Department of Plant Biology, University of Georgia, Athens, GA 30602, USA.
Department of Chemistry, University of Georgia, Athens, GA 30602, USA.
Plant Cell Physiol. 2024 Jun 27;65(6):1065-1079. doi: 10.1093/pcp/pcae028.
Grass xylan consists of a linear chain of β-1,4-linked xylosyl residues that often form domains substituted only with either arabinofuranose (Araf) or glucuronic acid (GlcA)/methylglucuronic acid (MeGlcA) residues, and it lacks the unique reducing end tetrasaccharide sequence found in dicot xylan. The mechanism of how grass xylan backbone elongation is initiated and how its distinctive substitution pattern is determined remains elusive. Here, we performed biochemical characterization of rice xylan biosynthetic enzymes, including xylan synthases, glucuronyltransferases and methyltransferases. Activity assays of rice xylan synthases demonstrated that they required short xylooligomers as acceptors for their activities. While rice xylan glucuronyltransferases effectively glucuronidated unsubstituted xylohexaose acceptors, they transferred little GlcA residues onto (Araf)-substituted xylohexaoses and rice xylan 3-O-arabinosyltransferase could not arabinosylate GlcA-substituted xylohexaoses, indicating that their intrinsic biochemical properties may contribute to the distinctive substitution patterns of rice xylan. In addition, we found that rice xylan methyltransferase exhibited a low substrate binding affinity, which may explain the partial GlcA methylation in rice xylan. Furthermore, immunolocalization of xylan in xylem cells of both rice and Arabidopsis showed that it was deposited together with cellulose in secondary walls without forming xylan-rich nanodomains. Together, our findings provide new insights into the biochemical mechanisms underlying xylan backbone elongation and substitutions in grass species.
草本木聚糖由β-1,4 连接的木糖基残基组成的线性链组成，这些残基通常仅形成被阿拉伯呋喃糖基（Araf）或葡萄糖醛酸（GlcA）/甲基葡萄糖醛酸（MeGlcA）残基取代的结构域，并且缺乏在双子叶植物木聚糖中发现的独特还原端四糖序列。草本木聚糖主链延伸的启动机制以及其独特的取代模式是如何确定的，这一机制仍难以捉摸。在这里，我们对包括木聚糖合酶、糖基转移酶和甲基转移酶在内的水稻木聚糖生物合成酶进行了生化特性分析。水稻木聚糖合酶的活性测定表明，它们需要短的木寡糖作为其活性的受体。虽然水稻木聚糖糖基转移酶有效地将葡萄糖醛酸基转移到未取代的木六糖受体上，但它们很少将 GlcA 残基转移到（Araf）取代的木六糖上，并且水稻木聚糖 3-O-阿拉伯糖基转移酶不能将阿拉伯糖基化 GlcA 取代的木六糖，这表明它们的内在生化特性可能有助于水稻木聚糖的独特取代模式。此外，我们发现水稻木聚糖甲基转移酶表现出低的底物结合亲和力，这可能解释了水稻木聚糖中部分 GlcA 的甲基化。此外，对水稻和拟南芥木质部细胞中木聚糖的免疫定位显示，它与纤维素一起沉积在次生壁中，而不会形成富含木聚糖的纳米结构域。总之，我们的发现为禾本科植物木聚糖主链延伸和取代的生化机制提供了新的见解。
