Syringa vulgaris L., or common lilac is a horticulturally important member of the Oleaceae, a family in which other economically significant genera (Olea, Fraxinus, Jasminum, Forsythia) also occur. Thirty species of Syringa are found distributed across the temperate and south temperate zones of Europe and Asia, while the common lilac has been introduced even more widely as an ornamental. A woody shrub (or small tree) in habit, lilac is typical of the Oleaceae in having opposite leaves, a calyx of four fused sepals and a corolla of four united petals. The highly scented flowers occur in thyrses, each branch bearing a terminal flower.
The high demand for common lilac and lilac hybrids in the woody ornamental trade has led to several studies of in vitro propagation of this and related species. New shoot formation could be induced from explants of actively growing shoot tips on MS medium supplemented with 0.1 mg/1 6-benzylaminopurine (BA) and 0.1-0.5 mg/1 indoleacetic acid. These were multiplied on MS medium containing high (7.5 mg/1) levels of BA and low (0.1 mg/1) levels of auxin (NAA). Low cytokinin levels led to callus formation, from which regeneration has yet to be reported in lilac. Successful in vitro multiplication of S. vulgaris has also been reported by Einset and Alexander and Pierik et al.. In Syringa chinensis, 0.1 mg/1 NAA and 3.0 mg/1 BA yielded the highest number of shoots and nodes per explant. Under conditions of high auxin and low cytokinin levels, explants of Syringa vulgaris and related species readily form fast-growing callus tissue.
Phytochemically, the Oleaceae are probably best known for the occurrence of iridoid glycosides and sugar alcohols. More recently, however, numerous reports have appeared concerning the presence, within this family and related families in the Asteridae, of a remarkably diverse family of aromatic metabolites generally referred to as phenylpropanoid glycosides. Since the original identification of echinacoside in roots of Echinacea angustifolia in 1950, many related phenylpropanoid glycosides have been isolated. The glucoside salidroside [1-O-β-D-glucosyl-2-(4`-hydroxyphenyl)ethanol] is often isolated from plants containing the more complex derivatives. Other related deacyl compounds have also been isolated and may represent either biosynthetic intermediates or degradation products. Table 1 illustrates the structures of the phenylpropanoid glycosides characterized to date, as well as the structure of salidroside.
There is little information concerning the physiological role of these compounds in the plant. Myricoside, extracted from roots of Clerodendrum myricoides, acts as a potent antifeedant against the African armyworm (Spodoptera exempta) at concentrations as low as 10 ppm. A more general role for these compounds may be inferred from the high UV-absorbtivity conferred by the caffeoyl moiety, which may enable these compounds to protect the plant from UV-induced mutation. In addition, the two orthodihydroxy ring substituents are readily oxidized following tissue damage, and may participate in the response to plant pathogenic interactions. Consistent with this hypothesis is the report that verbascoside is a phytoalexin in root tissue of Rehmannia glutinosa ().
Most investigations of the occurrence of these compounds have arisen from interests in chemotaxonomy, bitter principles, or components of plant materials used as folk medicines. As the result of studies of the potential medicinal properties of these compounds, a number of them [forsythoside A, forsythoside B, suspensaside, deshamnosyl verbascoside, purpureaside A, purpureaside C, and verbascoside ()] have been shown to possess antibacterial properties. Verbascoside and orobanchoside have been investigated as analgesic and antihypertensive compounds, and as potential DOPA agonists. The biological activity of analogous compounds has been documented as well, such as the cytotoxicity activity of caffeic acid phenylethyl ester on human tumor cell lines. The potential of these compounds as pharmacologic agents, the ease with which cell cultures can be established from many of the plants which produce them, and the yield of the phenylpropanoid glycosides by cultured cells, make these systems of particular interest both for the study of the diversity and control of plant secondary metabolism, and for possible commercial exploitation.
Syringa vulgaris: Conclusions and Prospects
Plant species which synthesize hydroxyphenylethanol glycosides generally appear to retain this capability when their tissues are cultured in vitro, in the cases reported thus far. The quantities of glycosides accumulated by the cultures exceed the levels of most other secondary metabolites produced in culture systems, which offers a number of opportunities. If compounds of this class are eventually found to be of commercial value, their production in large-scale culture systems could be considered, especially if the native plant source is a slow growing or inaccessible woody species. The level of glycoside synthesis and accumulation in the cultured cells also makes these systems attractive experimental objects in which to explore mechanisms of metabolite storage and transport in plant cells. Finally, the inducibility of several of the key enzymes involved in glycoside biosynthesis, coupled with the high levels of activity attained, makes it feasible to consider purifying the enzymes and retrieving the corresponding genes. In particular, the extensive involvement of specific glycosyltransferases in the synthesis of the more elaborate glycosides offers an opportunity to determine the structural and mechanistic requirements of this important class of enzymes, which is also crucial to glycoprotein biosynthesis in plants and animals. Considering current advances in the generation of antibodies and genetic probes and the experimental versatility afforded by cell suspension cultures, the elucidation of the molecular basis of coordinate induction of the biosynthetic enzymes, and the organization of the pathway within the cells, should be possible. An understanding of the biochemistry behind these specific high-level production patterns will be essential if the prospect of successfully manipulating plant secondary metabolism for industrial and agricultural purposes is to be realized.
Selections from the book: “Medicinal and Aromatic Plants III”, 1991.