Light regulation of assimilatory sulphate reduction in Arabidopsis thaliana

Adenosine 5'-phosphosulphate reductase (APR) is considered to be a key enzy me of sulphate assimilation in higher plants. We analysed the diurnal fluct uations of total APR activity and protein accumulation together with the mR NA levels of three APR isoforms nf Arabidopsis thaliana. The APR activity r eached maximum values 4h after light onset in both shoots and roots; the mi nimum activity was detected at the beginning of the night. During prolonged light, the activity remained stable and low in shoots, but followed the no rmal rhythm in roots. On the other hand, the activity decreased rapidly to undetectable levels within 24h of prolonged darkness both in shoots and roo ts. Subsequent re-illumination restored the activity to 50% in shoots and t o 20% in roots within 8h. The mRNA levels of all three APR isoforms showed a diurnal rhythm, with a maximum at 2h after light onset. The variation of APR2 mRNA was more prominent compared to APR1 and APR3. (SO42-)-S-35 feedin g experiments showed that the incorporation of S-35 into reduced sulphur co mpounds in vivo was significantly higher in light than in the dark. A stron g increase of mRNA and protein accumulation as well as enzyme activity duri ng the last 4h of the dark period was observed, implying that light was not the only factor involved in APR regulation. Indeed, addition of 0.5% sucro se to the nutrient solution after 38h of darkness led to a sevenfold increa se of root APR activity over 6h. We therefore conclude that changes in suga r concentrations are also involved in APR regulation.