Tobacco embryogenesis: storage-protein-accumulating cells of embryo, suspensor, and endosperm are able to undergo cytokinesis

It is widely accepted that seed storage proteins accumulate only in cells w hich have entered the cell expansion phase and do not continue to divide. H ere we present data indicating that the accumulation of storage globulins i n tobacco begins already during early embryogenesis in a period of sustaine d mitotic activity. Western blot analysis revealed that polypeptides of the legumin-like 12S globulins (M-r 60000, 40000, 20000) appear at mid/late gl obular stage, whereas the vicilin-like 7S globulin (M-r 50000) follows duri ng the transition from heart to torpedo stage. The accumulation of legumin- like polypeptides begins first in the endosperm during the mid globular sta ge followed in the embryo-suspensor complex during the heart-shaped stage. The vicilin-related fraction appears first in the endosperm and three days later in the embryo. Examination of individual cells from squash preparatio ns revealed that protein bodies are not confined to intermitotic cells, but are also present in cells undergoing mitosis. Protein bodies of dividing c ells situated outside the mitotic apparatus are not metabolized during cyto kinesis. The only cell type which loses its protein bodies completely prior to the first mitotic division is the primary hypophysis cell. Our finding that storage proteins can occur in dividing cells independent of their orig in and developmental capacity indicates that the cell expansion hypothesis of storage protein accumulation has to be revised.