Tartary buckwheat (TB) (Fagopyrum tataricum (L.) Gaertn) is a type of dicotyledon belonging to the genus Fagopyrum and the family Polygonaceae1. As a type of Chinese medicine homologous food, TB has been receiving considerable interest owing to its distinctive chemical composition and diverse bioactivities against diseases. TB is primarily rich in carbohydrates, proteins, vitamins, and carotenoids as well as in polyphenols such as phenolic acids and flavonoids1. Various biological and pharmacological activities of flavonoids, including antioxidative, antihypertensive2, and anti-inflammatory as well as anticancer and antidiabetic properties, have been demonstrated3.
Agrobacterium rhizogenes is a soil bacterium that contributes to the development of hairy root disease in several higher plants, especially dicotyledons, by infecting wound sites4,5. This process is initiated by the transfer of the T-DNA in the root-inducing (Ri) plasmid5,6 and is commonly accompanied by the integration and expression of an exogenous gene from the Ri plasmid and the subsequent steps of generating the hairy root phenotype7. A. rhizogenes-mediated transgenic hairy roots, as a powerful tool in the field of plant biotechnology, have been most widely used owing to their stable and high productivity and easy obtainment in a short period. Moreover, hairy roots induced by A. rhizogenes are efficiently distinguished by their plagiotropic root development and highly branching growth in a hormone-free medium8. They can be used in several fields of research, including artificial seed production, root nodule research, and in studying the interactions with other organisms such as mycorrhizal fungi, nematodes, and root pathogens7,9. In addition, hairy root transformation cultures have been extensively used as an experimental system to investigate the biochemical pathways and chemical signaling and to produce plant secondary metabolites that are used as pharmaceuticals, cosmetics, and food additives8,10. The valuable secondary metabolites, including indole alkaloids, aconites, tropane alkaloids, terpenoids, and flavonoids, synthesized in wild-type hairy roots have been investigated for several decades in numerous species, such as ginsenoside in Panax ginseng11, coumarine in Ammi majus12, and phenolic compounds in TB2,13.
Hairy roots have been produced using A. rhizogenes in 79 plant species from 27 families14. For instance, A. rhizogenes-mediated hairy root transformation has been reported in soybean15,16, Salvia17, Plumbago indica18, Lotus japonicus19, and chicory (Cichorium intybus L.)20. TB hairy root transformation has also been investigated2. Few detailed protocols are available regarding the development of hairy roots mediated by A. rhizogenes either carrying a binary vector or not. For instance, Sandra et al.21 introduced a method of producing transgenic potato hairy roots sustained in wild-type shoots. The fully developed hairy roots could be visualized 5-6 weeks after the injection of A. rhizogenes carrying the gus reporter gene into the stem internodes of potato plants. Another study had also reported a transgenic hairy root system induced by A. rhizogenes harboring the gusA reporter gene in jute (Corchorus capsularis L.)22. Furthermore, Supaart et al.23 obtained transgenic tobacco hairy roots using A. rhizogenes transformed with the expression vector pBI121 carrying the gene of Δ1-tetrahydrocannabinolic acid (THCA) synthase to produce THCA.
However, a step-by-step process for an effective generation of hairy root transformation, especially in TB, has been relatively less demonstrated. In this study, we have described a detailed protocol using A. rhizogenes carrying the reporter gene (GFP), a selective marker (Kan), and a gene of interest (b4, an identified from our group but unpublished gene from basic helix-loop-helix (bHLH) family) to generate hairy root genetic transformation in TB. The experiment lasted for 5-6 weeks, from the inoculation of seeds to generation of hairy roots, involving the explant preparation, infection, coculturing, subculturing, and subsequent propagation. Furthermore, A. rhizogenes containing a binary plasmid carrying the TB transgene of myeloblastosis transcription factor 116 (FtMYB116) was used to determine whether FtMYB116 can promote accumulation of flavonoids, particularly rutin, in TB at the gene and metabolic level through the TB hairy root transformation. FtMYB116, which is a light-induced transcriptional factor, regulates the synthesis of rutin under different light conditions5. Chalcone synthase (CHS), flavanone-3-hydroxylase (F3H), flavonoid-3'-hydroxylase (F3'H), and flavonol synthase (FLS)24 are key enzymes involved in the metabolic pathway of rutin biosynthesis. Therefore, this study demonstrates the overexpression of FtMYB116 in TB hairy roots and the expression of key enzyme genes as well as the content of rutin and other flavonoids such as quercetin.