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 Murashig e and Skoog's (MS) medium containing 1/10 of the normal N supply, were subj ected 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 treatment s. S-starved cells (-S) took up SO42- at eight to ten times the rate of S-s ufficient (+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 strong ly 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 sol e S-source had greatly increased SO42- uptake rates. This enhanced uptake w as repressed at similar rates by provision of SO42- or by the renewal of th e TL supply. The latter result was unexpected and cannot be explained at pr esent. 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. R epression by SO42- was observed although after a longer period than that fo r the repression of SO42- uptake. In +S cells, SO42- uptake and ATP-sulphur ylase activity were increased significantly by the addition of 0.5 mol.m(-3 ) O-acetyl-L-serine to the culture. Simultaneously, the cysteine pool incre ased 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. (C) Elsevier, Paris.