Following exposure to water, mature Arabidopsis seeds are surrounded by a gelatinous capsule, termed mucilage. The mucilage consists of pectin-rich polysaccharides, which are produced in epidermal cells of the seed coat. Although pectin is a major component of plant cell walls, its biosynthesis and biological functions are not fully understood. Previously, we reported that a transmembrane RING E3 ubiquitin ligase, FLYING SAUCER 1 (FLY1) regulates the degree of pectin methyl esterification for mucilage capsule formation. The Arabidopsis thaliana genome has a single FLY1 homolog, FLY2. In this study, we show that the FLY2 protein functions in mucilage modification together with FLY1. FLY2 was expressed in seed coat epidermal cells during mucilage synthesis, but its expression level was much lower than that of FLY1. While fly2 showed no obvious difference in mucilage capsule formation from wild type, the fly1 fly2 double mutants showed more severe defects in mucilage than fly1 alone. FLY2-EYFP that was expressed under the control of the FLY1 promoter rescued fly1 mucilage, showing that FLY2 has the same molecular function as FLY1. FLY2-EYFP colocalized with marker proteins of Golgi apparatus (sialyltransferase-mRFP) and late endosome (mRFP-ARA7), indicating that as FLY1, FLY2 controls pectin modification by functioning in these endomembrane organelles. Furthermore, phylogenetic analysis suggests that FLY1 and FLY2 originated from a common ancestral gene by gene duplication prior to the emergence of Brassicaceae. Taken together, our findings suggest that FLY2 functions in the Golgi apparatus and/or the late endosome of seed coat epidermal cells in a manner similar to FLY1.

中文翻译：

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拟南芥飞碟2与FLY1冗余地起作用，以建立正常的种皮粘液。

暴露于水后，成熟的拟南芥种子被称为粘液的胶状胶囊包围。粘液由富含果胶的多糖组成，这些多糖在种皮的表皮细胞中产生。尽管果胶是植物细胞壁的主要成分，但其​​生物合成和生物学功能尚不完全清楚。以前，我们报道了跨膜RING E3泛素连接酶FLYING SAUCER 1（FLY1）调节果胶甲基酯化程度，以形成粘液囊。拟南芥基因组只有一个FLY1同源物FLY2。在这项研究中，我们表明FLY2蛋白与FLY1一起在粘液修饰中起作用。FLY2在粘液合成过程中在种皮表皮细胞中表达，但其表达水平远低于FLY1。尽管fly2与野生型相比粘液囊的形成没有明显差异，但fly1 fly2双突变体的粘液缺陷比仅fly1更严重。在FLY1启动子的控制下表达的FLY2-EYFP拯救了fly1粘液，表明FLY2具有与FLY1相同的分子功能。FLY2-EYFP与高尔基体标记蛋白（唾液酸转移酶-mRFP）和晚期内体（mRFP-ARA7）共定位，表明作为FLY1，FLY2通过在这些内膜细胞器中起作用来控制果胶修饰。此外，系统发育分析表明，在十字花科出现之前，FLY1和FLY2通过基因复制起源于一个共同的祖先基因。在一起