Assimilate levels and the synthesis of specific root proteins

The effect of diurnal variations in assimilate concentration on in vivo protein synthesis was investigated in millet seedlings grown in sterile solution culture. Root proteins from intact seedlings were labeled with 35S met, phenol extracted and separated by 2D gel electrophoresis. Labeled proteins were visualized by autoradiography and quantified by excision and counting. During the course of a normal 12 h dark period a set of low MW (< 15 kd) proteins became highly labeled (≥ 5 fold) while labeling declined in at least 4 proteins of higher MW. Label in other clearly separated proteins was essentially constant throughout the dark period. These effects were magnified by extending the dark period for a further 15 h. Exposing the plants to 2h of light reversed these effects; ie, labeling declined in the low MW set and increased in the 4 higher MW proteins. The light treatment could be replaced by exposing the roots to 100 mM sucrose, suggesting that the effect of light is exerted primarily via an increase in root carbohydrate content. These results show that normal variations in the assimilate concentration of a sink tissue can lead to significant changes in the synthesis of specific proteins