Protein synthesis in a maize callus exposed to NaCl and mannitol

A maize (Zea mays, L) callus was exposed to media containing different levels of NaCl (0 to 3%) and mannitol (0 to 18.2%) for a period of 4 weeks, and the changes in growth and protein synthesis were determined. Cells are able to tolerate and grow in NaCl up to 1% (0.17 M) or mannitol up to 9.1% (0.5 M), but the relative overall growth rates are about 1/6 and 1/8 of the control, respectively. Protein synthesis, as assessed by pulselabeling of the cells with (35)S-methionine after exposure to the stress reagents at various times of incubation, suggests that the relative rates of amino acid uptake and its incorporation into proteins are inhibited as early as 4 h after exposure, and the extent of inhibition does not increase appreciably until after 1 week. Severe inhibition of uptake and protein synthesis results from prolonged exposures at growth-inhibitory concentrations of NaCl and mannitol. In general, the overall mean inhibition of cellular uptake and protein synthesis in the first 2-week period are approximately 50% and 35% for the NaCl (1%) and mannitol (7.3 %) treatments, respectively. No detectable differences are apparent in the abundant, steady state protein population as revealed by SDS-PAGE and on staining with Coomassie blue or silver, but random losses of individual proteins occur after 2 weeks at 2% and 3% NaCl and at 18.2% mannitol. Of the newly-synthesized proteins, discernible changes are found in 7 and 4 polypeptides in NaCl and mannitol treatments, respectively. Apparently three new proteins (74 kd, 28.5 kd, and 26.2 kd) are induced de novo under both treatments. Other proteins (39.5 kd, 39.0 kd, 30 kd, and 16.5 kd) show an increased or decreased level of synthesis. NaCl levels above 0.5% or mannitol levels above 3.6% do not after the pattern of newly-synthesized proteins. This altered expression of newly-made proteins in the maize callus occurs only after a week of exposure to salinity or osmotic stress and coincides with the cell growth phase.