Fig. 5

Regulatory mechanisms of miRNAs in terpenoid biosynthesis, phenolic biosynthesis, and alkaloid biosynthesis in medicinal plants. A large number of miRNAs participate in secondary metabolite synthesis by cleaving or repressing target mRNA. The modulating role of miRNA redirects secondary metabolites in plant cells for a specific biosynthetic pathway, thereby enhancing the production of therapeutic plant metabolites. The graph summarizes the current knowledge and understanding of miRNA and its role in the regulation, biosynthesis, and accumulation of secondary metabolites in plants, including alkaloids, terpenoids, and flavonoids. AACT, acetoacetyl CoA thiolase; HMGS, HMG-CoA synthase; HMGR, HMG-CoA reductase; MVK, mevalonate kinase; PMK, phosphomevalonate kinase; PMD, mevalonate diphosphate decarboxylase; IPPI (isopentenyl diphosphate isomerase); DXS, 1-deoxy-D-xylulose-5-phosphate synthase; DXR, 1-deoxy-D-xylulose 5-phosphate reductoisomerase; MCT, 2-C-methyl-D-erythritol 4-phosphate cytidylyltransferase; CMK, CDP-ME kinase; MDS, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase; HDS, (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase; HDR, (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase; FPPS, farnesyl diphosphate synthase; SQS, squalene synthase; SE, squalene epoxidase; β-AS, β-amyrin synthase; CAS, cycloartenol synthase; CYPs, cytochrome P450s; GTs, glandular trichomes. PAL, Phe ammonia-lyase; C4H, cinnamate-4-hydroxylase; 4CL, 4-coumaroyl:CoA-ligase; CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; FNS, flavones synthase; FLS, flavonol synthase; F3'H, flavonoid 3'-hydroxylase; DFR, dihydroflavonol 4-reductase; ANS, anthocyanidin synthase; UGT, UDP-glycosyltransferase; TYDC, tyrosine/dopa decarboxylase; HMGS, HMG-CoA synthase; NCS, norcoclaurine synthase; 6-OMT, 6-O-methyltransferase; CYP80B1, (S)-N-methylcoclaurine-3'-hydroxylase; 7-OMT, 7-O-methyltransferase; 4’-OMT, 4′-O- methyltransferase