Binding of phytochrome to plasma membranes in vitro

Nicotiana tabacum (cv. Xanthi), N. plumbaginifolia Viviani, N. sylvestris Speg. and Comes and Petunia axillaris × ( P. axillaris × P. hybrida ) (cv. Mitchell) mesophyll protoplast-derived cells were able to grow at low densities in chemically-defined media. Protoplasts of different origins (mesophyll, epidermis, pith, suspension culture) also gave rise to protoplast-derived cells that were able to grow at low densities. When auxin requirements at low densities were compared for different sources of auxins, IAA was found to be efficient in the same range of concentrations as NAA. This was unexpected since tobacco mesophyll protoplasts cannot be induced to divide when plated at high density in the presence of IAA. Optimal 2,4-D concentrations for low density growth were higher and clearly pH-dependent. On the contrary, picloram induced low density growth over a wide range of concentrations suggesting a distinct mechanism of action. These results confirm and extend previous observation on the tobacco mesophyll protoplast and show that the low-density growth technique has a potential use for the study of the action of phytohormones.