A plant plasma membrane ATP binding cassette-type transporter is involved in antifungal terpenoid secretion

ATP binding cassette (ABC) transporters, which are found in all species, ar e known mainly for their ability to confer drug resistance. To date, most o f the ABC transporters characterized in plants have been localized in the v acuolar membrane and are considered to be involved in the intracellular seq uestration of cytotoxins. Working on the assumption that certain ABC transp orters might be involved in defense metabolite secretion and their expressi on might be regulated by the concentration of these metabolites, we treated a Nicotiana plumbaginifolia cell culture with sclareolide, a close analog of sclareol, an antifungal diterpene produced at the leaf surface of Nicoti ana spp; this resulted in the appearance of a 160-kD plasma membrane protei n, which was partially sequenced. The corresponding cDNA (NpABC1) was clone d and shown to encode an ABC transporter, In vitro and in situ immunodetect ion showed NpABC1 to be localized in the plasma membrane, Under normal cond itions, expression was found in the leaf epidermis, In cell culture and in leaf tissues, NpABC1 expression was strongly enhanced by sclareolide and sc lareol. In parallel with NpABC1 induction, cells acquired the ability to ex crete a labeled synthetic sclareolide derivative. These data suggest that N pABC1 is involved in the secretion of a secondary metabolite that plays a r ole in plant defense.