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Possible Roles of Calcium and Calmodulin in the Biosynthesis and Secretion of -Amylase in Rice Seed Scutellar Epithelium ¹

Research Institute for Biochemical Regulation, School of Agriculture, Nagoya University, Chikusa, Nagoya 464, Japan

The scutellar epithelial cells of rice (Oryza sativa L. cv Kimmazé) seeds actively secrete -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 -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 -amylase.

A cation-specific requirement for Ca²⁺ was apparent, since both biosynthesis and extracellular secretion of -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 -amylase. A calmodulin antagonist, W-7, also inhibited the secretion of -amylase at concentrations where the enzyme synthesis was not much affected. Overall data indicate that Ca²⁺ movement and secretion of -amylase are tightly linked and it is likely that they are regulated by the cytoplasmic Ca²⁺ concentration under possible control by calmodulin.

¹ Supported by the research grant from Mombusho (Japanese Ministry of Education, Science and Culture), and the Mitsubishi Foundation (Tokyo). This is paper No. 13 in the series "Enzymic Mechanism of Starch Breakdown in Germinating Rice Seeds" and paper No. 12 is by Miyata and Akazawa (Plant Physiol 1982) (16). This research was performed when J. C. stayed at Nagoya University from November 1981 to September 1982 as a visiting scientist sponsored by the Japan Society for the Promotion of Science (JSPS).

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