Department of Plant Biology, University of Georgia, Athens, GA 30602, USA.
Plant Cell Physiol. 2013 Jul;54(7):1186-99. doi: 10.1093/pcp/pct070. Epub 2013 May 9.
Xylan, a major polysaccharide in plant lignocellulosic biomass, is acetylated at O-2 and/or O-3 and its acetylation impedes the use of biomass for biofuel production. Currently, it is not known what genes encode acetyltransferases that are responsible for xylan O-acetylation. In this report, we demonstrate an essential role for the Arabidopsis gene ESKIMO1 (ESK1) in xylan O-acetylation during secondary wall biosynthesis. ESK1 expression was found to be regulated by the secondary wall master regulator SND1 (secondary wall-associated NAC domain protein1) and specifically associated with secondary wall biosynthesis. Its encoded protein was localized in the Golgi, the site of xylan biosynthesis. The esk1 mutation caused reductions in secondary wall thickening and stem mechanical strength. Chemical analyses of cell walls revealed that although the esk1 mutation did not cause apparent alterations in the xylan chain length and the abundance of the reducing end sequence, it resulted in a significant reduction in the degree of xylan acetylation. The reduced acetylation of esk1 xylan rendered it more accessible and digestible by endoxylanase, leading to generation of shorter xylooligomers compared with the wild type. Further structural analysis of xylan showed that the esk1 mutation caused a specific reduction in 2-O- and 3-O-monoacetylation of xylosyl residues but not in 2,3-di-O-acetylation or 3-O-acetylation of xylosyl residues substituted at O-2 with glucuronic acid. Consistent with ESK1's involvement in xylan O-acetylation, an activity assay revealed that the esk1 mutation led to a significant decrease in xylan acetyltransferase activity. Together, these results demonstrate that ESK1 is a putative xylan acetyltransferase required for 2-O- and 3-O-monoacetylation of xylosyl residues and indicate the complexity of the biochemical mechanism underlying xylan O-acetylation.
木聚糖是植物木质纤维素生物质中的一种主要多糖，在 O-2 和/或 O-3 位被乙酰化，其乙酰化阻碍了生物质用于生物燃料生产。目前尚不清楚哪些基因编码负责木聚糖 O-乙酰化的乙酰转移酶。在本报告中，我们证明了拟南芥基因 ESKIMO1（ESK1）在次生壁生物合成过程中木聚糖 O-乙酰化中的重要作用。发现 ESK1 表达受次生壁主调控因子 SND1（次生壁相关 NAC 结构域蛋白 1）调控，并与次生壁生物合成特异性相关。其编码的蛋白质定位于高尔基体内，即木聚糖生物合成的部位。esk1 突变导致次生壁加厚和茎机械强度降低。细胞壁的化学分析表明，尽管 esk1 突变并没有导致木聚糖链长和还原末端序列丰度的明显改变，但它导致木聚糖乙酰化程度显著降低。esk1 木聚糖的低乙酰化使其更容易被内切木聚糖酶消化，导致与野生型相比生成更短的木二糖低聚物。进一步的木聚糖结构分析表明，esk1 突变导致木糖基残基的 2-O-和 3-O-单乙酰化特异性降低，但不影响 2,3-二-O-乙酰化或被葡萄糖醛酸取代的 O-2 位的 3-O-乙酰化的木糖基残基。与 ESK1 参与木聚糖 O-乙酰化一致，活性测定显示 esk1 突变导致木聚糖乙酰转移酶活性显著降低。综上所述，这些结果表明 ESK1 是一种假定的木聚糖乙酰转移酶，需要进行 2-O-和 3-O-单乙酰化的木糖基残基，并表明木聚糖 O-乙酰化的生化机制的复杂性。
