Ribonucleic Acid Synthesis in Germinating Castor Bean Endosperm

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Abstract
Total RNA extracted from castor bean endosperm at various stages during germination was separated into 3 fractions: low MW RNA (tRNA and 5S rRNA), rRNA (18S and 28S) and polyadenylated RNA. Although these fractions were readily detected in the endosperm of dry seeds, cellular content increased significantly during germination reaching a peak on the 3rd or 4th day. [14C]Adenine and [3H]uridine were used to label cellular RNA, and the capacity of the tissue to incorporate these precursors established that the maximum rate of synthesis of all major RNA fractions occurred after 2 days growth. When homogenates obtained from [14C]adenine-labeled tissue were fractionated to yield crude nuclear and cytoplasmic preparations, the kinetics of radioactive labeling indicated that newly synthesized RNA appears firstly in the nucleus and subsequently in the cytoplasm. Synthesis of the proteins responsible for the metabolic activity of germinating endosperm tissue apparently depends primarily on transcription during inhibition and germination rather than the activation of preformed RNA species already present in the dry seed.