Genomic Analysis Of The Grassland Soil Bacterium Bacillus SP.SP1 And Its Growth Promoting Effect On Forage

As an important ecosystem in the Eurasian continent,the grasslands of Inner Mongolia are currently suffering from varying degrees of damage,and the amount of forage has decreased sharply,which has seriously affected the development of agriculture and animal husbandry.As a new type of biological fertilizer,plant growthpromoting bacteria has become a hot research topic in the field of microbiology.The research on excellent forage Medicago sativa has mainly focused on the growthpromoting effect of inoculating rhizobia to form Medicago sativa-root nodule symbioic nitrogen fixation by inoculated rhizobacteria.Additionaly,Some studies have shown that adding phosphorus can promote the nitrogen fixation activity of legume nodules.However,the research on the growth-promoting effect and growth-promoting mechanism of heterotrophic bacteria on Medicago sativa remain poorly understood.Previously,our research group isolated several strains of bacteria from grassland soil,including Rhizobium,Bacillus,etc.,for which the growth-promoting effects are unknown.Based on this,this study took Medicago sativa L.cv.Aohan and Stipa breviflora as the research objects,and screened potential growth-promoting bacteria through the germination experiments of bacterial solution soaking and growthpromoting characteristics experiments,and then the growth characteristics,genome sequencing,and the promotion of forage grasses was carried out using the exsellent strian.The aim was to investigate the growth promotion mechanism of the strains by measuring plant and soil nutrients,nodule nitrogen fixation activity,and rhizosphere soil microbial community structure.The main results of the study are as follows:(1)Four strains(SP1,MHJ-6,SNQL,XHQ-10)significantly promoted the germination of Medicago sativa L.cv.Aohan and Stipa breviflora seeds,of which strain SP1 had the strongest capacity for organic phosphorus solubilisation(33.26 ?g/ml),inorganic phosphorus solubilisation(128.95 ?g/ml),indole acetic acid production(59.48 mg/L)and exopolysaccharide production(245.92mg/L).(2)Strain SP1 grows well under the conditions of salt concentration 0%?7%;p H 5?11 and 15??50?.The results of genome sequencing showed that strain SP1 had genes related to growth promotion,such as phosphate solubilization,IAA production,and exopolysaccharide,which supported the above experimental results.Based on the analysis of 16 S r RNA gene and the ANI value of the whole genome sequence,the strain SP1 belongs to Bacillus subtilis.(3)The results of pot inoculation experiments showed that strain SP1 significantly increased the aboveground dry weight,underground dry weight,plant height and root length of Medicago sativa under sterilization and non-sterilization soil conditions,but the growth-promoting ability under the two conditions was different.In addition,the strain SP1 also significantly promoted the aboveground and underground dry weight,plant height and root length of Stipa under non-sterilization soil condition.(4)In order to explore the growth-promoting mechanism of the strain on Medicago sativa,the plant and soil nutrients and the nitrogen fixation activity of root nodules were analyzed.Compared with the control,strain SP1 significantly increased soil nutrients such as available phosphorus,available K and available N,especially available P increased by 252.61%;the TC,TN and TP contents of aboveground plants increased by 206.98%,172.25% and 180.77%;in addition,the number of nodules?fresh weight and the nitrogen fixation activity increased by 51.85%?205.88% and161.24%,which may be related to the increase of available P.In conclusion,strain SP1 increased soil nutrients and nodule nitrogen fixation activity,thereby promoting plant growth,This indicated that the strain SP1 had multifunctional growth-promoting effects on Medicago sativa via increasing of phosphorus and nitrogen.(5)Results of 16 S r RNA gene amplicon sequencing showed that strain SP1 changed ?-diversity and ?-diversity of the bacterial community structure in the rhizosphere soil of Medicago sativa.At the genus level,the relative abundance of genera such as Lysobacter,Pseudomonas,Massilia increased after inoculation,and these flora may be indirectly involved in plant growth promotion.This study is expected to improve forage yield and restoration of degraded grassland status,and provide a theoretical basis for guiding the application of growthpromoting bacteria in forage planting.