Effects of phosphate on the Ca2 uptake and the sucrose-controlled secretion of ±-amylase molecules in cultured rice cells were investigated. Phosphate markedly stimulated Ca2, uptake into rice cells, particularly at the outer cell layer of the cell cluster. Phosphate also increased the synthesis and extracellular liberation of a - amylase II-4 in sucrose-supplemented cells. The distribution pattern of enzyme in rice cell clusters induced by phosphate was similar to that of Ca2+ uptake. Phosphate did not increase the level of mRNA of ±-amylase II-4, indicating that phosphate stimulates the translation and posttranslational secretory processes of ±-amylase II-4 in the presence of sucrose. Furthermore, phosphate enhanced both the Ca2r uptake and α-amylase II-4 synthesis in the microsomes. These results strongly suggested that the ratio of phosphate to sugar is important for regulating the Ca2+ uptake, and that phosphate and sugar precisely coordinate the Ca2- mediated synthesis and extracellular liberation of α-amylase II-4. © 2003, Japanese Society for Plant Cell and Molecular Biology. All rights reserved.