Plastid number and plastid structural changes associated with tobacco mesophyll protoplast culture and plant regeneration.

Mesophyll protoplasts were isolated from Nicotiana tabacum L. cv. Xanthi, and cell-colony formation induced in liquid culture. The plastid changes associated with the morphogenetic sequence from mesophyll protoplast to whole plant were examined. Minor ultrastructural changes in the plastids were evident after 1 d of culture, but by 8 d (four-to-eight-cell stage) the plastids were small, there was much less thylakoid membrane appression, and many prominent plastoglobuli were also present. Plastid-division figures were evident at this point of time and it was common to find plastids clustered around the nucleus. A typical proplastid was the dominant plastid type in the cultured cells from about 11 d until about five weeks when large amyloplasts and pregranal plastids were observed. Normally structured chloroplasts were present in the regenerated plant. There was no plastid division until the four-cell stage, with plastid numbers per cell approximately halving at each cell division, then stabilising around 12 per cell during cell-colony development, a number typical of meristematic cells. Though nucleoids were always present, their numbers in the plastids were reduced by the eight-cell stage.