Bacillus cereus G2 alleviate salt stress in Glycyrrhiza uralensis Fisch. by balancing the downstream branches of phenylpropanoids and activating flavonoid biosynthesis

The salinity environment is one of the biggest threats to Glycyrrhiza uralensis Fisch. (G. uralensis) growth, resulting from the oxidative stress caused by excess reactive oxygen species (ROS). Flavonoids are the main pharmacodynamic composition and help maintain ROS homeostasis and mitigate oxidative damage in G. uralensis in the salinity environment. To investigate whether endophytic Bacillus cereus G2 can improve the salt-tolerance of G. uralensis through controlling flavonoid biosynthesis, the transcriptomic and physiological analysis of G. uralensis treated by G2 in the saline environment was conducted, focused on flavonoid biosynthesis-related pathways. Results uncovered that salinity inhibited flavonoids synthesis by decreasing the activities of phenylalanine ammonialyase (PAL) and 4-coumarate-CoA ligase (4CL) (42% and 39%, respectively) due to down-regulated gene Glyur000910s00020578 at substrate level, and then decreasing the activities of chalcone isomerase (CHI) and chalcone synthase (CHS) activities (50% and 42%, respectively) due to down-regulated genes Glyur006062s00044203 and Glyur000051s00003431, further decreasing isoliquiritigenin content by 53%. However, salt stress increased liquiritin content by 43%, which might be a protective mechanism of salt-treated G. uralensis seedlings. Interestingly, G2 enhanced PAL activity by 27% whereas reduced trans-cinnamate 4-monooxygenase (C4H) activity by 43% which could inhibit lignin biosynthesis but promote flavonoid biosynthesis of salt-treated G. uralensis at the substrate level. G2 decreased shikimate O-hydroxycinnamoyltransferase (HCT) activity by 35%, increased CHS activity by 54% through up-regulating the gene Glyur000051s00003431 encoding CHS, and increased CHI activity by 72%, thereby decreasing lignin (34%) and liquiritin (24%) content, but increasing isoliquiritigenin content (35%), which could mitigate oxidative damage and changed salt-tolerance mechanism of G. uralensis.