Triterpenoid saponins are specialized metabolites, which are abundant in soybean seeds. They have a wide variety of effects on human health and physiology. The composition of sugar chain attached to the aglycone moiety of saponins can be controlled by genetic loci, such as Sg-1, 3, and 4. Among these, the homozygous recessive sg-4 impairs the accumulation of saponins that have an arabinose moiety at the second position of the C-3 sugar chain (i.e., saponins Ad and βa) in the hypocotyls. In this study, we found that sg-4 cultivars are disabled in Glyma.01G046300 expression in hypocotyls. This gene encodes a putative glycosyltransferase (UGT73P10) and is a homolog of GmSGT2 (UGT73P2) whose recombinant protein has been previously shown, in vitro, to conjugate the second galactose moiety at the C-3 position of soyasapogenol B monoglucuronide (SBMG). The sg-4 phenotype (absence of saponins Ad and βa in hypocotyls) was restored by introducing the Glyma.01G046300 genomic DNA fragment that was obtained from the Sg-4 cultivar 'Ibarakimame 7'. Although Glyma.01G046300 is expressed in the cotyledons even in the sg-4 cultivars such as 'Enrei', the induced premature stop codon mutation (W244*) resulted in impaired accumulation of saponin βa in this tissue also in the 'Enrei' genetic background. Furthermore, the recombinant Glyma.01G046300 protein was shown to conjugate the second Ara moiety at the C-3 position of SBMG using UDP-Ara as a sugar donor. These results demonstrate that Sg-4 is responsible for conjugation of the second Ara moiety at the C-3 position of soybean saponins.

中文翻译：

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参与大豆皂苷C-3位糖链结构形成的Sg-4糖基转移酶的遗传和功能表征

三萜皂苷是一种特殊的代谢物，在大豆种子中含量丰富。它们对人体健康和生理有多种影响。连接到皂苷苷元部分的糖链的组成可以由基因位点控制，如 Sg-1、3 和 4。其中，纯合隐性 sg-4 损害具有阿拉伯糖部分的皂苷的积累。下胚轴中 C-3 糖链的第二个位置（即皂苷 Ad 和 βa）。在本研究中，我们发现 sg-4 栽培品种在 Glyma.01G046300 在下胚轴中的表达被禁用。该基因编码推定的糖基转移酶 (UGT73P10) 并且是 GmSGT2 (UGT73P2) 的同源物，其重组蛋白先前已在体外显示，可在大豆皂醇 B 单葡糖苷酸 (SBMG) 的 C-3 位置缀合第二个半乳糖部分。通过引入从 Sg-4 栽培品种“Ibarakimame 7”获得的 Glyma.01G046300 基因组 DNA 片段，恢复了 sg-4 表型（下胚轴中没有皂苷 Ad 和 βa）。尽管 Glyma.01G046300 即使在 sg-4 栽培品种（如“Enrei”）中也表达在子叶中，但诱导的过早终止密码子突变（W244*）导致该组织中皂苷 βa 的积累受损，同样在“Enrei”遗传背景中. 此外，重组 Glyma.01G046300 蛋白显示使用 UDP-Ara 作为糖供体在 SBMG 的 C-3 位置缀合第二个 Ara 部分。这些结果表明，Sg-4 负责在大豆皂苷的 C-3 位置结合第二个 Ara 部分。