Differential regulation by phytochrome of the appearance of plastidic and cytoplasmatic isoforms of glutathione reductase in mustard (Sinapis alba L.) cotyledons.

An increase of glutathione reductase (GR; EC 1.6.4.2) activity during the transformation of mustard (Sinapis alba L.) cotyledons from storage organs to photosynthetically competent leaves was previously found to be controlled by light acting via phytochrome (Drumm, H., Mohr, H., Z. Naturforsch. 28c 559-563, 1973). Two isoforms of GR could be separated by disc electrophoresis. In the present study we have applied ionexchange chromatography to separate isoforms of GR during the development of the cotyledons. Furthermore, the technique of in situ photooxidation of plastids was used to distinguish between plastidic and cytoplasmatic isoforms. The isoform GR2 is the plastidic enzyme, as shown by its sensitivity to photooxidative treatment, while GR1 is a cytoplasmatic protein not affected by photooxidative treatment of plastids. Both isoforms are promoted by phytochrome but with different time courses. The appearance of GR1 is independent of the integrity of the plastids, as one might expect. However, unexpectedly, the phytochrome-mediated re-appearance of GR2 after a photooxidative treatment is much less affected by photooxidative destruction of the plastids, i.e. by the lack of a particular "plastidic factor", than was to be expected from previous experience with typical plastidic proteins. An interpretation of this finding must await measurements at the level of GR2 mRNA.