ARGONAUTE Genes in Salvia miltiorrhiza: Identification, Characterization, and Genetic Transformation

Polymerization of proanthocyanidins under the catalysis of miR397a-regulated laccases in Salvia miltiorrhiza and Populus trichocarpa

Proanthocyanidins (PAs) play significant roles in plants and are bioactive compounds with health benefits. The polymerization mechanism has been debated for decades. Here we show that laccases (LACs) are involved in PA polymerization and miR397a is a negative regulator of PA biosynthesis in Salvia miltiorrhiza and Populus trichocarpa. Elevation of miR397a level causes significant downregulation of LACs, severe reduction of polymerized PAs, and significant increase of flavan-3-ol monomers in transgenic S. miltiorrhiza and P. trichocarpa plants. Enzyme activity analysis shows that miR397a-regulated SmLAC1 catalyzes the polymerization of flavan-3-ols and the conversion of B-type PAs to A-type. Both catechin and epicatechin can serve as the starter unit and the extension unit during PA polymerization. Overexpression of SmLAC1 results in significant increase of PA accumulation, accompanied by the decrease of catechin and epicatechin contents. Consistently, CRISPR/Cas9-mediated SmLAC1 knockout shows the opposite results. Based on these results, a scheme for LAC-catalyzed PA polymerization is proposed. The work provides insights into PA polymerization mechanism.

The Smi-miR858a-SmMYB module regulates tanshinone and phenolic acid biosynthesis in Salvia miltiorrhiza

Tanshinones and phenolic acids are two major classes of bioactive compounds in Salvia miltiorrhiza. Revealing the regulatory mechanism of their biosynthesis is crucial for quality improvement of S. miltiorrhiza medicinal materials. Here we demonstrated that Smi-miR858a-Smi-miR858c, a miRNA family previously known to regulate flavonoid biosynthesis, also played critical regulatory roles in tanshinone and phenolic acid biosynthesis in S. miltiorrhiza. Overexpression of Smi-miR858a in S. miltiorrhiza plants caused significant growth retardation and tanshinone and phenolic acid reduction. Computational prediction and degradome and RNA-seq analyses revealed that Smi-miR858a could directly cleave the transcripts of SmMYB6, SmMYB97, SmMYB111, and SmMYB112. Yeast one-hybrid and transient transcriptional activity assays showed that Smi-miR858a-regulated SmMYBs, such as SmMYB6 and SmMYB112, could activate the expression of SmPAL1 and SmTAT1 involved in phenolic acid biosynthesis and SmCPS1 and SmKSL1 associated with tanshinone biosynthesis. In addition to directly activating the genes involved in bioactive compound biosynthesis pathways, SmMYB6, SmMYB97, and SmMYB112 could also activate SmAOC2, SmAOS4, and SmJMT2 involved in the biosynthesis of methyl jasmonate, a significant elicitor of plant secondary metabolism. The results suggest the existence of dual signaling pathways for the regulation of Smi-miR858a in bioactive compound biosynthesis in S. miltiorrhiza.

Noncoding RNAs in Medicinal Plants and their Regulatory Roles in Bioactive Compound Production

BACKGROUND

Noncoding RNAs (ncRNAs), such as microRNAs (miRNAs), small interfering RNAs (siRNAs) and long noncoding RNAs (lncRNAs), play significant regulatory roles in plant development and secondary metabolism and are involved in plant response to biotic and abiotic stresses. They have been intensively studied in model systems and crops for approximately two decades and massive amount of information have been obtained. However, for medicinal plants, ncRNAs, particularly their regulatory roles in bioactive compound biosynthesis, are just emerging as a hot research field.

OBJECTIVE

This review aims to summarize current knowledge on herbal ncRNAs and their regulatory roles in bioactive compound production.

RESULTS

So far, scientists have identified thousands of miRNA candidates from over 50 medicinal plant species and 11794 lncRNAs from Salvia miltiorrhiza, Panax ginseng, and Digitalis purpurea. Among them, more than 30 miRNAs and five lncRNAs have been predicted to regulate bioactive compound production.

