SummaryTobacco cells (Nicotiana tabacum L. Bright Yellow T‐13) exposed to harmful naphthols accumulate them as glucosylated and further modified compounds [Taguchi et al. (2003a) Plant Sci. 164, 231–240]. In this study, we identified the accumulated compounds to be 6′‐O‐malonylated glucosides of naphthols. Cells treated with various phenolic compounds accumulated the flavonoids mainly as malonylglucosides. To clarify the function of this malonylation in tobacco, we isolated the cDNA encoding a malonyltransferase (NtMaT1) from a cDNA library derived from tobacco cells. The heterologous expression of the gene in Escherichia coli revealed that the recombinant enzyme had malonyltransferase activity against several phenolic glucosides such as flavonoid 7‐O‐glucosides, flavonoid 3‐O‐glucosides and naphthol glucosides. The substrate preference of the enzyme was similar to that of the tobacco cell extract. Malonylation activity in the transgenic cells markedly decreased with the suppression of the expression of NtMaT1 mRNA in tobacco BY‐2 cells by RNA interference. The compounds administered to the transgenic cells were accumulated in the cells as glucosides or other modified compounds in place of malonylglucosides. These results show that NtMaT1 is the main catalyst of malonylation on glucosides of xenobiotic flavonoids and naphthols in tobacco plants.
摘要暴露于有害萘酚的烟草细胞(Nicotiana tabacum L. Bright Yellow T-13)会以葡萄糖基化和进一步修饰化合物的形式积累萘酚[Taguchi 等人(2003a),植物科学 164,231-240]。在本研究中,我们发现积累的化合物是萘酚的 6′-O-丙二酰化葡萄糖苷。用各种酚类化合物处理的细胞主要以丙二酰葡萄糖苷的形式积累黄酮类化合物。为了明确烟草中丙二酰化的功能,我们从烟草细胞的 cDNA 文库中分离出了编码丙二酰转移酶(NtMaT1)的 cDNA。将该基因在大肠杆菌中进行异源表达后发现,重组酶具有丙二酰转移酶活性,可作用于多种酚葡糖苷,如黄酮 7-O-葡糖苷、黄酮 3-O-葡糖苷和萘酚葡糖苷。该酶的底物偏好与烟草细胞提取物相似。通过 RNA 干扰抑制烟草 BY-2 细胞中 NtMaT1 mRNA 的表达,转基因细胞中的丙二酰化活性明显降低。转基因细胞中的化合物以葡萄糖苷或其他修饰化合物的形式在细胞中积累,取代了丙二酰葡萄糖苷。这些结果表明,NtMaT1 是烟草植物中异生物类黄酮和萘酚葡萄糖苷丙二酰化的主要催化剂。