Photosynthesis, antioxidant enzyme activity, and transcriptome sequencing analyses of Glycyrrhiza uralensis seedlings in response to drought stress

To explore the physiological and molecular mechanisms of Glycyrrhiza uralensis under drought stress simulated by polyethylene glycol 6000 (PEG-6000), transcriptome sequencing, photosynthetic characteristics, and antioxidant enzyme activity analyses were performed. The results showed that the values of net photosynthetic rate (P_n), stomatal conductance (G_s), and transpiration rate (T_r) were the lowest at 7 d under drought stress. Relative chlorophyll content soil and plant analysis development (SPAD) first increased and then decreased. Oxidative stress was induced by drought stress on G. uralensis. Catalase (CAT) activity of the leaves increased, whereas CAT activity of the roots decreased after 1 day of treatment. After 7 days of treatment, peroxide (POD) and superoxide dismutase (SOD) activities increased and decreased in the roots, respectively. Transcriptomic analysis identified a total of 7 500 differentially expressed genes (DEGs) in the aboveground parts and 5 298 DEGs in the underground parts. Gene ontology (GO) enrichment results showed that the DEGs of the aboveground and underground parts were both significantly enriched for cellular process, metabolic process, cell structure, catalytic activity, transporter activity, etc. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that DEGs in the aboveground and underground were both significantly enriched in transcription factors, protein kinases, phenylpropanoid biosynthesis, and cytochrome P450, etc. Transcriptional regulatory network analysis predicted that transcription factors such as ethylene responsive factor (ERF), basic helix-loop-helix (bHLH), NAM/ATAF/CUC (NAC), v-myb avian myeloblastosis viral oncogene homolog (MYB), and WRKY may be involved in regulating the expression of secondary metabolism genes. In conclusion, this study revealed the effects of drought stress on physiological characteristics of G. uralensis, and analyzed gene expression profiles in different parts of G. uralensis. Our results provide valuable information for understanding drought-resistance mechanisms of G. uralensis.