Abstract:

Drought， as one of the environmental factors that severely affects plant development and crop production， requires an efficient but environmentally neutral approach to mitigate its harmful effects on plants. Soil microorganisms are known to interact with plants to ameliorate the adverse effects of environmental factors， but there is still a need to verify whether soil microorganisms play a critical role in protecting plants under drought stress. In this study， to reveal the roles of soil microorganisms in the response of Medicago ruthenica to drought stress， we compared the growth phenotypes and physiological characteristics of two cultivars of M. ruthenica （Mengnong No.2 and Mengnong No.1） growing in sterilized and unsterilized soil under drought stress. In addition， we performed high-throughput sequencing analyses of 16S rRNA extracted from the rhizosphere soils of plants under drought stress or well-watered conditions to detect differences in microbial communities in the rhizosphere of M. ruthenica under drought stress， and to determine which bacterial taxa play a key role in the drought response. The results show that， compared with plants of Mengnong No.1 and Mengnong No.2 growing in sterilized soil， those growing in unsterilized soil showed higher values for plant height， dry weight of aboveground and belowground parts， and catalase activity， and lower values for proline and malondialdehyde contents. Meanhile， in the rhizosphere soils of well-watered plants of Mengnong No.2 and Mengnong No.1， Proteobacteriaaccounted for 72.83% and 67.65% of rhizosphere bacteria， respectively， and Rhizobium accounted for 5.28% and 3.65%， respectively. In Mengnong No.2 and Mengnong No.1 under drought stress， the relative abundance of Proteobacteria in the rhizosphere bacterial community was decreased by 8.42% and 4.76%， respectively， and the relative abundance of Rhizobium was increased by 1.69% and 2.35%， respectively. On the basis of these results， we identified Rhizobium as the core taxon related to drought resistance in M. ruthenica. Under well-watered conditions， Mengnong No.2 had more rhizobia in its rhizosphere than did Mengnong No.1， so it was better able to respond quickly to drought stress. In conclusion， the results of this study demonstrate that the soil microbiome plays an active role in plants’ resistance to drought stress， that rhizosphere microorganisms can improve the drought tolerance of M. ruthenica， and that microbial composition and abundance are regulated by drought stress and the host plant. The large differences in microbial communities in the rhizosphere between the two varieties of M. ruthenica suggest that the process of microbial enrichment is closely related to plant genotype.

Key words: Medicago ruthenica, drought stress, rhizosphere bacteria, plant-microbial interactions, 16S rRNA sequencing