玉米β-澱粉酶不參與種子萌芽後之胚乳澱粉的降解作用

β-Amylase is not Involved in Degradation of Endosperm Starch During Seed Germination of Maize

大麥等小麥族的植物，於種子發育階段會在胚乳中合成並累積大量的β-澱粉酶蛋白質分子，藉由種子萌芽後的活化作用才表現β-澱粉酶的酵素活性；因此，β-澱粉酶被視為萌發種子中分解澱粉的重要酵素之一。玉米穀粒之β-澱粉酶和大麥有不同的表現模式。利用免疫組織定位法，我們發現在發育中的玉米穀粒，只有果皮組織含有β-澱粉酶分子，胚乳則無；而在萌芽的玉米穀粒中，盾片的上皮細胞是不會合成β-澱粉酶的，只有糊粉層細胞能夠合成β-澱粉酶。不同於α-澱粉酶酵素分子在合成後會從糊粉層細胞分泌到胚乳，新合成的玉米β-澱粉酶分子則被保留在糊粉層細胞內。由PAS和碘染的分析結果顯示，只有粉質胚乳組織含有澱粉粒，而糊粉層細胞則沒有澱粉的存在。由澱粉薄層和電泳活性分析結果推論，α-澱粉酶是分解玉米胚乳澱粉的重要酵素；而β-澱粉酶雖在萌芽的糊粉層中合成，但自始它就留在糊粉層細胞內，不可能參與澱粉的分解作用。

For species of Triticeae, e.g., barley, β-amylase was synthesized in endosperm during seed development and processed to active form during seed germination. Thus, β-amylase was suggested to be one of the enzymes to degrade endosperm starch for seed germination. β-Amylase expression in maize kernels apparently is different from that of barley. By immunolocalization, we found that maize β-amylase is expressed in the pericarp but not in the endosperm during seed development; and is synthesized in the aleurone cells but not in the scutellar epithelium cells during seed germination. The newly synthesized maize β-amylase is retained inside of the aleurone cells. On the contrary, maize α-amylase is synthesized and secreted from the aleurone cells to the starchy endosperm. By PAS and iodine stain, we showed that starch is present in the starchy endosperm and absent in the aleurone cells of maize seeds. Results of starch film assay and native activity gel analysis indicated that α-amylases are the enzymes for endosperm starch degradation during maize seed germination. Because maize β-amylase is synthesized and retained in the aleurone cells, it is unlikely that β-amylase acts on endosperm starch during seed germination.