CONCLUSION

The regulation may achieve through various regulatory modules and pathways, such as the miR397-LAC module, the miR12112-PPO module, the miR156-SPL module, the miR828-MYB module, the miR858-MYB module, and other siRNA and lncRNA regulatory pathways. Further functional analysis of herbal ncRNAs will provide useful information for quality and quantity improvement of medicinal plants.

SmPPT, a 4-hydroxybenzoate polyprenyl diphosphate transferase gene involved in ubiquinone biosynthesis, confers salt tolerance in Salvia miltiorrhiza

SmPPT, which encodes 4-hydroxybenzoate polyprenyl diphosphate transferase involved in ubiquinone biosynthesis, confers salt tolerance to S. miltiorrhiza through enhancing the activities of POD and CAT to scavenge ROS. Ubiquinone (UQ), also known as coenzyme Q (CoQ), is a key electron transporter in the mitochondrial respiratory system. UQ is composed of a benzene quinone ring and a polyisoprenoid side chain. Attachment of polyisoprenoid side chain to the benzene quinone ring is a rate-limiting step catalyzed by 4-hydroxybenzoate polyprenyl diphosphate transferase (PPT). So far, only a few plant PPT-encoding genes have been functionally analyzed. Through genome-wide analysis and subsequent molecular cloning, a PPT-encoding gene, termed SmPPT, was identified from an economically and academically important medicinal model plant, Salvia miltiorrhiza. SmPPT contained many putative cis-elements associated with abiotic stresses in the promoter region and were responsive to PEG-6000 and methyl jasmonate treatments. The deduced SmPPT protein contains the PT_UbiA conserved domain of polyprenyl diphosphate transferase and an N-terminal mitochondria transit peptide. Transient expression assay of SmPPT-GFP fusion protein showed that SmPPT was mainly localized in the mitochondria. SmPPT could functionally complement coq2 mutation and catalyzed UQ6 production in yeast cells. Overexpression of SmPPT increased UQ production and enhanced salt tolerance in S. miltiorrhiza. Under salinity stress conditions, transgenic plants accumulated less H₂O₂ and malondialdehyde and exhibited higher peroxidase (POD) and catalase (CAT) activities compared with wild-type plants. It indicates that SmPPT confers salt tolerance to S. miltiorrhiza at least partially through enhancing the activities of POD and CAT to scavenge ROS.

Functional Analysis of Polyprenyl Diphosphate Synthase Genes Involved in Plastoquinone and Ubiquinone Biosynthesis in Salvia miltiorrhiza

Polyprenyl diphosphate synthase (PPS) plays important roles in the biosynthesis of functionally important plastoquinone (PQ) and ubiquinone (UQ). However, only few plant PPS genes have been functionally characterized. Through genome-wide analysis, two PPS genes, termed SmPPS1 and SmPPS2, were identified from Salvia miltiorrhiza, an economically significant Traditional Chinese Medicine material and an emerging model medicinal plant. SmPPS1 and SmPPS2 belonged to different phylogenetic subgroups of plant trans-long-chain prenyltransferases and exhibited differential tissue expression and light-induced expression patterns. Computational prediction and transient expression assays showed that SmPPS1 was localized in the chloroplasts, whereas SmPPS2 was mainly localized in the mitochondria. SmPPS2, but not SmPPS1, could functionally complement the coq1 mutation in yeast cells and catalyzed the production of UQ-9 and UQ-10. Consistently, both UQ-9 and UQ-10 were detected in S. miltiorrhiza plants. Overexpression of SmPPS2 caused significant UQ accumulation in S. miltiorrhiza transgenics, whereas down-regulation resulted in decreased UQ content. Differently, SmPPS1 overexpression significantly elevated PQ-9 content in S. miltiorrhiza. Transgenic lines showing a down-regulation of SmPPS1 expression exhibited decreased PQ-9 level, abnormal chloroplast and trichome development, and varied leaf bleaching phenotypes. These results suggest that SmPPS1 is involved in PQ-9 biosynthesis, whereas SmPPS2 is involved in UQ-9 and UQ-10 biosynthesis.