Possible Roles of Calcium and Calmodulin in the Biosynthesis and Secretion of α-Amylase in Rice Seed Scutellar Epithelium

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Abstract
The scutellar epithelial cells of rice (Oryza sativa L. cv Kimmazé) seeds actively secrete α-amylase in an early stage of germination. Employing an in vivo system of freshly dissected scutellar tissues, effect of Ca2+ on the biosynthesis and the secretion of α-amylase have been studied. The maximum biosynthetic rate was saturated at about 0.5 mm external Ca2+ concentrations, whereas the secretion continued to increase to concentrations above 10 mm Ca2+. In the presence of 1 mm Ca2+, 0.01 μm A-23187 significantly increased both the biosynthesis and the secretion of α-amylase. A cation-specific requirement for Ca2+ was apparent, since both biosynthesis and extracellular secretion of α-amylase were inhibited by 0.1 mm EGTA but were increased above basal rate only with Ca2+ and Sr2+; K+, Mg2+, and Ba2+ being ineffective. La3+ and ruthenium red (selective inhibitors of [Ca2+ + Mg2+]-ATPase) were found to profoundly inhibit the secretion of α-amylase. A calmodulin antagonist, W-7, also inhibited the secretion of α-amylase at concentrations where the enzyme synthesis was not much affected. Overall data indicate that Ca2+ movement and secretion of α-amylase are tightly linked and it is likely that they are regulated by the cytoplasmic Ca2+ concentration under possible control by calmodulin.
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