Light-triggered organization of the stigma in dark-grown nondividing cells of Euglena gracilis.

Dark-grown cells of Euglena gracilis Klebs var. bacillaris Cori contain amorphous stigma material. When these cells are placed on resting medium for 3 days in darkness, the cells cease division; the organization of a normal stigma from the amorphous material requires continuous illumination for 72-96 hr. We have now found that if dark-grown cells are placed on resting medium for 8 days, a 40-min light pulse is sufficient to cause normal organization of the stigma in a subsequent 72-hr dark period. Thus stigma development is light-dependent at 3 days of resting but becomes light-triggered at 8 days. Other examples of light-triggered phenomena in Euglena are discussed and a model based on turnover of protein molecules repressing development that are ordinarily removed by exposure to light is presented; it is suggested that as the cells become more starved their ability to replace repressor molecules removed by light becomes limited and the system thereby becomes light-triggered rather than light-dependent.