Plants produce a large number of secondary metabolites, such as alkaloids, terpenoids, polyphenols, quinones and many further compounds having combined structures of those groups. Physiological roles of those metabolites for plants are still under investigation, but they play, at least in part, important functions as protectants for plant bodies against herbivores and pathogens, as well as from physical stresses like ultraviolet light and heat. In order to accomplish these functions, biosyntheses and accumulation of secondary metabolites are highly regulated in a temporal and spatial manner in plant organs, where they can appropriately accumulate. In this mini-review, I introduce the mechanism of accumulation and membrane transport of these metabolites, in particular, focusing on ATP-binding cassette transporters involved.
Higher plants produce a vast number of secondary metabolites, in addition to primary metabolites, via complex pathways, which are regulated in highly sophisticated manners . Many of them show strong biological activities, e.g., inhibition of DNA and protein synthesis, inhibition of the nerve system, cardiac activity, modulation of microtuble structure, etc. Bioactive secondary metabolites have been, therefore, utilized as natural medicines and often such plants containing those compounds have been used as medicinal plants and prescribed in many recipes as forms of crude drugs [2,3]. In most cases these bioactive natural compounds are found in particular organs, which are called ‘‘medicinal part’’ in pharmacognosy, and their contents in such organs are often seasonally regulated . The physiological roles of these secondary metabolites for plants have not been completely elucidated, but reasonable explanations have been made for some secondary metabolites, i.e., they may function as biological protectants from herbivores, pathogen attacks and abiotic environmental stresses such as UV irradiation [5,6]. For instance, nicotine of tobacco or caffeine of coffee tree was reported to act as strong insecticides [7,8]. Some secondary metabolites are known to function as mediators necessary for the interaction with other organisms, as being allelopathic substances or insect attractants to facilitate pollination . To achieve those functions, accumulation or secretion of those compounds has to be highly regulated, for instance, flavonoids acting as UV protectant are specifically accumulated in epidermal cells , and insect attractants are emitted from flower petals . Biosynthetic genes responsible for the formation of those secondary metabolites may be highly expressed in such tissues where the metabolites are mainly accumulated, while translocation of natural compounds among plant organs often occurs as well, e.g., biosynthetic genes for nicotine, a pyrrolidine alkaloid of Nicotiana species, are mostly expressed in root tissues (source organ) whereas it is transported to the aerial part and accumulated in leaves (sink organ) . The membrane transport of plant secondary metabolites is a newly developing research area , and it has been found that ATP-binding cassette (ABC) transporters are involved in some plant systems. In this mini review, I overview the involvement of ABC transporters for the membrane transport of endogenous secondary metabolites in plants and also those mediating the transport of plant products in heterologous systems as well. The comparison of ABC transporters in both systems is discussed.