Investigating The Mechanism Of Root-Associated Bacterial Communities In Enhancing Astragalus Membranaceus Resistance To Drought Stress

Astragalus membranosus root is rich in pharmacodynamic ingredients and has a variety of pharmacological activities,which is favored by the market.Although some studies have shown that moderate drought stress can increase the accumulation of secondary metabolites in medicinal plants,plant biomass will be seriously affected in water shortage environment,and the industrialization development of Astragalus membranosus is facing severe challenges.Therefore,this study takes Astragalus membranaceus as the research object.A synthetic community which can promote plant growth,improve the quality of Astragalus membranaceus has been constructed,and the mechanism of root-associated bacterial communities in enhancing Astragalus membranaceus resistance to drought stress has been investigated.The main conclusions are as follows:(1)Four treatments(normal watering,mild stress,moderate stress and severe coercion)have been set.The growth index of Astragalus membranaceus showed that the root length,plant height,above ground fresh weight,underground fresh weight,above ground dry weight and underground dry weight of Astragalus membranaceus decreased significantly after drought stress,and with the increase of stress,plant biomass decreased.However,the accumulation of CA,CAG,FO,AST Ⅰ,AST Ⅱ and AST Ⅳ in Astragalus root increased after drought stress,the expression levels of key enzyme genes corresponding to these secondary metabolites were up-regulated correspondingly,and the accumulation of most secondary metabolic increased significantly under mild and moderate stress.In addition,the accumulation of CA,CAG,ASV Ⅰ,ASV Ⅱ and ASV Ⅳ in rhizosphere of Astragalus membranaceus under moderate stress were significantly higher than those of mild stress and severe coercion.It was found that Astragaloside IV could effectively assist plants to resist drought stress and promote plant growth under moderate stress.(2)The changes of microbial communities in the root system of Astragalus membranosus were analyzed.It was found that the α diversity of bacterial communities in the bulk soil,rhizosphere soil and root of Astragalus membranaceus increased after drought stress,and the structure and composition of bacterial communities changed significantly.There was a strong correlation between biomass and secondary metabolites content of Astragalus membranaceus.The co-occurrence network became more complex after drought.The key species of the rhizosphere co-occurrence network were Nocardioides,Ensifer,Rhizobium,etc.,and the key species of the root co-occurrence network were Aeromicrobium,Nitrospira,Parablastomonas,etc.(3)A total of 232 strains of bacteria were isolated and purified from the root and rhizosphere soil of Astragalus membranaceus,which belonge to 37 genus and 4 phylum.Compared with the culturable microbial species,13 key species in the rhizosphere bacterial community co-occurring network after drought stress were screened to construct synthetic community.Pot experiment showed that the plant height,aboveground fresh weight,aboveground dry weight and average root diameter of Astragalus membranaceus were increased by 25.0%,47%,61.5% and 18.2% by adding synthetic community under moderate stress.The addition of synthetic community could effectively relieve drought pressure and promote plant growth under moderate stress.(4)The synthetic community was simplified to 7 strains by 16 S three-generation fulllength sequencing and gene sequence alignment.Pot experiment showed that the plant height,aboveground fresh weight,underground fresh weight and aboveground dry weight of Astragalus membranaceus were increased by 14.3%,50.1%,76.4% and 55.3% by the addition of the simplified community under moderate stress.The simplified community had the drought-resistant and growth-promoting function similar to the synthetic community.(5)The mechanism of drought resistance of Astragalus membranaceus was investigated,it was found that the simplified community have the ability to produce IAA,ACC deaminase,membrane and extracellular polysaccharide,and the ability to produce IAA was significantly improved compared with single strain.The antioxidant enzyme activity and chlorophyll fluorescence indexes of plant leaves were measured.It was found that the content of MDA was decreased by 19.6%,the activity of CAT was increased by 20.5%,and Fv/Fm and Y(Ⅱ)were increased by 50.9% and 90.5% after the addition of simplified community.By producing IAA,ACC deaminase and extracellular polysaccharide,the simplified community can reduce MDA content and increase CAT activities,Fv/Fm and Y(Ⅱ)of leaves,so as to reduce the damage of drought stress on Astragalus membranaceus and improve its drought resistance.In conclusion,the addition of simplified community can increase the accumulation of saponins and flavonoids in Astragalus membranaceus,and alleviate the inhibitory effect of drought on plant growth.This study can provide a new idea for the improvement of yield and quality in Astragalus membranaceus planting,and provide a scientific reference for further research on the mechanism of synthetic community assisting plants to cope with drought stress.