Daucus carota (family Umbelliferae) is one of the most commonly used plant materials in tissue culture studies. Although various aspects of growth and organization have been extensively studied, relatively few attempts have been made at in vitro production of specific plant ingredients in carrot cells. Carrot root is characterized by its high content of carotenoid pigments. Carotenoids are also found in petals, seeds, and fruits of various plant species. Many of the yellow, orange, and red colors seen in these plant organs are often due to the presence of these compounds. The distribution of carotenoids in higher plants has been summarized by Goodwin () and Spurgeon and Porter (). However, their occurrence is not restricted to those storage organs but includes all parts of the plant. They serve as light-harvesting molecules in photosynthetic organelles and also play a role in protecting prokaryotes from the deleterious effects of light. Carotenoids are also essential for vision. The light-absorbing molecules of the visual system in many organisms, 11-cis-retinal, is derived from β-carotene. Carotenoids have been shown to be anti-carcinogenous in rats and mice, and it also appears to be the case in humans (). Although β-carotene is currently produced synthetically for commercial use, carrot cell cultures offer a very useful model system in studying the in vitro production of important plant metabolites. Besides carotenoids, carrot cells produce phytoalexin when they are challenged by microorganisms. Phytoalexins are defined as the low molecular weight antimicrobial compounds produced by the plant in response to infection. The iso-coumarin phytoalexin 6-methoxymellein has been reported to accumulate in carrot roots infected with fungi (). We found that it can be produced in culture of carrot cells in vitro when they are properly treated with certain chemicals. This chapter deals with the in vitro production of carotenoids and carrot phytoalexin, 6-methoxymellein in Daucus carota.
Carrot cell suspension cultures have proved to be an excellent experimental system for investigating the mechanisms by which cell growth and biochemical processes are regulated. Carotenoid synthesis in carrot cells is regulated by various factors including medium composition, growth regulators and possibly by a certain bioregulator which affects gene expression. High-yielding strains have been developed through induced mutation and screening of the variant cells. The amounts of carotenoids in these cells were equal to or greater than the intact parent tissue. This method may be applicable to other plant tissue cultures; however, many difficulties still remain to be overcome. For instance, carrot cells were selected according to their color, but the detection of cells accumulating noncolored material is not easy. More work is, therefore, required to establish a widely applicable technique for the selection of high-yielding plant cells.
Biosynthesis of the carrot phytoalexin 6-methoxymellein is elicited by oligo-uronides or peptides. These elicitors are released from the plant cell walls by pectinolytic and proteolytic enzymes secreted by invading fungi. 6-Methoxymellein is synthesized from acetyl-CoA and malonyl-CoA via the polyketide route and 6-hydroxymellein accumulated as an intermediary compound. Evidence has been accumulated to suggest that a calcium messenger system mediates induction of secondary metabolic pathway by these elicitors in cultured plant cells.
It is generally recognized that many plant cells often fail to produce certain secondary metabolites when they are transferred to the in vitro culture system. Recent work, however, showed that the addition of certain elicitors to these “repressed culture” sometimes induces the biosynthetic activity of secondary products (). These findings indicate that increased synthesis of secondary metabolites in response to elicitation of various types is not restricted to defense reactions against invading microorganisms. In the search for an effective elicitor to stimulate the production of desired plant metabolites, more attention should be focused on understanding the role of elicitors and their mode of action in these processes.
Selections from the book: “Medicinal and Aromatic Plants V”, 1993.