 18 had the ability of producing IAA,ethylene,ACC deaminase,protease,amylase and cellulase;only strains AY3,AY9 and AG18 could enhanced host plant tolerance to osmotic stress in seed germination and radicle growth,the remaining 3 strains did not have the function.3.Ethylene production was detected during endophytic bacteria-induced seed germination.Exogenous ethephon and ACC(1-aminocyclopropanecarboxylic acid)could completely break the seed dormancy and promote germination,while exogenous ethylene biosynthesis inhibitor,AOA(aminooxyacetic acid),and physiological inhibitor,AgNO3,can inhibit endophytic bacteria-induced seed germination.The results indicated that ethylene was involved in the endophytic bacteria-induced germination of Ammodendron bifolium.4.The ethylene production was 0.005-0.016 ?l g-1 h-1(about 12-38.4 ?l l-1)during the interaction between the growth-promoting endophytic bacteria and the seeds,while the ethylene production by the strain itself reached the highest level of 0.038 ?l g-1 h-1(about 5 ?l l-1),and this level was much lower than the ethylene yield of the interaction.It was suggested that the amount of ethylene production by strain itself may not be enough to induce seed germination,but it could induce ethylene production in seeds and promote germination after interaction with seeds.5.The content of protein in the seed reserves was the most abundant,accounting for 26.9%of seed dry weight,followed by carbohydrates and fats,which accounted for 16.8%and 15.5%of seed dry weight,respectively.The rich content of sucrose and unsaturated fatty acid can reflect the resistance of the seeds.6.The degradation of protein and soluble sugar occurred at the beginning of endophytic bacteria-induced seed germination,and the degradation of lipid and starch occurred after the germination of the seeds.The results indicated that the degradation and transformation of storage reserves in seeds can provide nutrition and energy for endophytic bacteria-induced seed germination and post-germination growth,and also can reflect seed germination strategy.7.Correlation analysis showed that seed germination and radicle growth were closely related to extracellular hydrolytic enzyme production by endphytic bacteria and the degradation of storage reserves during the interaction of endophytic bacteria and seeds.However,3 growth-promoting endophytic bacteria were not capable of producing all hydrolytic enzymes,some enzymes in 3 strains were not detected,such as lipase,and still can promote fat degradation;the 3 strains were also able to produce ethylene,but promoted ethylene production during the interaction of endophytic bacteria and seeds.Therefore,the mechanism of plant growth promotion cannot be explored only by analyzing the characteristics of the strain itself,and were studied by the angle of interaction between the strain and the host plant.Different strains may take different mechanisms of action to produce the same growth-promoting effect on the host plant in this paper.The above results showed that the species composition of endophytic bacteria from Ammodendron bifolium displayed a rich diversity,all tested strains exhibited different growth-promoting characteristic,and growth-promoting and resistance ability,and the degradation of storage reserves in seeds and ethylene production during interaction may be the reasons of endophytic bacteria-induced seed germination and radicle growth. |
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