Induction of glycollate oxidase activity in mustard seedlings under the influence of continuous irradiation with red and far-red light.
1. A method was developed for the measurement of glycollate oxidase activity in buffered extracts of white mustard seedlings (Sinapis alba L.). 2. After 36 hours germination in darkness, the activity of the enzyme was followed during the further development of the seedlings kept in darkness or irradiated continuously with red or far-red light. In irradiated seedlings as well as in dark-grown controls, most of the activity was found in the cotyledons. 3. Kinetics of activity induction were studied for both the whole seedlings and the cotyledons. In permanent darkness enzymatic activity increased up to 60 hours after sowing; then a nearly constant activity level was maintained. When the seedlings were exposed to continuous far-red light, an increase of enzymatic activity was observed. This light-mediated increase showed a lag-phase wich was dependent on the age of the seedlings. When irradiation was started 36 hours after sowing, it took at least 12 hours for the differences between light and dark activity to become significant; in seedlings irradiated 60 hours after sowing, the lag-phase -if any- was less than 6 hours. The magnitude of the photoresponse diminished with increasing age of the seedlings. After interruption of irradiation in the phase of vigorous activity induction, the activity kept rising for about 8 hours and resulted in a stable intermediary activity level. Continuous far-red light, started 36 hours after sowing, caused a threefold activity increase in cotyledons after 42 hours and the maximal level of activity was light-independent. An activity induction was also obtained under continuous irradiation with red light. When irradiation was started 36 hours after sowing, a lag-phase of about 6 hours was found. Then the activity rose to a constant level inferior to that in continuous far-red light. With 60 hours old seedlings, no significant differences were observed in the response to either red or far-red light. 4. The involvement of phytochrome in the photoresponse was shown by means of the conventional induction-reversion experiments. 5. The induction pattern of glycollate oxidase in mustard is compared with that of other glyoxysome enzymes and dicussed with respect to the problem of compartment biogenesis. Dissimilarities in the induction patterns of the individual enzymes indicate the absence of a uniform regulatory mechanism for the entire glyoxysome compartment of the cells.
Laboratorium voor Plantenfysiologie, Rijksuniversiteit Gent, Belgium.
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M
Van Poucke
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F
Barthe