Silicon modulates nitrogen and secondary metabolism in <i>Glycyrrhiza uralensis</i> under drought and salt stress conditions

Glycyrrhiza uralensis Fisch (G. uralensis) is a key species for windbreak and sand fixation, possessing notable pharmacological and economic value. However, the yield of G. uralensis is considerably impacted due to its cultivation in arid, semi-arid, and salt-affected regions. Silicon (Si) has been reported to improve plant tolerance to drought and salt stress by regulating nitrogen and secondary metabolism. Herein, the effects of Si treatment on nitrogen and secondary metabolism of G. uralensis seedlings under drought (D), salt (S), and drought-salt (SD) stresses were investigated in combination with physiological and transcriptomic analyses. The results indicated that stress conditions significantly inhibited the growth of G. uralensis seedlings by suppressing nitrogen and secondary metabolism. Si treatment counteracted these inhibitions to some extent. Specifically, Si treatment increased soluble protein content by approximately 15% by regulating the nitrogen metabolism of G. uralensis under D stress. Furthermore, Si treatment elevated the content of glycyrrhetinic acid by about 89% under SD stress by increasing the content of primary metabolites and regulating the expression of enzymes involved in the biosynthesis of glycyrrhizic acid and liquiritin, including 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), squalene synthase (SQS), and β-amyrin synthase (β-AS). In summary, our findings suggest that Si could alleviate the adverse effects induced by drought and/or salt stresses on the growth of G. uralensis seedlings by regulating nitrogen metabolisms, which further triggered the accumulation of secondary metabolites, ultimately improving the stress resistance of cultivated G. uralensis seedlings. This work provides direction for Si to improve stress resistance.