Post-transcriptional regulation of storage protein synthesis during early embryogenesis

Messenger RNA synthesis during somatic embryogenesis of alfalfa is controlled at the post-transcriptional level during the globular to torpedo stages of development, and transcriptionally during the cotyledonary stages. Storage protein messages are sequestered in messenger ribonucleoprotein (mRNP) complexes during the early stages of embryogenesis, in association with specific binding proteins. Reconstitution of the messenger-protein complex can be achieved in vitro using medicagin (11S) mRNA; cell-free extracts from pre-cotyledonary embryos contain different binding proteins from those at the cotyledonary stages. Those associated with mRNA at the early stages may prevent translation, whereas those at the late stages may enhance large polysome formation. The mRNA-bound proteins from pre-cotyledonary-stage embryos exhibit 50-fold more protein kinase activity than those from cotyledonary embryos, and several proteins in the mRNP fraction can be phosphorylated. While the physiological function of the phosphorylated proteins is unknown, it is possible that selective phosphorylation and dephosphorylation of proteins determines recruitment of individual mRNAs into polysomes, thus regulating translation during embryogenesis.