Metabolic engineering to enhance the accumulation of bioactive flavonoids licochalcone A and echinatin in Glycyrrhiza inflata (Licorice) hairy roots

A Cotyledon-based Virus-Induced Gene Silencing (Cotyledon-VIGS) approach to study specialized metabolism in medicinal plants

BACKGROUND

Virus-induced gene silencing (VIGS) is widely used in plant functional genomics. However, the efficiency of VIGS in young plantlets varies across plant species. Additionally, VIGS is not optimized for many plant species, especially medicinal plants that produce valuable specialized metabolites.

RESULTS

We evaluated the efficacy of five-day-old, etiolated seedlings of Catharanthus roseus (periwinkle) for VIGS. The seedlings were vacuum-infiltrated with Agrobacterium tumefaciens GV3101 cells carrying the tobacco rattle virus (TRV) vectors. The protoporphyrin IX magnesium chelatase subunit H (ChlH) gene, a key gene in chlorophyll biosynthesis, was used as the target for VIGS, and we observed yellow cotyledons 6 days after infiltration. As expected, the expression of CrChlH and the chlorophyll contents of the cotyledons were significantly decreased after VIGS. To validate the cotyledon based-VIGS method, we silenced the genes encoding several transcriptional regulators of the terpenoid indole alkaloid (TIA) biosynthesis in C. roseus, including two activators (CrGATA1 and CrMYC2) and two repressors (CrGBF1 and CrGBF2). Silencing CrGATA1 led to downregulation of the vindoline pathway genes (T3O, T3R, and DAT) and decreased vindoline contents in cotyledons. Silencing CrMYC2, followed by elicitation with methyl jasmonate (MeJA), resulted in the downregulation of ORCA2 and ORCA3. We also co-infiltrated C. roseus seedlings with TRV vectors that silence both CrGBF1 and CrGBF2 and overexpress CrMYC2, aiming to simultaneous silencing two repressors while overexpressing an activator. The simultaneous manipulation of repressors and activator resulted in significant upregulation of the TIA pathway genes. To demonstrate the broad application of the cotyledon-based VIGS method, we optimized the method for two other valuable medicinal plants, Glycyrrhiza inflata (licorice) and Artemisia annua (sweet wormwood). When TRV vectors carrying the fragments of the ChlH genes were infiltrated into the seedlings of these plants, we observed yellow cotyledons with decreased chlorophyll contents.

CONCLUSIONS

The widely applicable cotyledon-based VIGS method is faster, more efficient, and easily accessible to additional treatments than the traditional VIGS method. It can be combined with transient gene overexpression to achieve simultaneous up- and down-regulation of desired genes in non-model plants. This method provides a powerful tool for functional genomics of medicinal plants, facilitating the discovery and production of valuable therapeutic compounds.

Engineering the plant metabolic system by exploiting metabolic regulation

Plants are the most sophisticated biofactories and sources of food and biofuels present in nature. By engineering plant metabolism, the production of desired compounds can be increased and the nutritional or commercial value of the plant species can be improved. However, this can be challenging because of the complexity of the regulation of multiple genes and the involvement of different protein interactions. To improve metabolic engineering (ME) capabilities, different tools and strategies for rerouting the metabolic pathways have been developed, including genome editing and transcriptional regulation approaches. In addition, cutting-edge technologies have provided new methods for understanding uncharacterized biosynthetic pathways, protein degradation mechanisms, protein-protein interactions, or allosteric feedback, enabling the design of novel ME approaches.

Histone Deacetylase GiSRT2 Negatively Regulates Flavonoid Biosynthesis in Glycyrrhiza inflata

Glycyrrhiza inflata Batalin is a medicinal licorice species that has been widely used by humans for centuries. Licochalcone A (LCA) is a characteristic flavonoid that accumulates in G. inflata roots with high economical value. However, the biosynthetic pathway and regulatory network of its accumulation remain largely unknown. Here we found that a histone deacetylase (HDAC) inhibitor nicotinamide (NIC) could enhance the accumulation of LCA and total flavonoids in G. inflata seedlings. GiSRT2, a NIC-targeted HDAC was functionally analyzed and its RNAi transgenic hairy roots accumulated much more LCA and total flavonoids than its OE lines and the controls, indicating a negative regulatory role of GiSRT2 in the accumulation of LCA and total flavonoids. Co-analysis of transcriptome and metabolome of RNAi-GiSRT2 lines revealed potential mechanisms in this process. An O-methyltransferase gene, GiLMT1 was up-regulated in RNAi-GiSRT2 lines and the encoded enzyme catalyzed an intermediate step in LCA biosynthesis pathway. Transgenic hairy roots of GiLMT1 proved that GiLMT1 is required for LCA accumulation. Together, this work highlights the critical role of GiSRT2 in the regulation of flavonoid biosynthesis and identifies GiLMT1 as a candidate gene for the biosynthesis of LCA with synthetic biology approaches.