During the life cycle of flowering plants, nuclear fusion, or karyogamy, occurs three times: once during female gametogenesis, when the two polar nuclei fuse in the central cell, and twice during double fertilization. In Arabidopsis thaliana, nuclear fusion events during sexual reproduction proceed without the breakdown of the nuclear envelope, indicating that nuclear membrane fusion is essential for the completion of this process. Arabidopsis gamete expressed 1 (GEX1) is a membrane protein that is conserved among plant species. GEX1 shares homology with the yeast karyogamy protein Kar5, which is primarily expressed in the nuclear membrane. The GEX1 family represents a putative karyogamy factor. Herein, we show that GEX1 is required for the nuclear fusion events in Arabidopsis reproduction. GEX1-deficient mature female gametophytes were found to contain two unfused polar nuclei in close proximity within the central cell. Electron microscopy showed that the outer membrane of the polar nuclei was connected via the endoplasmic reticulum, whereas the inner membrane remained unfused. These results indicate that GEX1 is involved in polar nuclear membrane fusion following the fusion of the outer nuclear membrane. Furthermore, sperm nuclear fusion events were defective in the fertilized egg and central cell following plasmogamy in the fertilization of gex1-1 female gametophytes by gex1-1 pollen. An analysis of GEX1 localization in the female gametophyte using a transgenic line expressing GFP-tagged GEX1 driven by the GEX1 promoter showed that GEX1 is a nuclear membrane protein in the egg and central cell. Time-lapse live-cell imaging showed that in developing female gametophytes, the nuclear GFP-GEX1 signal was first detectable in the central cell shortly before the polar nuclei came in close contact, and then in the egg cell. Thus, we suggest that the GEX1-family proteins are nuclear membrane proteins involved in karyogamy in the reproduction of eukaryotes including flowering plants.

在开花植物的生命周期中，核融合或核配发生三次：一次在雌配子发生期间，当两个极核在中央细胞中融合时，两次在双受精期间。在拟南芥，有性生殖过程中的核融合事件在没有核膜破裂的情况下进行，这表明核膜融合对于完成这一过程至关重要。拟南芥配子表达 1 (GEX1) 是一种在植物物种中保守的膜蛋白。 GEX1 与酵母核配蛋白 Kar5 具有同源性，Kar5 主要在核膜中表达。 GEX1 家族代表了假定的核配因子。在此，我们证明 GEX1 对于核聚变事件是必需的拟南芥生殖。发现 GEX1 缺陷的成熟雌配子体在中央细胞内含有两个非常接近的未融合的极核。电镜显示极核的外膜通过内质网连接，而内膜未融合。这些结果表明GEX1参与外核膜融合后的极性核膜融合。此外，在受精卵受精后，受精卵和中心细胞中的精子核融合事件存在缺陷。基因1-1雌配子体由基因1-1花粉。使用表达 GFP 标记的 GEX1 的转基因系对雌配子体中的 GEX1 定位进行分析GEX1启动子显示GEX1是卵子和中央细胞中的核膜蛋白。 延时活细胞成像显示，在发育中的雌配子体中，在极核紧密接触之前不久，首先在中央细胞中检测到核 GFP-GEX1 信号，然后在卵细胞中检测到。因此，我们认为 GEX1 家族蛋白是参与真核生物（包括开花植物）繁殖中核配子的核膜蛋白。