Digitalis plants are of great importance in pharmacy due to their production of cardioactive glycosides. They are most frequently employed in the treatment of heart diseases. Heart glycosides appear in several plant families which mostly are not related to each other, but they occur in many Digitalis species. The majority of investigations refer to D. purpurea and D. lanata.
Digitalis, also known as foxglove, belongs to the family Scrophulariaceae. Inspired by the form of their flowers, Leonhart Fuchs (1542) for the first time used the name Digitalis in his herbal. The species of Digitalis are biennial or perennial herbs. The foliage consists of a rosette of leaves with inflorescences of about 1 m height. Their morphology and their flower can be seen from, exemplified by Digitalis lanata.
Digitalis glycosides belong to the cardenolide type and are therefore named cardenolides. In the stereo-ring system, an unsaturated five-membered lactone ring is substituted in position 17, thus differentiating cardenolides from the bufa-dienolides presenting a six-membered lactone ring in this position. Apart from many cardiotonic glycosides, many ineffective glycosides occur in Digitalis plants. Different groups, substituted on the basic steroid structure, contribute decisively to the efficiency of these compounds. Although aglycones actually constitute the active part, the substituted sugars support their effect strongly.
The cardiac glycosides exert similar physiological effects in spite of very different pharmacokinetic properties. Because of their small active concentration range, the pharmaceutical preparations must fulfill certain requirements. At present in the treatment of heart insufficiency α- and β-acetyldigoxin and β-methyldigoxin are preferred. The therapeutic effect of Digitalis was first described by the Scottish physician W. Whithering (1741-1791); however, the use of Digitalis drugs dates back much earlier.
Within the family Scrophulariaceae, the occurrence of cardiac glycosides could be proven with certainty in Digitalis species only. The highest content of these substances is found in the leaves. Together with cardiac glycosides, digitanol glycosides and steroidal saponines are detectable. Among other secondary products that occur in Digitalis, flavones, anthrachinones, and organic acids have been detected.
For commercial use Digitalis plants are grown large-scale from seed (in Europe: The Netherlands, Hungary; also in the U.S.A., in Argentina and in Africa and Asia). The seeds germinate easily. Special techniques are required to harvest the seeds and dry them in large quantities. There are many comprehensive review articles on Digitalis concerning the plants, as well as the secondary products, especially cardioactive glycosides.
Digitalis spp.: Regeneration
In vitro cultures also facilitate the vegetative propagation of plants which are difficult to reproduce vegetatively in the conventional way. Regeneration of plants in vitro may also start from a single cell. This happens in somatic embryogenesis and in microculture of individual cells, i.e., when embryogenic processes are induced in somatic cells and/or when single cells are isolated, bringing about an induction of cell division and organogenesis. In addition, in vitro cultures offer the possibility of reproducing the regenerates quickly to achieve numerous individual plants (.).
Although especially efficient strains of D. lanata could be obtained from seedlings by selection methods, the possibility of the vegetative propagation in vitro cannot be neglected; the metabolic patterns of the individual plants of D. lanata, for example, varied greatly. Clonal cultivation, directly concentrated toward selected individual plants, could possibly eliminate the variation and create uniform plants for further propagation. In order to obtain more qualified plants, several methods of in vitro culture are increasingly applied. Their advantages are not only obvious in clonal propagation of test plants, but also in the commercial use of plants.
Various Digitalis species cultivated in vitro showed clear differences in their regenerative behavior. Besides the variability inherent in a species, also organ differences became noticeable. Most studies deal with D. lanata and D. purpurea. The following refers to morphogenetic and organogenetic behavior of Digitalis species.
In tissues of D. lanata seedlings it was easy to induce roots, leaves, and shoots and to propagate them under the influence of suitable cultivating conditions. Whereas in juvenile tissues, clonal propagation of D. lanata via shoot induction could be achieved, only the regeneration of roots, but no normal shoot formation was observed in leaf tissue of adult plants and in callus tissue.
When starting from adult D. lanata plants, two special in vitro techniques are available for regeneration and propagation: (1) Induction of somatic embryogenesis, and (2) the cultivation of the apical meristems, the method developed by Morel and frequently used since then.
For somatic embryogenesis, the following can be said: by cultivating tissue ex-plants of different parts of the flowers, especially structured callus was formed at the filaments which turned out to be embryogenic. After changing the hormonal conditions and cultivating in liquid media under light, these special calli formed green globular structures first and subsequently developed embryos, seedlings, and plantlets under the influence of changed culture conditions.
Regarding meristem culture, Schoner and Reinhard reported successful cultivation assays of apical meristem tips of adult D. lanata plants, in relation to growth, development, further propagation, and cardenolide formation. The authors considered this technique as a possible approach to obtain clones of D. lanata from tissues of adult plants.
Great efforts were undertaken to obtain haploid plants of D. lanata, but without satisfactory results. Therefore the experiments were either prematurely stopped or continued under other aspects. This follows from discussions and/or from the reports on those studies which have led to the elaboration of somatic embryogenesis in D. lanata, originally planned as cultivation program.
Concluding Remarks and Future Prospects
Plants are the main source of many valuable pharmaceutical products. This is particularly true of the cardiac glycosides of Digitalis. The use of the in vitro culture offers great advantage for plant improvement and for realizing the possibility of propagating the plants clonally. Using in vitro culture for the production of natural compounds brings the advantage of independence from the delivery of basal plant materials. This fact cannot be sufficiently overestimated, especially in connection with valuble substances in medicine.
From a review of the present literature it can be seen that several methods have been elaborated for clonal propagation of Digitalis referring to tissues of seed-lings, adult plants and also of single cells. Concerning the production of cardenolides, it is noteworthy that highly differentiated callus tissues preserved their ability for cardenolide formation. Biotransformation of cardenolides can be carried out also by less differentiated callus cells. This offers the further advantage of transforming cardenolides not frequently used in pharmacy into those compounds which are favorably applied.
The best-known techniques of plant in vitro culture have been applied for Digitalis and successfully adapted. This underscores the importance of Digitalis on the one hand and the possibilities of the in vitro culture for investigations in basic research and in practical usage on the other hand.
The performance of sensitive analytical methods such as immunological assays, together with improved and new in vitro cultivation techniques, can be of great advantage for future studies and applications.
The results obtained indicate some successful applications of plant tissue and cell cultures of Digitalis, especially the following:
- 1. Biogenetic studies for obtaining knowledge of enzyme activities in cardenolide biosynthesis;
- 2. Breeding and propagating high yield Digitalis plants;
- 3. Use of Digitalis cell cultures in the production of pharmaceutically important glycosides.
Selections from the book: “Medicinal and Aromatic Plants I”, 1988.