The effect of phosphate deprivation on protein synthesis and fixed carbon storage reserves in Brassica nigra suspension cells

Abstract

Two-dimensional gel electrophoresis was used to analyze labelled proteins from cell suspension cultures of Brassica nigra which differed in their phosphate status. Differences in protein synthesis profiles were detected during cellular transition from phosphate sufficiency to deficiency and between cells which were maintained continuously in either phosphate-sufficient or phosphate-deficient states. Four proteins were identified which increased in their de novo synthesis rates under phosphate-starved conditions. Their approximate molecular mass values and isoelectric points were 64 kDa, pi 5.2; 41 kDa, pi 5.6; 27 kDa, pi 5.7; and 27 kDa, pi 5.2. Conversely, high levels of phosphate supply induced the production of a protein of 33 kDa and pi 5.1. Morphometric analysis revealed that lipid droplets and starch granules in phosphate-deprived suspension cells occupied 13.9 ± 5.0% (mean ± 95% confidence interval) of the total cell volume, whereas in cells supplied with 10 mM KH2PO4 these carbon reserves occupied only 5.7 ± 1.2% of the cell. Chemical analysis of total lipid, phenolics, and protein showed that phosphate-deprived cells had much higher levels of lipid and phenolics, whereas total protein content was independent of phosphate status. These observations suggest possible roles of the proteins preferentially synthesized in B. nigra suspension cells under conditions of phosphate deprivation. Key words: Brassica nigra, carbon metabolism, phosphate deprivation, protein synthesis, ultrastructure.