Bacillus pumilus improved drought tolerance in Glycyrrhiza uralensis G5 seedlings through enhancing primary and secondary metabolisms

It has been reported that drought stress adversely affects the growth and yield of Glycyrrhiza uralensis, Chinese liquorice, in agricultural production. Bacillus pumilus, an important plant growth-promoting bacterium, play a significant role in improving plant tolerance to abiotic stress. However, the role of Bacillus pumilus G5 in resisting drought stress is largely unknown. In the present study, we found that drought stress significantly inhibited the growth and reduced the biomass of G. uralensis seedlings by restraining C- and N-metabolism, while this could be effectively reversed by B. pumilus G5 inoculation. Specifically, B. pumilus G5 significantly increased the content of primary metabolites such as soluble sugar, soluble protein, and free amino acids by regulating the C and N metabolic processes in G. uralensis seedlings. Moreover, B. pumilus G5 increased the content of glycyrrhizic acid, one of the important secondary metabolites, likely mediated through the increased content of primary metabolites and by recovering the expression of three key enzymes, HMGR, SQS, and β-AS, in the biosynthesis of glycyrrhizic acid. Interestingly, the regulating effect of B. pumilus G5 inoculation on promoting the accumulation of glycyrrhizic acid and increasing the expression of synthesis-related genes is spatially selective. In summary, our findings suggest that B. pumilus G5 could alleviate adverse effects induced by drought stress on the growth of G. uralensis seedlings by regulating C- and N-metabolisms that further triggered the accumulation of secondary metabolites, and this finally improved the drought tolerance of cultivated G. uralensis seedlings.