Uptake and assimilation of sulphate by sulphur deficient Zea mays cells: The role of O-acetyl-L-serine in the interaction between nitrogen and sulphur assimilatory pathways

Cell suspension cultures of maize (Zea mays) growing on a modified Murashige and Skoogʹs (MS) medium containing 1/10 of the normal N supply, were subjected to SO42– starvation for 4 d. During the period of the experiment, the batches of cells were growing at similar rates in both +S and –S treatments. S-starved cells (–S) took up SO42– at eight to ten times the rate of S-sufficient (+S) cells. The high uptake rate of –S cells was repressed within 1–2 h after SO42– was re-supplied. The response to S-starvation was strongly diminished in cells which had been deprived of a N-source for 4 d. Cells grown for several culture cycles with homocysteine thiolactone (TL) as sole S-source had greatly increased SO42– uptake rates. This enhanced uptake was repressed at similar rates by provision of SO42– or by the renewal of the TL supply. The latter result was unexpected and cannot be explained at present. ATP-sulphurylase (EC 2.7.7.4) activity was also de-repressed: in –S cells, the measured activity being more than four times that in +S cells. Repression by SO42– was observed although after a longer period than that for the repression of SO42– uptake. In +S cells, SO42– uptake and ATP-sulphurylase activity were increased significantly by the addition of 0.5 mol.m–3 O-acetyl-L-serine to the culture. Simultaneously, the cysteine pool increased in the same proportion as the former activities. The addition of other amino acids, viz. glutamine or alanine, had either negative effects or no effect on SO42– uptake.