Sugar-Controlled Ca2+ Uptake and -Amylase Secretion in Cultured Cells of Rice (Oryza sativa L.)

Abstract

Sugar starvation-induced synthesis and extracellular liberation of -amylase molecules in suspension-cultured cells of rice (Oryza sativa L.) required Ca 2+ , although the level of translatable a-amylase mRNA was not affected in the presence of Ca 2+ . Sugar depletion markedly stimulated Ca 2+ uptake by rice cells and sucrose supplementation reduced it. Immunohistocheinical and electron probe microanalyzer studies indicated an apparent resemblance between the distribution pattern of Ca 2+ and that of a-amylase molecules induced in the sugar-depleted cells. Ca 2+ uptake was reduced by sucrose, maltose, fructose, and glucose similarly at more than 5 mM, but was unaffected by mannitol (88 mM), 6-deoxy-D-glucose (10 mM), and 3-0methyl-D-glucose (10 mM). Furthermore, an effective Ca 2+ channel blocker, La 3+ significantly inhibited the Ca 2+ uptake and the synthesis and extracellular liberation of aamylase molecules in the absence of sucrose, while a general P-type ATPase inhibitor, vanadate greatly stimulated both in the presence of sucrose. We concluded that, by controlling the Ca 2+ uptake, metabolic sugars regulate the protein synthesis and posttranslational secretory processes of a-amylase molecules in rice cells.