Acquisition of plastid movement responsiveness to light during mesophyll cell differentiation.

A culture of Nicotiana tabacum leaf protoplasts, regenerating in vitro, was used to study light-induced plastid translocations. Experiments were carried out for 5 months starting with protoplasts, through single cells, microcolonies and callus to the differentiated mesophyll of regenerated plants. Although the actin cytoskeleton was fully developed at every stage of culture, blue light-mediated directional movements of chloroplasts were observed only after the full differentiation of callus into leaf tissues. These chloroplast rearrangements were similar to those observed in control plants grown from seeds. Under strong blue light, chloroplasts gathered at the cell walls parallel to the light direction (profile position); under weak blue light, they gathered at the walls perpendicular to the light direction (face position). No light-dependent plastid arrangements were found in undifferentiated cell cultures even after cell wall recovery. A characteristic pattern of plastids in the dividing cells was independent of light signals. Only trace chloroplast responses to strong blue light were detected in the first leaves regenerating from callus. We hypothesize that factors which control the developmental status of a cell, i.e. division and differentiation, take over the control of plastid redistribution from light signals.