Transcriptome and metabolome analysis reveal the dynamic changes and biosynthesis pathways of alkaloids in Sophora alopecuroides L. under drought stress

Sophora lopecuroides L. is a medicinal plant containing a variety of biologically active substances, in which alkaloids are the main active substances, and a variety of alkaloids have been identified in S. alopecuroides. Drought stress can stimulate the accumulation of biologically active substances in medicinal plants. In this respect, the imposition of appropriate degree of drought stress may be an effective way to increase the alkaloid content in S. alopecuroides. However, the changes in the types and abundance of alkaloids in S. alopecuroides under drought stress and the molecular mechanism of regulating alkaloid biosynthesis are still unclear. In this study, the metabolome and transcriptome analysis were conducted to explore the influence of mild (D1), moderate (D2), and severe (D3) drought stress on the types and abundance of alkaloids in S. alopecuroides, and to identify the key regulatory genes in the alkaloid synthesis pathways. The results showed that the total alkaloid content in S. alopecuroides in the D1 and D2 group significantly decreased by 13.49% and 10.40%, respectively compared with that in the control group, but the total alkaloid content in S. alopecuroides in the D3 group significantly increased by 8.16% compared with that in the control group. Sixty-eight alkaloids were identified in S. alopecuroides, with quinolizidine alkaloids being the main type of alkaloids. Of the 25 differentially accumulated metabolites (DAMs), the number of DAMs with increased relative abundance in the D1, D2, and D3 group were 0, 7, and 20, respectively. The relative abundance of 6 DAMs increased in both D2 and D3 group, and 14 DAMs only increased in the D3 group such as methoxyindoleacetic acid, indole, and piperidine. Besides, the biosynthetic pathways of three major types of alkaloids (quinolizidine-, piperidine-, and plumerane-type alkaloids) were illustrated, and key genes (i.e., asd, DapL, PrAOs, DDC, and ALDH) associated with the alkaloid biosynthesis were identified. This study also found that 22 TFs (SaERF1∼SaERF5, SaWRKY1∼SaWRKY3, SabHLH1∼SabHLH3, SaMYB1∼SaMYB6, SabZIP1∼SabZIP5) were positively correlated with the alkaloid biosynthesis. This study deepens our understanding of the molecular mechanism of S. alopecuroides in response to drought stress, and helps to develop biotechnology strategies to manipulate the level of alkaloids in S. alopecuroides.