Nicotiana plumbaginifolia NpPDR1, a plasma membrane pleiotropic drug resistance-type ATP-binding cassette transporter formerly named NpABC1, has been suggested to transport the diterpene sclareol, an antifungal compound. However, direct evidence for a role of pleiotropic drug resistance transporters in the plant defense is still lacking. In situ immunolocalization and histochemical analysis using the gusA reporter gene showed that NpPDR1 was constitutively expressed in the whole root, in the leaf glandular trichomes, and in the flower petals. However, NpPDR1 expression was induced in the whole leaf following infection with the fungus Botrytis cinerea, and the bacteria Pseudomonas syringae pv tabaci, Pseudomonas fluorescens, and Pseudomonas marginalis pv marginalis, which do not induce a hypersensitive response in N. plumbaginifolia, whereas a weaker response was observed using P. syringae pv syringae, which does induce a hypersensitive response. Induced NpPDR1 expression was more associated with the jasmonic acid than the salicylic acid signaling pathway. These data suggest that NpPDR1 is involved in both constitutive and jasmonic acid-dependent induced defense. Transgenic plants in which NpPDR1 expression was prevented by RNA interference showed increased sensitivity to sclareol and reduced resistance to B. cinerea. These data show that NpPDR1 is involved in pathogen resistance and thus demonstrate a new role for the ATP-binding cassette transporter family.

ATP binding cassette (ABC) transporters, which are found in all species, are known mainly for their ability to confer drug resistance. To date, most of the ABC transporters characterized in plants have been localized in the vacuolar membrane and are considered to be involved in the intracellular sequestration of cytotoxins. Working on the assumption that certain ABC transporters might be involved in defense metabolite secretion and their expression 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 Nicotiana spp; this resulted in the appearance of a 160-kD plasma membrane protein, which was partially sequenced. The corresponding cDNA (NpABC1) was cloned and shown to encode an ABC transporter. In vitro and in situ immunodetection showed NpABC1 to be localized in the plasma membrane. Under normal conditions, expression was found in the leaf epidermis. In cell culture and in leaf tissues, NpABC1 expression was strongly enhanced by sclareolide and sclareol. In parallel with NpABC1 induction, cells acquired the ability to excrete a labeled synthetic sclareolide derivative. These data suggest that NpABC1 is involved in the secretion of a secondary metabolite that plays a role in plant defense.

ATP binding cassette (ABC) transporters, which are found in all species, are known mainly for their ability to confer drug resistance. To date, most of the ABC transporters characterized in plants have been localized in the vacuolar membrane and are considered to be involved in the intracellular sequestration of cytotoxins. Working on the assumption that certain ABC transporters might be involved in defense metabolite secretion and their expression 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 Nicotiana spp; this resulted in the appearance of a 160-kD plasma membrane protein, which was partially sequenced. The corresponding cDNA ( NpABC1 ) was cloned and shown to encode an ABC transporter. In vitro and in situ immunodetection showed NpABC1 to be localized in the plasma membrane. Under normal conditions, expression was found in the leaf epidermis. In cell culture and in leaf tissues, NpABC1 expression was strongly enhanced by sclareolide and sclareol. In parallel with NpABC1 induction, cells acquired the ability to excrete a labeled synthetic sclareolide derivative. These data suggest that NpABC1 is involved in the secretion of a secondary metabolite that plays a role in plant defense. ń INTRODUCTIONThe ATP binding cassette (ABC) superfamily of membrane transporters is found in all prokaryotic and eukaryotic species. All members of this family are involved in the active transport of many chemically and structurally unrelated compounds and use ATP hydrolysis as a source of energy (Higgins, 1992). Several of these proteins have been identified by their ability to confer drug resistance, hence their designation as multidrug resistance (MDR) or pleiotropic drug resistance (PDR) proteins. For example, the human MDR1 (Chen et al., 1986), which is also designated P-glycoprotein, is implicated in resistance of cell lines to antitumor drugs (Ueda et al., 1987), PDR5 is a multidrug transporter found in Saccharomyces cerevisiae (Balzi et al., 1994), and Candida PDR1 (CDR1) and CDR2, identified in the yeast Candida albicans , contribute to fungicide resistance (Prasad et al., 1995;Sanglard et al., 1997). An analog of human MDR1 also has been found in prokaryotes (van Veen et al., 1996). Other ABC transporters were characterized by their ability to secrete internal compounds; this is the case with the S. cerevisiae protein STE6, which is involved in the transport of a mating pheromone peptide (Kuchler et al., 1989), and mammalian MDR2, which acts as a phospholipid translocator (Ruetz and Gros, 1994).Multidrug resistance-related proteins (MRPs) are ABC proteins that can be distinguished from the proteins described above by their possession of an N-terminal hydrophobic extension and an internal regulatory domain. In mammals, the MRPs HmMRP1 (Cole et al., 1992) and cMOAT (Buchler et al., 1996) transport glutathione S -conjugates and are implicated in the de...