Protein degradation in C-3 and C-4 plants subjected to nutrient starvation. Particular reference to ribulose bisphosphate carboxylase/oxygenase and glycolate oxidase

In a previous study [Esquivel et al., J. Exp. Bet. 49 (1998) 807-816], the degradation patterns of ribulose bisphosphate carboxylase:oxygenase (RuBP c arboxylase/oxygenase; EC4.1.1.39) and of glycolate oxidase (EC1.1.3.1) were shown to be species specific under normal metabolic conditions, suggesting that they do nor depend on the type of photosynthetic metabolism In this w ork, we have extended this study to analyse protein degradation under condi tions of sulphur or nitrogen deprivation in the second leaves of intact whe al (C-3), maize (C-4) and sorghum (C-4) plants. The plants were grown and t he leaf proteins double-labelled with radioactive precursors. Immediately a fter the labelling period, the plants were transferred to unlabelled medium and chased, under selected conditions, for up to 15 days to measure protei n turnover. Sulphur and nitrogen deficiencies greatly reduce plant growth. However, the fresh weight of the second leaves utilised in the experiments remain essentially unaltered during the 15-day chase period. Changes in chl orophyll, total soluble protein and RuBP carboxylase/oxygenase activity in the plants grown under control conditions, or deprived of sulphur or nitrog en, produce similar patterns in the three species examined. Under sulphur d eficiency, degradation of wheat RuBP carboxylase/oxygenase is faster than t hat of the total soluble protein. However, sulphur deficiency in the two C- 4 plants does not affect the rate of enzyme catabolism. On the other hand, nitrogen deprivation slightly increases the degradation of wheat and sorghu m RuBP carboxylase/oxygenases, but strongly enhances the degradation of the enzyme in maize. Glycolate oxidase appears to be a fast turning over enzym e in all plants tested, exhibiting a large enhancement in its rate of degra dation with sulphur or nitrogen deficiency. The results obtained suggest th at in C-3 plants, total soluble protein, RuBP carboxylase/oxygenase and gly colate oxidase degradation are more affected by sulphur deficiency than by nitrogen deprivation. The opposite appears to be true for C-4 plants. These observations raise the possibility that these responses may be somewhat de pendent on the type of photosynthetic metabolism, but raise uncertainties a bout considering RuBP carboxylase/oxygenase as a leaf storage protein. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.