Polysaccharide methylation, especially that of pectin, is a common and important feature of land plant cell walls. Polysaccharide methylation takes place in the Golgi apparatus and therefore relies on the import of S-adenosyl methionine (SAM) from the cytosol into the Golgi. However, so far, no Golgi SAM transporter has been identified in plants. Here we studied major facilitator superfamily members in Arabidopsis that we identified as putative Golgi SAM transporters (GoSAMTs). Knockout of the two most highly expressed GoSAMTs led to a strong reduction in Golgi-synthesized polysaccharide methylation. Furthermore, solid-state NMR experiments revealed that reduced methylation changed cell wall polysaccharide conformations, interactions and mobilities. Notably, NMR revealed the existence of pectin ‘egg-box’ structures in intact cell walls and showed that their formation is enhanced by reduced methyl esterification. These changes in wall architecture were linked to substantial growth and developmental phenotypes. In particular, anisotropic growth was strongly impaired in the double mutant. The identification of putative transporters involved in import of SAM into the Golgi lumen in plants provides new insights into the paramount importance of polysaccharide methylation for plant cell wall structure and function.

多糖甲基化，尤其是果胶的甲基化，是陆地植物细胞壁的一个常见且重要的特征。多糖甲基化发生在高尔基体中，因此依赖于S-腺苷甲硫氨酸 (SAM) 从胞质溶胶进入高尔基体。然而，到目前为止，尚未在植物中发现高尔基体 SAM 转运蛋白。在这里，我们研究了拟南芥中的主要促进者超家族成员我们确定为推定的高尔基体 SAM 转运体 (GoSAMTs)。敲除两个最高表达的 GoSAMT 导致高尔基体合成的多糖甲基化强烈减少。此外，固态核磁共振实验表明，甲基化减少会改变细胞壁多糖的构象、相互作用和迁移率。值得注意的是，核磁共振揭示了完整细胞壁中果胶“蛋盒”结构的存在，并表明它们的形成因甲基酯化的减少而增强。墙体结构的这些变化与大量生长和发育表型有关。特别是，双突变体的各向异性生长受到严重损害。