In this study, a large number of useful gene sequences were obtained from leaves and roots transcriptome of B. chinense under drought stress using a high-throughput technique. These results provide abundant genetic resources for further analysis of functional annotation and metabolic pathway under drought stress. In addition, a total of 33,728 SSR loci were identified from the transcriptomic data of B. chinense, which provided a rich theoretical basis for further research on genetic diversity, molecular marker-assisted breeding, and genetic map construction for this species. The data also provides abundant genetic resources for further functional annotation and analysis of metabolic pathways associated with drought stress.
The genes plants over-express under drought stress fall into three categories. One is to express genes related to detoxification and antioxidant stress, such as superoxide dismutase synthesis genes. Secondly, genes are related to the synthesis of osmotic regulatory substances, such as proline synthesis genes. The third is to protect the expression of genes protecting the expression of biological macromolecules and membrane structure, such as chaperones proteins and post-embryogenic rich proteins [21–23]. In this study, the active genes involved in scavenging reactive oxygen and superoxide in both leaves and roots of B. chinense, such as superoxide dismutase activity, peroxidase activity, glutathione peroxidase activity and glutathione transferase were over-expressed. Among them, the genes involved in coding peroxidase activity have bidirectional regulation, indicating that peroxidase activity scavenging reactive oxygen species is one of the more important mechanisms of drought resistance [24].
Previous studies have shown that drought stress can regulate hormone synthesis and signal transduction in vivo [25–27]. Under drought stress, the genes encoding plant hormone signal transduction were significantly up-regulated in the leaves and roots of B. chinense. In addition, drought stress had a bidirectional regulation effect on DEGs encoding biosynthesis and metabolism of auxin, gibberellin, and cytokinin. For examples, drought stress significantly down-regulated expression of gibberellin20-oxidase (GA20ox) (TRINITY_DN2977_c0_g2), a key rate-limiting enzyme encoding gibberellin in B. chinense, especially in roots. GA20ox is a multifunctional enzyme that not only participates in GA biosynthesis, but also controls the synthesis of GA1 and GA4, and maintains the dynamic balance of gibberellin in cells [28, 29]. Spielmeyer found that the deletion of Os GA20ox2 gene in rice SD1 mutants resulted in reduced GA bioactivity in the stem and dwarfing of the plants [30]. However, the effects of drought stress on plant growth need in B. chinense to be further explored.
KEGG analysis showed that drought stress mainly affected photosynthesis and phylpropanoid synthesis in leaves and dieterpenoid synthesis and unsaturated fatty acids synthesis in roots of B. chinense. The genes involved in photosystem I (PS I) and photosystem II (PS II) were down-regulated under drought stress. This may be due to the inhibition of photosynthesis, the reduction of transcription and translation rate, the reduction of assimilate levels, and the degradation of related proteins and mRNA. This is consistent with the stress results of short-term drought on PS II studied in Arabidopsis thaliana [31]. Meanwhile, terpenoids were synthesized in root system to protect plants from drought stress [32]. It is noteworthy that after drought stress, the DEGs in roots of B. chinense were twice as much as in leaves, which may be due to the direct contact between roots and soil and the direct influence of soil moisture [33].
B. chinense leaves contain a lot of flavonoids, but the content of saikosaponins is low, while their roots are the opposite [19]. Previous studies have shown that genes encoding the phenylpropane pathway are involved in flavonoid synthesis [34], which also explains the significant expression of phenylpropanoid biosynthesis in leaves of B. chinense after drought stress. As pyruvate is a raw material for the synthesis of saikosaponins, the expression level of pyruvate directly affects the content of saikosaponins. After drought stress, pyruvate metabolism in the root system was significantly over-expressed, which is consistent with the previous study [35]. Drought stress promotes the transformation of primary metabolites into secondary metabolites in plants to resist damage caused by adversity stress. Appropriate drought stress is generally considered conducive to the accumulation of active components in plants and the improvement of the quality of medicinal materials [36], and this data for B. chinense reached the same conclusion.