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

Abstract

In a previous study [Esquı́vel et al., J. Exp. Bot. 49 (1998) 807–816], the degradation patterns of ribulose bisphosphate carboxylase/oxygenase (RuBP carboxylase/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 not depend on the type of photosynthetic metabolism. In this work, we have extended this study to analyse protein degradation under conditions of sulphur or nitrogen deprivation in the second leaves of intact wheat (C 3), maize (C 4) and sorghum (C 4) plants. The plants were grown and the leaf proteins double-labelled with radioactive precursors. Immediately after the labelling period, the plants were transferred to unlabelled medium and chased, under selected conditions, for up to 15 days to measure protein 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 chlorophyll, total soluble protein and RuBP carboxylase/oxygenase activity in the plants grown under control conditions, or deprived of sulphur or nitrogen, produce similar patterns in the three species examined. Under sulphur deficiency, degradation of wheat RuBP carboxylase/oxygenase is faster than that 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 sorghum RuBP carboxylase/oxygenases, but strongly enhances the degradation of the enzyme in maize. Glycolate oxidase appears to be a fast turning over enzyme in all plants tested, exhibiting a large enhancement in its rate of degradation with sulphur or nitrogen deficiency. The results obtained suggest that in C 3 plants, total soluble protein, RuBP carboxylase/oxygenase and glycolate 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 dependent on the type of photosynthetic metabolism, but raise uncertainties about considering RuBP carboxylase/oxygenase as a leaf storage protein.