Transposon tagging of tobacco mosaic virus resistance gene N: its possible role in the TMV-N-mediated signal transduction pathway.

Plants can recognize and resist invading pathogens by signaling the induction of rapid defense responses. Often these responses are mediated by single dominant resistance genes (R genes). The products of R genes have been postulated to recognize the pathogen and trigger rapid host defense responses. Here we describe isolation of the classical resistance gene N of tobacco that mediates resistance to the well-characterized pathogen tobacco mosaic virus (TMV). The N gene was isolated by transposon tagging using the maize Activator (Ac) transposon. We confirmed isolation of the N gene by complementation of the TMV-sensitive phenotype with a genomic DNA fragment. Sequence analysis of the N gene shows that it encodes a protein with an amino-terminal domain similar to that of the cytoplasmic domains of the Drosophila Toll protein and the interleukin 1 receptor in mammals, a putative nucleotide-binding site and 14 imperfect leucine-rich repeats. The presence of these functional domains in the predicted N gene product is consistent with the hypothesis that the N resistance gene functions in a signal transduction pathway. Similarities of N to Toll and the interleukin 1 receptor suggest a similar signaling mechanism leading to rapid gene induction and TMV resistance. IMAGES:

植物可通过信号诱导快速防卫反应，识别并抵御入侵的病原物。此类防卫反应通常由单个显性抗病基因（resistance genes, R genes）介导。此前研究推测，R基因的编码产物可识别病原物，并触发宿主快速的防卫反应。本文报道了烟草经典抗病基因N的克隆过程，该基因可介导对研究较为透彻的病原物——烟草花叶病毒（tobacco mosaic virus, TMV）的抗性。研究人员利用玉米激活子（maize Activator, Ac）转座子标签技术克隆得到该N基因。通过将含有该基因的基因组DNA片段转化对TMV敏感的烟草植株，使其恢复抗病表型，从而验证了所克隆的即为N基因。对N基因的序列分析显示，其编码的蛋白质氨基末端结构域与果蝇Toll蛋白（Drosophila Toll protein）的胞质结构域以及哺乳动物白细胞介素1受体（interleukin 1 receptor）的对应区域具有相似性，同时包含一个推定的核苷酸结合位点，以及14个不完美的富亮氨酸重复序列（leucine-rich repeats）。预测的N基因编码产物中存在这些功能结构域，这与“N抗病基因参与信号转导通路”的假说相符。N基因与Toll蛋白及白细胞介素1受体的同源性，提示二者可能具有相似的信号传导机制，进而介导快速的基因诱导反应与TMV抗性。图像：