ULTRASTRUCTURE AND FORMATION OF PHLOEM IN CUCURBITA-PEPO CALLUS

Sieve elements and companion cells that form in Cucurbita callus were ultrastructurally similar to the sieve tube members and companion cells of the phloem in intact plants. Companion cells, however, bordered only a third of these sieve elements. Plasmodesmatal connections were rarely observed between callus sieve elements and companion cells, whereas no plasmodesmata were observed between these cells and surrounding parenchyma cells. The paucity of companion cells and symplastic isolation of callus sieve elements suggested that if phloem loading were to occur sugars would be transported into the sieve elements entirely from the apoplast. The degeneration and eventual loss of nuclei, vacuoles, and groundplasm from differentiating callus sieve elements followed similar developmental patterns observed in the phloem of intact plants. The rearrangement of the endoplasmic reticulum, the formation of P-protein, and the development of sieve plate pores were also similar to that observed in intact plants. In squash callus, no parietal network of dispersed P-protein filaments was observed lining the lumina of mature sieve elements, Undispersed P-protein bodies, however, were found along their walls. Sieve plate pores between callus sieve elements were not occluded with either P-protein or callose. © 1991.