Indigenous PGPB Inoculant from Qinghai-Tibetan Plateau Soil Confer Drought-Stress Tolerance to Local Grass Poa annua

Grassland degradation in Qinghai-Tibetan Plateau (QTP) is becoming a serious environmental problem which has attracted widespread public attention. Application of plant growth promoting bacteria (PGPB) inoculant is considered as a potential strategy in helping plants cope with environmental stress. This study aimed to explore an effective functional microbial inoculant for the restoration of degenerated grasslands. Three PGPB strains (i.e., Paenibacillus tundrae TS19, Bacillus mycoides TS26 and Brevibacterium frigoritolerans TS22) characterized by outstanding osmotic tolerance, siderophore production, and plant growth promotion capacity were isolated from QTP grassland. The influence of single and mixed inoculations of three strains on the growth of Poa annua were evaluated. The results showed that water deficiency significantly inhibited plant growth and led to increased reactive oxygen species (ROS), superoxide dismutase (SOD) activity, and proline content in plant. The inoculation of PGPB could alleviate the drought stress by modifying root morphology and activating SOD activity. It is interesting that mixed inoculation of P. tundrae TS19 and B. mycoides TS26 was more effective under low and moderate drought stress, while single inoculation of P. tundrae TS19 had the best performance under high drought stress. The suitable inoculant-combination should be chosen according to the drought stress level and drought-tolerant capacity of inoculant. This study provides valuable microbial resources and guidance of inoculants selection for remediation of degraded grassland in QTP.