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Drought stress has become the primary severe threat to global agriculture production, including medicinal plants. Plant growth-promoting bacteria (PGPB) and environmentally friendly element silicon (Si) have emerged as effective methods in alleviating drought stress in various plants. Here, the effects of the plant endophytic G5 interaction with Si on regulating nitrogen absorption, assimilation, and metabolism pathways were investigated in the morphophysiological and gene attributes of Glycyrrhiza uralensisexposed to drought. Results showed that G5+Si application improved nitrogen absorption and assimilation by increasing the available nitrogen content in the soil, further improving the nitrogen utilization efficiency. Then, G5+Si triggered the accumulation of the major adjustment substances proline, ?-aminobutyric acid, putrescine, and chlorophyll, which played an important role in contributing to maintaining balance and energy supply in G. uralensisexposed to drought. These findings will provide new ideas for the combined application of PGPR and Si on both soil and plant systems in a drought habitat.

Soil salinity is a severe abiotic stress that reduces crop productivity. Recently, there has been growing interest in the application of microbes, mainly plant-growth-promoting bacteria (PGPB), as inoculants for saline land restoration and plant salinity tolerance. Herein, the effects of the plant endophyte G2 on regulating soil N cycle, plant N uptake and assimilate pathways, proline and glycine betaine biosynthesis, and catabolic pathways were investigated in Glycyrrhiza uralensisexposed to salinity. The results indicated that G2 improved the efficiency of N absorption and assimilation of plants by facilitating soil N cycling. Then, G2 promoted the synthesis substrates of proline and glycine betaine and accelerated its synthesis rate, which increased the relative water content and reduced the electrolyte leakage, eventually protecting the membrane system caused by salt stress in G. uralensis. These findings will provide a new idea from soil to plant systems in a salinity environment.

ABSTRACTThis article studies the effect and mechanism of Si on drought and salt tolerance of Glycyrrhiza uralensisby focusing on the pathways of proline metabolism and nitrogen assimilation. G. uralensiswas analysed under control, 70 mM NaCl, 15% PEG-6000 and 70 mM NaCl+15% PEG-6000 with or without Si. The results showed that Si increased P5Cs and δ-OAT activities by 16.29% and 5.85% under drought-salt stress, respectively. Si increased PDH by 28.14% under salt stress and increased proline by 27.6% and 33.0% under drought and drought-salt stresses in proline metabolism. In addition, Si increased NO3−by 36.9%, 42.63% and 44.07% under drought, salt and drought-salt stresses, respectively, increased GOGAT by 50.9% under salt stress, increased GDH by 34.78% and 25.44% under salt and drought-salt stresses in the process of N assimilation. Moreover, Si decreased H2O2and MDA contents by 15.27%, 15.57%, 25.18% and 43.05%, 7.59%, 21.44% under drought, salt and drought-salt stresses, respectively. In sum, Si could increase the proline content by increasing P5Cs activity; meanwhile, Si also regulated the synthesis pathway of proline by improving N assimilation. Thus, the tolerance of G. uralensisto drought-salt stress was further enhanced by eliminating excessive ROS and reducing lipid peroxidation impairment.