A novel ankyrinrepeat membrane protein, IGN1, is required for persistence of nitrogen-fixing symbiosis in root nodules of Lotus japonicus
A novel ankyrinrepeat membrane protein, IGN1, is required for persistence of nitrogen-fixing symbiosis in root nodules of Lotus japonicus
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DOI：
10.1104/pp.106.095356
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发表时间：
2007-03-01
期刊：
影响因子：
7.4
通讯作者：
Kouchi, Hiroshi
中科院分区：
文献类型：
--
作者：
Kumagai, Hirotaka;Hakoyama, Tsuneo;Kouchi, Hiroshi
豆科植物与根瘤菌的固氮共生关系是通过两个共生伙伴之间复杂的相互作用而建立的。与共生初期的相互识别和相互作用类似，寄主豆科植物根瘤内根瘤菌的固氮活性在根瘤发育后期也受到特定相互作用的控制。我们从日本莲花中分离出一个新的Fix 2突变体，无效的绿色结节1 (ign1)，它形成含有内共生细菌的正常结节，但不具有固氮活性。基于图谱的克隆突变基因使我们能够鉴定出IGN1基因，该基因编码一种具有跨膜区域的新型锚蛋白重复序列蛋白。IGN1在枇杷各器官均有表达，且在结瘤过程中表达不增强。免疫分析和绿色荧光蛋白-IGN1融合结构的表达分析表明，IGN1蛋白在质膜中定位。ign1结节表现出极快的早衰。在结核形成的早期（接种后8-9 d）观察到具有多种类杆菌的不规则扩大的共生体，随后共生体被破坏，结核感染的细胞质解体，类杆菌聚集。虽然IGN1基因的确切生化功能仍有待阐明，但这些结果表明，IGN1是类细菌和共生体分化和/或持续存在所必需的，因此对功能性共生至关重要。
Nitrogen-fixing symbiosis of legume plants with Rhizobium bacteria is established through complex interactions between two symbiotic partners. Similar to the mutual recognition and interactions at the initial stages of symbiosis, nitrogen fixation activity of rhizobia inside root nodules of the host legume is also controlled by specific interactions during later stages of nodule development. We isolated a novel Fix 2 mutant, ineffective greenish nodules 1 (ign1), of Lotus japonicus, which forms apparently normal nodules containing endosymbiotic bacteria, but does not develop nitrogen fixation activity. Map-based cloning of the mutated gene allowed us to identify the IGN1 gene, which encodes a novel ankyrin-repeat protein with transmembrane regions. IGN1 expression was detected in all organs of L. japonicus and not enhanced in the nodulation process. Immunoanalysis, together with expression analysis of a green fluorescent protein-IGN1 fusion construct, demonstrated localization of the IGN1 protein in the plasma membrane. The ign1 nodules showed extremely rapid premature senescence. Irregularly enlarged symbiosomes with multiple bacteroids were observed at early stages (8-9 d post inoculation) of nodule formation, followed by disruption of the symbiosomes and disintegration of nodule infected cell cytoplasm with aggregation of the bacteroids. Although the exact biochemical functions of the IGN1 gene are still to be elucidated, these results indicate that IGN1 is required for differentiation and/or persistence of bacteroids and symbiosomes, thus being essential for functional symbiosis.