Light‐ and sucrose‐dependent gene expression in photomixotrophic cell suspension cultures and protoplasts of rape (Brassica napus L.)

Abstract

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 Pfr destruction and rapid Pfr 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 re‐induction 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.