Phytochrome-mediated induction of ascorbate oxidase in different organs of a dicotyledonous seedling (Sinapis alba L.).

The time courses of the level of ascorbate oxidase (AO; EC 1.10.3.3.) were followed in the different organs (cotyledons, hypocotyl, taproot) of the developing mustard seedling. Phytochrome (operationally, far-red light, cf. [20]) rapidly and strongly enhances the rate of apparent ascorbate oxidase (AO)(1) synthesis in cotyledons and hypocotyl, while in the taproot the detectable amount of AO is only small. However, the relative increase of AO as mediated by continuous far-red light is the same in all organs. Far-red → dark kinetics indicate that the phytochrome-induced enzyme is much less stable in the hypocotyl than in the cotyledons, at least during the experimental period. It is concluded that the effect of phytochrome on enzyme induction is precisely the same in cotyledons and hypocotyl, while the processes of enzyme degradation are specific for the organ. Thus the time courses of enzyme levels can be determined by the "nature" of the particular organ, even if no isoenzymes are involved and the "mechanism" of the inductive process is the same in the different organs.