Synthesis and Breakdown of Plant Cell Wall Polysaccharides by a Two-Step Process

阐明植物细胞壁多糖的合成和分解过程，是了解植物生长调控机制的重要途径。根据初步结果，我们推测植物细胞壁基质多糖是通过中等大小的中间产物经过两步合成和降解的。在本研究中，我们分析了两种主要基质多糖木糖葡聚糖和（1 * 3），(1 * 4)- β -葡聚糖的代谢，得到了支持这一假设的数据。对木葡聚糖和（1 * 3）、(1 * 4)- β -葡聚糖与粗细胞壁酶反应产物的HPLC和GLG分析表明，在一定条件下可制得中等大小的多糖。木葡聚糖中间体多糖的大小约为50 kDa，（1 * 3）、(1 * 4)- β -葡聚糖中间体多糖的大小均大于100 kDa。从红豆上胚轴细胞壁中纯化出了木葡聚糖水解酶，并对其进行了鉴定。在分子结构上发现它们属于内切木葡聚糖转移酶（EXT）家族。然而，纯化的一种酶仅催化水解，而另一种酶仅在有限条件下显示出内转移酶活性。我们还从红豆上胚轴细胞壁中纯化出一种能够触发木葡聚糖第二步分解的α -聚焦酶。从水稻胚芽鞘细胞壁中纯化出（1 * 3）、(1 * 4)- β -葡聚糖酶。其中一种酶显示出内切葡聚糖酶活性，似乎参与了葡聚糖分解的第一步。分解的第二步可由外聚糖酶进行。重力和光照均抑制木葡聚糖水解酶和（1 * 3）、(1 * 4)-葡聚糖酶的活性。这种酶活性的降低可能导致这些多糖的分子大小增加，导致壁伸长性降低，从而抑制植物生长。少

Processes of the synthesis and breakdown of plant cell wall polysaccharides should be clarified in order to understand the mechanism of regulation of plant growth. From preliminary results, we hypothesized that matrix polysaccharides of plant cell wall are synthesized and degraded by a two-step process via intermediate products with a medial size. In the present study we analyzed the metabolism of two major matrix polysaccharides, xyloglucans and (1 * 3), (1 * 4) -beta-glucans, and obtained the data supporting this hypothesis.1. From the analysis with HPLC and GLG of products by a reaction of xyloglucans and (1 * 3), (1 * 4)-beta-glucans with crude cell wall enzymes, it was shown that the polysaceharides with an intermediate size were produced under certain conditions. The size of the intermediate polysaccharides was about 50 kDa for xyloglucans and was higher than 100 kDa for (1 * 3), (1 * 4)-beta-glucans.2. Xyloglucan hydrolases responsible for the first step of xyloglucan breakdown … More were purified from the cell wall of azuki bean epicotyls and characterized. They were found to belong to endo xyloglucan-transferase (EXT) family in molecular structure. However, one enzyme purified catalyzed only hydrolysis and the other showed the endo-transferase activity only under limited conditions. An alpha-fucosidase capable of triggering the second step of xyloglucan breakdown was also purified from the cell wall of azuki bean epicotyls.3. Two types of (1 * 3), (1 * 4)-beta-glucanases were purified from the cell wall of rice coleoptiles. One enzyme showed an endoglucanase activity and appears to be involved in the first step of the breakdown of the glucans. The second step of the breakdown may be performed by an exoglucanase.4. Both gravity and light suppressed activities of xyloglucan hydrolases and (1 * 3), (1 * 4)-beta-glucanases. Such a decrease in enzyme activities may bring about the increase in molecular size of these polvsaccharides, leading to the decrease in the wall extensibility, thereby suppressing plant growth. Less