Posttranscriptional regulation of α-amylase II-4 expression by gibberellin in germinating rice seeds

Hormonal regulation of expression of α-amylase II-4 that lacks the gibberellin-response cis-element (GARE) in the promoter region of the gene was studied in germinating rice (Oryza sativa L.) seeds. Temporal and spatial expression of α-amylase II-4 in the aleurone layer were essentially identical to those of α-amylase I-1 whose gene contains GARE, although these were distinguishable in the embryo tissues at the early stage of germination. The gibberellin-responsible expression of α-amylase II-4 was also similar to that of α-amylase I-1. However, the level of α-amylase II-4 mRNA was not increased by gibberellin, indicating that the transcriptional enhancement of α-amylase II-4 expression did not occur in the aleurone. Gibberellin stimulated the accumulation of 45Ca2+ into the intracellular secretory membrane system. In addition, several inhibitors for Ca2+ signaling, such as EGTA, neomycin, ruthenium red (RuR), and W-7 prevented the gibberellin-induced expression of α-amylase II-4 effectively. While the gibberellin-induced expression of α-amylase II-4 occurred normally in the aleurone layer of a rice dwarf mutant d1 which is defective in the α subunit of the heterotrimeric G protein. Based on these results, it was concluded that the posttranscriptional regulation of α-amylase II-4 expression by gibberellin operates in the aleurone layer of germinating rice seed, which is mediated by Ca2+ but not the G protein.