Xyloglucan endo-transglycosylase (XET) functions in gelatinous layers of tension wood fibers in poplar -: A glimpse into the mechanism of the balancing act of trees
Xyloglucan endo-transglycosylase (XET) functions in gelatinous layers of tension wood fibers in poplar -: A glimpse into the mechanism of the balancing act of trees
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DOI：
10.1093/pcp/pcm055
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发表时间：
2007-06-01
影响因子：
4.9
通讯作者：
Mellerowicz, Ewa J.
中科院分区：
文献类型：
--
作者：
Nishikubo, Nobuyuki;Awano, Tatsuya;Mellerowicz, Ewa J.
张力木材是落叶树木的一种特殊组织，它的功能是弯曲木质树干，以优化它们在空间中的位置。与另一侧的纤维相比，在茎的一侧发展的张力木纤维具有更强的收缩能力，从而产生弯矩。人们认为，含有几乎纯纤维素的胶状(G)细胞壁层是拉伸木纤维收缩的关键。通过对杨树分离的G层的糖组成和连接分析，发现它们除了含有纤维素外，还含有一些基质成分，其中木葡聚糖含量最高。通过CCRC-M1单抗标记、XXXG-SR原位掺入标记和PttXET16-34多克隆抗体标记，分别检测到木葡聚糖、木葡聚糖内糖基转移酶(XET)活性和木葡聚糖内糖基转移酶/水解酶(Xth)基因产物，说明木葡聚糖在G层发育过程中已整合到G层中。此外，几个Xth转录本被改变，与正常木材相比，在发育的抗张木材中普遍上调。在成熟的G纤维中，XTH基因产物存在于G层，而XET活性在邻近的S-2壁层中明显存在。我们认为XET活性是G-纤维收缩所必需的，它通过修复G-层和S-2层之间的木葡聚糖交联键，从而维持它们之间的接触。令人惊讶的是，Xth基因产物和XET活性在成熟的G纤维中持续了几年，这表明这种酶在细胞死亡后修复交联链的功能，因为它们在收缩过程中被断开。
Tension wood is a specialized tissue of deciduous trees that functions in bending woody stems to optimize their position in space. Tension wood fibers that develop on one side of the stem have an increased potency to shrink compared with fibers on the opposite side, thus creating a bending moment. It is believed that the gelatinous (G) cell wall layer containing almost pure cellulose of tension wood fibers is pivotal to their shrinking. By analyzing saccharide composition and linkage in isolated G-layers of poplar, we found that they contain some matrix components in addition to cellulose, of which xyloglucan is the most abundant. Xyloglucan, xyloglucan endo-transglycosylase (XET) activity and xyloglucan endo-transglycosylase/hydrolase (XTH) gene products were detected in developing G-layers by labeling using CCRC-M1 monoclonal antibody, in situ incorporation of XXXG-SR and the polyclonal antibody to poplar PttXET16-34, respectively, indicating that xyloglucan is incorporated into the G-layer during its development. Moreover, several XTH transcripts were altered and were generally up-regulated in developing tension wood compared with normal wood. In mature G-fibers, XTH gene products were detected in the G-layers while the XET activity was evident in the adjacent S-2 wall layer. We propose that XET activity is essential for G-fiber shrinking by repairing xyloglucan cross-links between G- and S-2-layers and thus maintaining their contact. Surprisingly, XTH gene products and XET activity persisted in mature G-fibers for several years, suggesting that the enzyme functions after cell death repairing the cross-links as they are being broken during the shrinking process.