Distribution, Classification, and Importance of the Genus Coronilla
The genus Coronilla s.l. (Fabaceae) consists of about 50 species of perennial shrubs and perennial or annual herbs occurring in North and Central Europe, the Canary Islands, the Mediterranean region, North Asia, China, and Somaliland. The genus was divided by Uhrova (1935) into four sections, namely the two monospecific sections Emerus and Ballia, the sections Eucoronilla (divided into five series), and Scorpioides.
In the more recent revisions by Zoz and Jahn, the latter taking into consideration also chemotaxonomic aspects, Uhrova’s scheme was followed, with only minor differences in the treatment of the third section, called by them Coronilla. A complete systematic revision on the basis of morphoanatomical, cytological, geographical, and chemical characters led Schmidt to propose a new classification of the genus. In Schmidt’s scheme, the genus is divided into the two monospecific sections Emerus and Ballia, the section Coronilla with a reduced number of species, and the section Scorpioides, formerly including only C scorpioides (L.) Koch and C. repanda (Poir.) Guss., and now including eight additional species, namely: C. coronata L., C. ramosissima (Ball) Ball, C. juncea L., C. speciosa Uhr., C. valentina L., C glauca L., C. minima L., and C. vaginalis Lam.
Recently, the species traditionally included in the genus Coronilla have been divided into three genera (): Coronilla (2n = 12) including the species of Schmidt’s sections Ballia (C. viminalis Salisb.) and Scorpioides (C. coronata L., Cjuncea L., C. minima L., C. ramosissima (Ball) Ball, C. repanda (Poir.) Guss., C. scorpioides (L.) Koch, C. vaginalis Lam., C. valentina L. s.l. (including C. glauca L., C. pentaphylla Desf., C. speciosa Uhr.); Securigera (2n = 12) to which the species included by Schmidt in the section Coronilla (ex.: C. varia) have been transferred together with the recently described species C. somalensis Thulin (); Hippocrepis, to which C. emerus L. and C. emeroides Boiss. & Spruner have been transferred, mainly on the basis of cytological evidence (2n = 14).
With the new delimitation suggested, Coronilla, Securigera, and Hippocrepis constitute very distinct, natural groups which well deserve to be treated as separate genera. Lassen’s classification is followed in the present chapter.
The present interest in this genus as a medicinal plant is due to the occurrence in several species of cardiac glycosides and the hydroxycoumarins (Fig. 1) umbelliferone (I), scopoletin (II) and daphnoretin (III) and the furocoumarin psoralen (IV).
Table Coumarin compounds identified in Coronilla, Securigera, and Hippocrepis
|C. coronata L.||+||+||+||+|
|C. hyrcana Prilipko||+||+||+||–|
|C. juncea L.||–||+||+||+|
|C. minima L.||+||+||+||+|
|C. montana Scop.||–||–||–||+|
|C. repanda (Poiro) Guss.||+||+||+||+|
|C. scorpioides (L.) Koch||+||+||+||+|
|C. vaginalis Lam.||+||+||+||+|
|C. valentina L. s.l.||+||+||+||+|
|C. viminalis Salisb.||+||+||+||traces|
|S. cretica (L.) Lassen||+||+||+||traces|
|S. elegans (Pancic) Lassen||+||+||+||–|
|S. orientalis (Mill.) Lassen||+||+||+||+|
|S. parviflora (Desv.) Lassen||+||+||+||+|
|H. emerus (L.) Lassen||–||–||–||–|
Furocoumarins (psoralens) are well-known photoreactive compounds. They are commonly used in cosmetics and suntan preparations. Psoralens are also being increasingly used in dermatology for the photochemotherapy of numerous diseases like: vitiligo, psoriasis, mycosis fungoides, atopic eczema, alopecia areata, and others.
Coventional Practices for Propagation and Production of Coumarin Compounds
Most of the species belonging to the genus Coronilla s.l. grow in very restricted wild populations without forming uniform vegetation, so that it is difficult to obtain large quantities of plant material. The species have scarce agronomical interest. Only the crownvetch [Coronilla varia L. = Securigera varia (L.) Lassen] is widely planted for erosion control (Hofbauer 1989) while it has limited use as a forage crop owing to lack of persistence under continuous grazing, low palatability, and the presence of several aliphatic nitro compounds (3-nitrop-ropanoyl-D-glucopyranose esters, NPA) which are toxic for nonruminants.
An accurate bibliographic research carried out by Borin, Gianquinto, Zuin, and Da Giau (Institute of General Agronomy, University of Padua) has shown that no records of standardized practices for propagation and cultivation are available, with the exception of crown vetch, for which a detailed protocol was suggested by Duke.
As regards the production of psoralen, the occurrence of this compound in the vegetative organs of C. vaginalis () and of other Coronilla species, although in smaller amounts than those recorded for fruits and seeds, raises the question of whether cultivation for seed production or as a forage crop is preferable.
The furocoumarins are widespread in higher plant families and can be found as secondary metabolites in Apiaceae, Rutaceae, Moraceae, and Fabaceae. At present, the pharmaceutical demand for furocoumarins is supplied by Ammi majus L. (Apiaceae) and Citrus bergamia Risso & Poit. (Rutaceae).
As a rule, mixtures of furocoumarin compounds occur in plant material. In contrast, Coronilla and Securigera species contain only one furocoumarin, namely the linear compound psoralen. Therefore the species belonging to Coronilla s.l. might be more interesting alternative natural sources, allowing psoralen recovery more easily than the classical ones. The total chemical synthesis of psoralens, although possible, is prohibitively expensive.
In this work, the possibilities of in vitro furocoumarin production are illustrated. The first investigated species were Coronilla vaginalis and C. viminalis, chosen on account of their different furocoumarin contents. In fact, while C. vaginalis seeds and fruits have to be considered as a fairly good natural source of psoralen, no psoralen was detected by us in seeds from native specimens and in seeds and vegetative organs of cultivated C. viminalis plants. Traces of psoralen were, however, recorded in C. viminalis seeds by Stoll et al.. The aim of this choice was also to verify whether C. viminalis revealed unexpressed biosynthetic capabilities under tissue culture conditions.
Encouraging results were obtained as regards secondary metabolite production by Coronilla species. Furocoumarins are biosynthesized by calli and cell suspensions not only in C. vaginalis but even in C. viminalis, a species which is known to produce only traces of or no psoralen at all in vivo. Our work suggests that in this species the expression of the genetic information responsible for psoralen biosynthesis is induced by in vitro culture conditions. Another encouraging result is that considerable amounts of furocoumarins can be recovered from the culture medium, excretion from the cells and accumulation in the culture fluid being prerequisites for a large-scale economically feasible production in vitro.
Research is now extending to other Coronilla species; the first results indicate that one of them [C. scorpioides (L.) Koch] is particularly suitable for furocoumarin production by cell suspension cultures.
Selections from the book: “Medicinal and Aromatic Plants IX” (1996).