Cellular and Molecular Biology Program, State Key Laboratory of Agrobiotechnology, School of Life Sciences, The Chinese University of Hong Kong, New Territories, Hong Kong, China.
Department of Mathematics, The Chinese University of Hong Kong, New Territories, Hong Kong, China.
Plant J. 2017 Nov;92(4):596-610. doi: 10.1111/tpj.13704. Epub 2017 Oct 16.
Root border cells lie on the surface of the root cap and secrete massive amounts of mucilage that contains polysaccharides and proteoglycans. Golgi stacks in the border cells have hypertrophied margins, reflecting elevated biosynthetic activity to produce the polysaccharide components of the mucilage. To investigate the three-dimensional structures and macromolecular compositions of these Golgi stacks, we examined high-pressure frozen/freeze-substituted alfalfa root cap cells with electron microscopy/tomography. Golgi stacks in border cells and peripheral cells, precursor cells of border cells, displayed similar morphological features, such as proliferation of trans cisternae and swelling of the trans cisternae and trans-Golgi network (TGN) compartments. These swollen margins give rise to two types of vesicles larger than other Golgi-associated vesicles. Margins of trans-Golgi cisternae accumulate the LM8 xylogalacturonan (XGA) epitope, and they become darkly stained large vesicles (LVs) after release from the Golgi. Epitopes for xyloglucan (XG), polygalacturonic acid/rhamnogalacturonan-I (PGA/RG-I) are detected in the trans-most cisternae and TGN compartments. LVs produced from TGN compartments (TGN-LVs) stained lighter than LVs and contained the cell wall polysaccharide epitopes seen in the TGN. LVs carrying the XGA epitope fuse with the plasma membrane only in border cells, whereas TGN-LVs containing the XG and PGA/RG-I epitopes fuse with the plasma membrane of both peripheral cells and border cells. Taken together, these results indicate that XGA is secreted by a novel type of secretory vesicles derived from trans-Golgi cisternae. Furthermore, we simulated the collapse in the central domain of the trans-cisternae accompanying polysaccharide synthesis with a mathematical model.
根边缘细胞位于根冠的表面，大量分泌含有多糖和蛋白聚糖的黏液。边缘细胞中的高尔基堆叠具有肥大的边缘，反映了升高的生物合成活性，以产生黏液的多糖成分。为了研究这些高尔基堆叠的三维结构和大分子组成，我们使用电子显微镜/断层扫描检查了高压冷冻/冷冻取代的紫花苜蓿根冠细胞。边缘细胞和外周细胞（边缘细胞的前体细胞）中的高尔基堆叠显示出相似的形态特征，例如内质网和反式内质网和反式高尔基体网络（TGN）区室的肿胀。这些肿胀的边缘导致两种类型的囊泡比其他与高尔基体相关的囊泡大。反式高尔基潴泡边缘积累 LM8 木糖半乳糖醛酸聚糖（XGA）表位，并且在从高尔基体释放后变成深色染色的大囊泡（LVs）。木葡聚糖（XG）、半乳糖醛酸/鼠李半乳糖醛酸-I（PGA/RG-I）的表位在最靠近反式的潴泡和 TGN 区室中被检测到。来自 TGN 区室（TGN-LVs）的 LVs 比 LVs 染色更浅，并且含有在 TGN 中看到的细胞壁多糖表位。仅在边缘细胞中，携带 XGA 表位的 LVs 与质膜融合，而含有 XG 和 PGA/RG-I 表位的 TGN-LVs 与外周细胞和边缘细胞的质膜融合。总之，这些结果表明 XGA 是由源自反式高尔基潴泡的新型分泌囊泡分泌的。此外，我们使用数学模型模拟了伴随多糖合成的反式潴泡中央域的塌陷。
