Ultrastructural changes during growth and embryogenesis in carrot cell cultures

Clumps in suspension cultures of wild carrot have two histological regions; an inner region of highly vacuolate, nondividing cells and a peripheral region of small meristematic cells. Significant ultrastructural features of the meristematic cells include the presence of many starch-filled plastids, a dense background of free ribosomes, rough ER bearing conspicuous polyribosomal aggregates, and numerous multivesicular bodies apparently derived from dictyosomes. Multivesicular bodies fuse with the plasmalemma, releasing their contents into the wall space. Cell enlargement is accompanied by an increase in rough ER, eventual decrease in free ribosomal content, and two types of structural alteration in the walls which eventually cause clump disintegration and release of smaller fragments. Ultrastructural changes accompanying formation of embryos in auxin-free media include a striking increase in free ribosomal content, decrease in ER, loss of polyribosomal aggregates, and appearance of microtubules. The mechanism of embryo polarization is discussed.