Effects Of PGPR On Physiology And Root Microecology Of Caragana Korshinskii Seedlings Under Drought Stress

Drought is the main stress factor of plant growth and development,plant growth-promoting Rhizobacteria(PGPR)promotes plant growth and resists the effect of drought stress against plants in a variety of methods.From the perspective of bacteria-plant interaction,studying the effects of PGPR on plant physiological characteristics,growth and root microecology under drought stress can lay a theoretical foundation for in-depth exploration of the mechanism of PGPR promoting plant growth and alleviating plant drought stress.In this study,the growth promoting bacteria Enterobacter sp.(AC3),Pseudomonas sp.(Fel l)and Serratia sp.(WP8)in the rhizosphere of Caragana korshinskii were used as test strains to prepare single agent and compound agent and inoculate C.korshinskii seeds.The effected of PGPR on physiological indexes,rhizosphere soil properties,bacterial community structure and endophytic bacterial community composition of C.korshinskii seedlings under different drought stress intensities were studied.The main results are as follows:(1)Measure the physiological and biochemical indexes and biomass of C.korshinskii seedlings after 20 days of drought stress treatment.The results showed that with the intensification of drought stress,the SOD enzyme activity of each treatment group did not change much,while the POD and CAT activities showed a slow increase trend.Compared with the control,the activities of antioxidant enzymes POD and CAT in the leaves of C.korshinskii in the inoculated group were significantly increased,the content of proline Pro and soluble protein also gradually increased,and the content of malondialdehyde MDA decreased,and the change was the most under severe stress obvious.It shows that under drought conditions,PGPR can alleviate drought damage to plants by regulating seedling protective enzyme activity,increasing the content of osmotic adjustment substances,and reducing the accumulation of membrane lipid peroxidation products.Inoculation with PGPR can significantly promote the growth of seedlings of C.korshinskii,and the plant height,root length,fresh weight and dry weight of the ground/underground are all significantly increased.Among them,the increase in underground biomass is the most obvious.Compared with the uninoculated control,the underground fresh weight and dry weight of the single bacterial agent group increased by 37.04%and 57.61%,respectively;the compound bacterial agent group increased by 96.30%and 100.00%,respectively.The compound bacterial agent significantly affects the growth of the underground part of C.korshinskii.(2)To analyze the effects of PGPR on the physical and chemical properties of the rhizosphere soil and the composition of the rhizosphere bacterial community of C.korshinskii under drought stress.The results showed that under drought stress,compared with the uninoculated control,the soil total nitrogen,available potassium,available nitrogen and organic matter content of the inoculated treatment group were significantly increased.The results of 16S rRNA high-throughput sequencing showed that the Shannon index of the control group was higher than that of the inoculation group under drought stress,while the Simpson was lower than that of the inoculation group,indicating that PGPR inoculation reduced the diversity of bacteria in the rhizosphere of C.korshinskii.A total of 38 phylum,129 classes,312 orders,490 families,and 834 genus of rhizosphere bacteria were detected in the rhizosphere soil samples of C.korshinskii.Among them,Proteobacteria(24.52%-33.36%)and Actinobacteria(18.99%-27.56%)are the dominant phylum of C.korshinskii rhizosphere soil,followed by Chloroflexi(12.27%-18.87%),Acidobacteria(6.89%-14.12%),Bdellovibrionota(0.55%-13.31%),etc.At the genus level,Bdellovibrio is the dominant genus in the inoculated group,with relative abundance of 3.46%-12.98%;the dominant genus in the non-inoculated group are some unclassified genus,and Streptomyces(1.07%-2.23%),Pedomicrobium(1.14%-1.90%)and Sphingomonas(1.12%-2.12%)are the common genus in all samples,and their relative abundance.There is not much difference between the groups.PCoA showed that the soil bacterial communities of the inoculation group and the control group had a clear classification trend.According to Adonis analysis,the effect of inoculation treatment(F=1.6,P<0.01)on bacterial community composition was more significant than that of water treatment.Correlation analysis shows that the relative soil water content and pH are the main factors affecting the soil bacterial community.(3)To analyze the effects of PGPR on the endophytic bacterial community composition in the roots of C.korshinskii under drought stress by 16S rRNA high-throughput sequencing technology.The results showed that the Alpha diversity of endophytic bacteria in the roots of different treatment groups was different,but under severe stress,the species diversity and richness of endophytic bacteria in the inoculated group were significantly higher than that of the non-inoculated group.At the phylum level,Proteobacteria is the absolute dominant phylum in all samples,with relative abundance as high as 93.64%-99.54%,while Actinobacteria,Firmicutes,Bacteroidetes,etc.are found in root tissues The proportions are low,and the relative abundance of endophytic bacteria is closely related to PGPR treatment and drought stress.At the class level,the Gammaproteobacteria are the absolute predominance of the uninoculated control group(85.03%-98.80%)and the severely stressed inoculation group(53.22%-80.20%).Alphaproteobacteria(69.63%-92.60%)are the dominant bacteria in the normal watering,mild and moderate stress inoculation group.At the genus level,the bacterial composition of the inoculation group and the control group has changed significantly:Pseudomonas is the absolute dominant genus of the control group,with a relative abundance of 80.13%-95.65%;while Mesorhizobium is the absolute dominance of the inoculation group(except the severe stress group)is 69.44%-92.37%.PCoA showed that inoculation with PGPR and severe stress caused significant differences in the composition of the endophytic bacterial community in the roots.The results of LEfSe showed that the genus Mesorhizobium,Serratia,and Pseudomonas were the most significantly different functional groups in the inoculated single bacterial agent group,the compound bacterial agent group,and the non-inoculated group.