CHANGES IN LEVELS OF GENE TRANSCRIPTS AND THEIR CORRESPONDING PROTEINS IN NODULES OF SOYBEAN PLANTS SUBJECTED TO DARK-INDUCED STRESS

Experiments were carried out to investigate the effects of stress on the levels and activities of key nodule proteins and the amounts of the mRNAs encoding these proteins. Soybean plants, subjected to 3 d of continuous darkness and then returned to the normal light/dark regime, were used as a model system. Samples of nodules were taken during the stress/recovery cycle and total RNA and total soluble protein were extracted. The levels of mRNAs encoding leghaemoglobin (Lb), sucrose synthase (SS) and glutamine synthetase (GS) were estimated using Northern and slot blots. Within 24 h of complete darkness the levels of all three transcripts had fallen significantly (by 90%, 70% and 50% for SS, Lb and GS mRNAs, respectively) and continued to decline for the next 2 d of darkness. When the plants were returned to the normal light/dark regime, all transcript levels rapidly increased again. The corresponding proteins were analysed for biological activity (enzyme activities in the cases of SS and GS, and haem content for Lb) and amount of protein (using Western blotting and specific antibodies raised against Lb, SS and GS). The amounts and activities of Lb and GS were unchanged, whilst those of SS declined significantly over the 3 d dark period and increased again during the period following return to the normal light/dark regime. Neither the imposition of a 30 min light break each 24 h during the continuous dark treatments, nor the addition of 50 mol m-3 sucrose to the daily supply of nutrients had any effect on the changes described.