Proteins of Axial Organs of Dormant and Germinating Horse Chestnut Seeds: 2. Protein-Synthesizing Capacity of Embryo Axes

Abstract

This is the first characterization of the protein-synthesizing capacity of axial organs of recalcitrant horse chestnut seeds, which were subjected to cold wet stratification, during deep dormancy, dormancy release, and germination. The embryo axis length, fresh weight, and protein content remained almost unchanged during the entire period of stratification but increased after radicle protrusion. Isolated embryo axes were capable of radioactive amino acid uptake and incorporation, starting from the first hours of their incubation at 28°C. The capacity for protein synthesis was easily detected since the first days of stratification; its rate did not change until germination and increased after radicle protrusion. Cycloheximide suppressed protein synthesis by 95%; during stratification, protein synthesis was insensitive to α-amanitin (7 μg/ml) but was inhibited by 40% during germination. This indicates the presence of preformed translatable mRNAs in stratifying recalcitrant seeds, whereas in germinating seeds protein synthesis depends on transcription to a greater extent. Proteins synthesized by the axes were located in various subcellular compartments but mostly in the cytosol; among cytosolic proteins, thermolabile proteins dominated. The number and set of synthesized polypeptides remained almost unchanged during stratification. Most pronounced changes in protein composition occurred during germination and concerned mainly minor components of cytosolic heat-stable proteins. It seems evident that, under dormancy and during dormancy release, a constant set of genes was expressed due to preformed mRNA activity. Noticeable changes in gene expression occurred at the level of protein synthesis only during seed germination. Thus, the competent translational machinery is present in axis cells of dormant mature recalcitrant seeds capable of performing protein synthesis under favorable conditions. It seems probable that inability of these seeds to germinate is due to the absence of special gene expression, but not to disfunctioning of protein-synthesizing machinery.