Seed Storage Protein Transcription and mRNA Levels in Brassica napus during Development and in Response to Exogenous Abscisic Acid.

Transcription rates and mRNA levels for Brassica napus seed storage protein families, cruciferin and napin, have been determined in embryos developing in the seed, as well as in embryos cultured with and without abscisic acid. Cruciferin and napin mRNAs are high during the cell expansion phase of embryo development, representing as much as 11 and 8%, respectively, of the total embryo mRNA. During the same time cruciferin and napin gene transcription rates, as measured in isolated nuclei, are also high. The data indicate that cruciferin mRNA is more stable than napin mRNA because while the napin transcription rate is higher than the cruciferin transcription rate, the cruciferin mRNA accumulates to higher levels. However, late in embryo development, both cruciferin and napin mRNAs seem to be less stable than earlier because comparable transcription rates result in lower mRNA levels. When embryos are cultured in the presence of abscisic acid, the levels of cruciferin and napin mRNAs are two- to threefold higher than in embryos cultured on basal medium. The transcription rates show a similar increase in the presence of abscisic acid, suggesting that abscisic acid is responsible for the increased mRNA level at least in part through an increase in the transcription rate of the two genes.