植物細胞壁具有調節植物細胞體積，與維持植物細胞形狀兩個主要功能，在植物生理上扮演十分重要的角色。然而，植物細胞需要增長，一定需要某種機制能使初生細胞壁鬆弛與膨大，Shieh與Cosgrove於1992年觀察到一種大小約為28 kD與29 kD蛋白質，可使黃瓜的下胚軸細胞壁延展，他們認定此蛋白質主要功能為使植物細胞擴張生長(diffused growth)，而後將其命名為Expansin。研究顯示Expansin蛋白大小約26 kD，主要分為Expansin A與Expansin B兩個家族。
　　Expansin蛋白並不具有水解纖維素等聚糖長鏈的能力，Expansin A證實具有打斷纖維素與聚糖長鏈分子間氫鍵的能力，更進一步，利用細菌合成之類似簡單細胞壁聚合物，測試Expansin蛋白活性，觀察到Expansin蛋白可以使兩條由氫鍵接合之重疊聚糖長鏈變長，因此使由聚糖長鏈結合之纖維素間距變大，推斷可藉此控制使細胞壁鬆弛，讓各種細胞壁增長，進而使細胞壁膨大。我們已討論在有氧與無氧以及與不同的荷爾蒙作用稻米芽鞘之生長狀態下，利用西方轉漬法偵測鞘葉裡Expansin B蛋白表現是與細胞拉長之間的關係，發現無氧下生長的芽鞘生長速率較快，約為0.05cm/h，但Expansin B並沒有被偵測到。有氧下生長則較慢，約為0.015cm/h，但Expansin B卻大量表現。在有氧生長時加入荷爾蒙，Expansin B的整體表現比不加入時少，同時也發現有氧生長時芽鞘厚於無氧生長，分別為每日0.88 mm及0.72 mm，這些結果顯示Expansin B與細胞生長之間並無前人描述Expansin的功能，它的其他生理功能因而有待釐清。

　　Plant cell wall is an important structure that determines cell shape, glues cells together, and provides essential mechanical strength and rigidity. During growth, plant cells secret a protein called expansin, which unlocks the network of wall polysaccharides, permitting turgor-pressure cell enlargement. The protein with molecular weight about 26 kD isolated first from young cucumber hypocotyls and subsequently from other plant tissue showed being capable of inducing pH-dependent wall extension and stress relaxation. The proteins were divided into two groups, Expansin A and Expansin B. Expansin was incapable of having xyloglucan endotransglycosylase activity, but showed the ability to weaken the mechanical strength of cellulose paper without hydrolyzing cellulose through the disruption of hydrogen bonds between glycans. This was believed is the mechanism leading to the growth of cell walls.

　　From the rice coleoptiles grown under hypoxia condition we had obtained the Expansin B gene, from which its cDNA was obtained. With this cDNA as a probe, an Expansin-like B gene, was detected in rice coleoptiles grown under anaerobic condition. This was the only gene detected in the family of Expansin. The protein of Expansin-like B was obtained with E. coli as the host, which was further used to generate its antibody from rabbit. With this antibody, the relationship between the growth of coleoptiles cultivated in aerobic and anaerobic conditions with or without the presence of growth hormone and the expression of Expansin-like B were studied. It was found that in anaerobic the growth rate of coleoptiles was about 0.05 cm/h, however, Expansin was undetectable. The growth rate in aerobic was much slower, 0.015 cm/h, but Expansin was detected. With the presence of growth hormone in aerobic, the expression of Expansin was less than that without the presence of hormone. We also found that the enlargement of the stem of coleoptiles was higher in aerobic than in anaerobic with 0.88 mm and 0.72 mm per day, respectively. These results suggest that Expansin B may not involve in the growth of coleoptiles, especially in anaerobic. The biological function of Expansin B in the growth of coleoptiles are under investigation.

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