Formation of the Pollen-aggregating Threads in Strelitzia reginae

Abstract

Morphogenesis of the specialized thread-forming (TF) cells in the Strelitzia reginaeanther was investigated; particular attention was given to the cell walls and the degree of vacuolation. The mass of both cell wall and cytoplasm increased until just before dehiscence. However, cell growth and degradation were largely synchronous processes in the TF cells: before any wall thickening could be observed, degradation of primary cell wall material was already initiated. This degradation continued, with the result that the mature thread cells were eventually fully separated from their surrounding cells. Four stages of development, mainly relating to the degree of cell separation, were established. At stage 1, TF cells began to separate from the subepidermis, while at stage 2 some initial cell wall thickening was taking place. The walls of the TF cell were, at stage 3, thickened considerably (about 1 μm), especially along the radial axes. The texture of these walls was loose due to the presence of large intermicrofibrillar regions, and the previously vacuolated cells were filled with cytoplasm. Longitudinal sections revealed conical gaps in the thick cell wall over the plasmodesmata. Just before dehiscence (late stage 3), the TF cells separated from each other and the subepidermis to such an extent that only plasmodesmata and fibrillar wall remnants kept the files of TF cells in place. The released uniseriate threads were classified as stage 4. (Occasionally the threads were multicellular but only where the transverse walls had not separated from each other.) The threads had thinner cell walls than the TF cells at stage 3 and were vacuolated.