LIGHT-DEPENDENT AND SUCROSE-DEPENDENT GENE-EXPRESSION IN PHOTOMIXOTROPHIC CELL-SUSPENSION CULTURES AND PROTOPLASTS OF RAPE (BRASSICA-NAPUS L)

Light-dependent gene expression was analysed in photomixotrophic cell suspension cultures of rape (Brassica napus L.) growing in media containing either 2.0% or 0.6% sucrose. During growth in darkness phytochrome type I and NADPH-protochlorophyllide oxidoreductase (Pchlide reductase) accumulated in both cell culture lines to a similar extent. Illumination with continuous white, blue or red light, but not with far-red light, resulted in disappearance of both chromoproteins within 24 h in both cell cultures. Further analysis showed that the phytochrome system of rape cell cultures reacts in a similar way to that of re-etiolated dicotyledonous plants, showing rapid P(fr) destruction and rapid P(fr) dark reversion. In contrast, the light-dependent expression of genes encoding the major chlorophyll a- and b-binding protein (CAB) and the re-accumulation of chlorophyll were found to be strongly dependent on sucrose concentration in culture media. Whereas cells grown in darkness in medium containing 2.0% sucrose showed, after exposure to continuous white light, a very weak reinduction of CAB mRNA, CAB protein and chlorophyll accumulation , the cells in medium containing 0.6% sucrose reacted very strongly. It was also possible to demonstrate that phytochrome (by high irradiance response, HIR, and by low fluence response, LF) and the blue/UV-A receptor are involved in the light-dependent gene expression of CAB. Similar to complete cells, protoplasts derived from the two different cell cultures showed an almost identical sucrose concentration-dependent and light-quality-dependent regulation of CAB mRNA accumulation. As the dark-grown photomixotrophic cells and protoplasts reflect some typical photoregulatory characteristics known from dark-grown plants it is supposed that this system will be an excellent tool for studying biochemical and molecular biological aspects of light-dependent signal transduction in cells of higher plants.