Scoparia dulcis L. (Sweet Broomweed)

Sweet broomweed (Scoparia dulcis L., Scrophulariaceae) is a perennial herb widely distributed in the torrid zone. The original habitat of this plant is tropical America. Stems are erect, branching, and sometimes woody at the base, 25-80 cm tall. Roots are pale yellow and straight, 10-15 cm long, with many lateral roots. Leaves are lanceolate, elliptical, or obovate, 5-20 mm long, with serrations at the edge, and are opposite or verticillate. The plant has small, white flowers with four calices. The corrola is actinomorphic and split in four. Flowers are 4-5 mm in diameter and bear four stamens and a pistil. Flowering time is summer and autumn. After flowering, ovate or globular capsules mature (2-3 mm in diameter), which contain many powder-like seeds.

In tropical and subtropical regions of Asia, Africa, and South and Central America, the fresh or dried plant of S. dulcis has traditionally been used as a medicament for stomach disorders, bronchitis, diabetes, hypertension, hemorroids and hepatosis, and as an analgesic and antipyretic.

The antidiabetic activity of the Indian S. dulcis is attributed to the glycoside ammelin obtained from the fresh plant. The methanolic and water extracts from roots of Formosan S. dulcis have been found to exert hypotensive activity, and 6-methoxybenzoxazolinone (MBOA) was isolated as an active principle. MBOA, originally isolated from gramineous plants such as Job’s tears (Coix lacryma jobi), wheat (Triticum aestivum), and corn (Zea mays), was thereafter reported to show antiinflammatory property. The analgesic and antiinflammatory activities of the ethanolic extract of this plant were recently observed, and a triterpene, glutinol, was detected as a major bioactive component by Brazilian workers.

Our studies on biologically active substances from Paraguayan and Formosan collections of S. dulcis led to the isolation of three different types of diterpenoids: (1) labdane type: scoparic acid A, scoparic acid B, scoparic acid C, and scopadiol (); (2) scopadulane type: scopadulcic acid A, scopadulcic acid B, and scopadulciol

(3) aphidicolane type: scopadulin. Among these compounds, scopadulcic acid B was found to possess various biological activities such as inhibitory activities against replication of herpes simplex virus type 1 and gastric H\ K + -ATPase, antitumor activity and antitumor-promoting activity. Because of these many-sided biological activities, synthetic investigations have been carried out and the total synthesis of ( + ) scopadulcic acid B was accomplished recently. On the other hand, scoparic acid A was found to be a potent β-glucuronidase inhibitor, and scopadulciol was also shown to inhibit gastric H + , K+-ATPase. Furthermore, we isolated cytotoxic flavonoid, hymenoxin, beta-glucuronidase inhibitors, isovitexin and 8-hydroxytricetin-7-glucuronide, and antiviral flavonoid, acacetin.

HPLC analyses of diterpenoids in individual plants collected in Paraguay, Taiwan, China, Indonesia, and Thailand revealed that there are three chemo-types based on the major component, i.e., scopadulcic acid B (SDB) type, scoparic acid A (SA) type, and scopadiol (SDX) type. When each chemotype plant was grown from seeds and the diterpenoids content of the leaves was analyzed, the diterpene patterns of the plants raised were the same as those of the parent plants. In addition, these bioactive diterpenoids, as well as MBOA, were found to be present in largest amount in leaves, and younger leaves contained more than the older ones.

Conclusions and Prospects

Our phytochemical and pharmaceutical studies on S. dulcis resulted not only in isolation of characteristic ingredients, SDB, and related substances, but also in observation of the presence of three chemotypes in this plant. Results of diter-penoid analyses in callus and multiple shoots induced from two chemotypes of S. dulcis suggested that SDB might be biosynthesized via SA. Although the capacity for SDB production was negligibly low in callus tissues, it was found to be restored in the differentiated tissues such as multiple shoots derived from SDB-type plants and leaves of regenerated plants. In addition, it was indicated that the fertile regenerated plants were easily obtained from any cultured tissues induced so far. Since production of diterpenoids by cultured tissues is very limited, it is better to produce plants from the cultured tissues.

On the other hand, the production of MBOA by cultured tissues was relatively good. Thus, multiple shoot and hairy root cultures seemed to be suitable for MBOA production.

The fertile regeneration of S. dulcis is advantageous in some cases of genetic engineering. However, the plants transformed with an Ri vector showed an unfavorable “hairy root syndrome” and produced a lower amount of SDB compared with normal plants. Furthermore, the regulatory mechanism of SDB production has not yet been clarified on the molecular level. Therefore, it would be necessary to identify the DNA sequences responsible for genetically determined chemotypes such as the SA and SDB types of S. dulcis.

Selections from the book: “Medicinal and Aromatic Plants IX” (1996).