The dynamics of seedling and cotyledon cell development in Arabidopsis thaliana during reserve mobilization

We have analyzed seedling and cotyledon cell development in Arabidopsis thaliana during reserve mobilization qualitatively and quantitatively and related development to the time scale ''hours after imhibition'' (HAI). Cell division and elongation occur simultaneously and are observable 18 HAI just behind the radicle apex where a zone of cell elongation is formed prior to radicle emergence. Structurally and biochemically, the transition from a storage to a photosynthetic tissue occurs 48-60 HAI, evidenced by major changes in organelle sizes and numbers; symplast and apoplast volume increases throughout germination, and plastid number and size increase substantially, with a simultaneous decrease in lipid. Vacuome organization changes drastically as it is converted from a protein store to a central, sap-filled vacuolar system; protein bodies initially swell and then fuse during hydrolysis forming a single large structure. The constant cell number in the developing cotyledons demonstrates the relative independence of the organelles in a population of nondividing cells. Mobilization of reserves occurs in a short and well-defined period during seedling emergence 36-60 HAI and occurs first in the endosperm adjacent to the radicle apex. In the seedling, changes begin near the radicle apex and proceed acropetally through the cotyledons 42-84 HAI. Reserves are hydrolyzed preferentially from all the epidermal cells, but mobilization is not associated with vascular strands; protein is mobilized internally from the vacuole. Our data on increasing size, relatively constant microbody numbers and shape, and absence of observable microbody destruction during the presumptive glyoxysome-peroxisome transition period support the ''interconversion model'' of microbody ontogeny.