Dpto. Biología Funcional, Facultad de Biología, Universidade de Santiago, 15782, Santiago de Compostela, Spain.
Plant Mol Biol. 2019 Jul;100(4-5):495-509. doi: 10.1007/s11103-019-00875-1. Epub 2019 Apr 26.
Brachypodium distachyon has a full set of exoglycosidases active on xyloglucan, including α-xylosidase, β-galactosidase, soluble and membrane-bound β-glucosidases and two α-fucosidases. However, unlike in Arabidopsis, both fucosidases are likely cytosolic. Xyloglucan is present in primary walls of all angiosperms. While in most groups it regulates cell wall extension, in Poaceae its role is still unclear. Five exoglycosidases participate in xyloglucan hydrolysis in Arabidopsis: α-xylosidase, β-galactosidase, α-fucosidase, soluble β-glucosidase and GPI-anchored β-glucosidase. Mutants in the corresponding genes show alterations in xyloglucan composition. In this work putative orthologs in the model grass Brachypodium distachyon were tested for their ability to complement Arabidopsis mutants. Xylosidase and galactosidase mutants were complemented, respectively, by BdXYL1 (Bd2g02070) and BdBGAL1 (Bd2g56607). BdBGAL1, unlike other xyloglucan β-galactosidases, is able to remove both galactoses from XLLG oligosaccharides. In addition, soluble β-glucosidase BdBGLC1 (Bd1g08550) complemented a glucosidase mutant. Closely related BdBGLC2 (Bd2g51280), which has a putative GPI-anchor sequence, was found associated with the plasma membrane and only a truncated version without GPI-anchor complemented the mutant, proving that Brachypodium also has soluble and membrane-bound xyloglucan glucosidases. Both BdXFUC1 (Bd3g25226) and BdXFUC2 (Bd1g28366) can hydrolyze fucose from xyloglucan oligosaccharides but were unable to complement a fucosidase mutant. Fluorescent protein fusions of BdXFUC1 localized to the cytosol and both proteins lack a signal peptide. Signal peptides appear to have evolved only in some eudicot lineages of this family, like the one leading to Arabidopsis. These results could be explained if cytosolic xyloglucan α-fucosidases are the ancestral state in angiosperms, with fucosylated oligosaccharides transported across the plasma membrane.
柳枝稷具有一套完整的木葡聚糖外切糖苷酶，包括α-木糖苷酶、β-半乳糖苷酶、可溶性和膜结合型β-葡萄糖苷酶以及两种α-岩藻糖苷酶。然而，与拟南芥不同的是，这两种岩藻糖苷酶可能都存在于细胞质中。木葡聚糖存在于所有被子植物的初生壁中。虽然在大多数植物中它调节细胞壁的延伸，但在禾本科植物中，其作用仍不清楚。五种外切糖苷酶参与拟南芥中的木葡聚糖水解：α-木糖苷酶、β-半乳糖苷酶、α-岩藻糖苷酶、可溶性β-葡萄糖苷酶和 GPI 锚定的β-葡萄糖苷酶。相应基因的突变体显示木葡聚糖组成的改变。在这项工作中，模式禾本科植物柳枝稷的假定同源物被测试其在拟南芥突变体中的互补能力。木糖苷酶和半乳糖苷酶突变体分别由 BdXYL1（Bd2g02070）和 BdBGAL1（Bd2g56607）互补。与其他木葡聚糖β-半乳糖苷酶不同的是，BdBGAL1 能够从 XLLG 寡糖中去除两个半乳糖。此外，可溶性β-葡萄糖苷酶 BdBGLC1（Bd1g08550）互补了一个葡萄糖苷酶突变体。与之密切相关的 BdBGLC2（Bd2g51280）具有一个假定的 GPI-锚序列，被发现与质膜相关，只有没有 GPI-锚的截断版本才能互补突变体，证明柳枝稷也有可溶性和膜结合型木葡聚糖葡萄糖苷酶。BdXFUC1（Bd3g25226）和 BdXFUC2（Bd1g28366）都可以从木葡聚糖寡糖中水解岩藻糖，但不能互补岩藻糖苷酶突变体。BdXFUC1 的荧光蛋白融合定位在细胞质中，并且这两种蛋白都缺乏信号肽。信号肽似乎只在这个家族的一些被子植物的进化支中进化，如拟南芥。如果细胞质木葡聚糖α-岩藻糖苷酶是被子植物的原始状态，带有岩藻糖基的寡糖穿过质膜运输，那么这些结果是可以解释的。
