Characterization of Xyloglucan Galactosyltransferase Homologs in Arabidopsis

高等植物的细胞外基质(或“细胞壁”)在植物的整个生命周期中起着许多保护和生长调节的作用。此外，植物细胞壁材料是地球上最丰富的新合成生物质来源，使其成为重要的可再生能源，并成为从纺织生产到木材加工等工业应用的原材料。尽管从基础和应用的角度来看，植物细胞壁都很重要，但在分子水平上对细胞壁材料的合成知之甚少。植物细胞壁主要由多糖组分组成，这些多糖组分由称为糖基转移酶的酶相互连接。植物细胞壁研究中最重要和最具挑战性的领域之一是鉴定编码这些糖基转移酶的基因，并确定它们的功能。为了通过遗传方法解决这个问题，我们分离了植物模式生物拟南芥的突变体，这些突变体的细胞壁材料的组成发生了变化。其中一个突变体被证明无法合成正常的木葡聚糖(大多数高等植物中的主要半纤维素)，因为特定的糖基转移酶存在缺陷。对拟南芥基因组序列的评估表明，存在与该酶结构基本相似的10个编码区。这10个基因预计将编码细胞壁合成中的额外糖基转移酶。在我们未来的研究中，将通过三个互补的方法来研究大多数这些酶的功能和性质：(A)确定这些预测的糖基转移酶驻留在植物的哪些部分。(B)鉴定这些酶活性有缺陷的突变系。随后将对它们的细胞壁成分进行分析，并对其生长习性、生理和壁属性的变化进行评估。(C)在微生物中生产预测的糖基转移酶，然后进行酶分析以确定其功能。拟议的实验旨在加深我们对高等植物细胞壁合成的了解，并有可能操纵重要农作物的细胞壁组成，以提高饲料作物的消化率，促进纸制品和服装纤维的生产，并改善人体营养中细胞壁成分的特性。

The extracellular matrix (or "cell wall") of higher plants plays numerous protective and growth-regulating roles during the life cycle of a plant. Furthermore, plant cell wall material represents the most abundant source of newly synthesized biomass on earth making it an important renewable source of energy, and the raw material for industrial applications ranging from textile production to wood processing. Despite this importance of plant cell walls both from a basic and applied point of view, little is known about the synthesis of cell wall material on the molecular level. Plant cell walls consist primarily of polysaccharides representing long strings of sugar moieties that are attached to each other by enzymes referred to as glycosyltransferases. One of the most important and challenging areas in plant cell wall research is the identification of genes encoding these glycosyltransferases, and the determination of their function. To address this issue by a genetic approach, mutants of the plant model organism Arabidopsis thaliana were isolated which showed alterations in the composition of their cell wall material. One of these mutants turned out to be unable to synthesize normal xyloglucan (the major hemicellulose in most higher plants) because of a defect in a specific glycosyltransferase. An evaluation of the Arabidopsis genome sequence indicated the presence of ten coding regions with substantial structural similarity to this enzyme. These ten genes are expected to encode additional glycosyltransferases in cell wall synthesis. In our future research, the functions and properties of most of these enzymes will be investigated via three complementary approaches: (A) Determination in which parts of the plant these predicted glycosyl-transferases reside. (B) The identification of mutant lines which are defective in the activity of these enzymes. This will be followed by an analysis of their cell wall composition and an evaluation for changes in growth habit, physiology, and wall properties. (C) Production of the predicted glycosyltransferases in microorganisms followed by enzyme assays to determine their functions. The proposed experiments are designed to further our understanding of cell wall synthesis in higher plants with the potential to manipulate the cell wall composition of important crop plants to improve digestibility of forage crops, facilitate the production of fibers for paper products and clothing, and to improve the properties of cell wall components in human nutrition.