INTRODUCTION 159 SOURCES OF MICROBODIES: MICROBODY CONSTITUENTS 162 Glyoxysomes...... ....... ........ ... ...... ... ..... 162 Leaf Peroxisomes 164 Microbodies from Other Tissues ...... 164 DISTRIBUTION OF ENZYMES WITHIN MICROBODIES 165 ISOENZYMES IN GLYOXYSOMES AND LEAF PEROXISOMES 166 Isoenzymes of MDH 166 Isoenzymes of Citrate Synthetase 167 Isoenzymes of Aspartate-a.-Ketoglutarate Transaminase 168 FUNCTIONS OF GLYOXYSOMES AND LEAF PEROXISOMES 168 DEVELOPMENT OF MICROBODIES 170 Glyoxysome Biogenesis ..... ,", . . . . . . . . . . . . . . ,. 170 Biogenesis of Leaf Peroxisomes 176 Microbody Transition in Fatty Cotyledons 177 Hormonal Influences 185 CONCLUSIONS , ...... , 186

Microbodies in Higher Plants

9 – Cytokinins and Senescence

The Role of Phytohormones (Cytokinins) in Chloroplast Development

1. In developing rye (Secale cereale L.) leaves the formation of plastidic ribosomes was selectively prevented in light as well as in darkness, when the seedlings were grown at an elevated temperature of 32° instead of 22° where normal development ocurred. Plastid ribosome deficient parts of lightgrown leaves were chlorotic at 32°. — 2. At both temperatures the leaves contained under all conditions (light or dark, on H2O or nutrient solution) equal or very similar amounts of total amino nitrogen. In light, the contents of total protein and dry weight were lower at 32° than at 22°, especially when the plants were grown on nutrient solution. — 3. Mitochondrial marker enzymes had normal or even higher activities in 32°-grown leaves. Respiration rates were similar for segments of leaves grown on water in light either at 32° or at 22° but by 20–30% lower for 32°-grown plants when they had been raised in darkness or on nutrient solution. In contrast to 22°-grown tissue, respiration of 32°-grown leaf segments was rather insensitive to KCN. Comparative inhibitor studies indicated the presence of both the cyanide-sensitive and the cyanide-insensitive pathway of respiration in 32°-grown leaves. — 4. Leaf microbody marker enzymes were present in leaves grown at 32°. From chlorotic parts of 32°-light-grown leaves a typical microbody fraction was isolated on sucrose densitygradients. — 5. Leaves of seedlings grown at 32° contained only very low levels of ribulosediphosphate carboxylase activity and of fraction I protein. Photosynthetic 14CO2-fixation of such leaves was only a few per cent of that observed in normal leaves, and no photosynthetic oxygen evolution was observed in chlorotic leaf segments. However, ten other soluble enzymes which are exclusively or partially localized in chloroplasts reached high activities under all conditions at 32° (Table 4). — 6. From chlorotic parts of 32°-light-grown leaves as well as from etiolated 32°-grown leaves a fraction of intact plastids was isolated and purified by sucrose gradient centrifugation which contained several soluble chloroplast enzymes. From the results we conclude that cytoplasmic protein synthesis must contribute a functional chloroplast envelope including the mechanism for the recognition and uptake of chloroplast proteins which are synthesized on cytoplasmic ribosomes.

Biochemical differentiation of plastids and other organelles in rye leaves with a high-temperature-induced deficiency of plastid ribosomes.

In green as well as in etiolated leaves of rye (Secale cereale L. ev. ‘Halo’), exposed to strong light at low temperature (0.4°C) catalase was inactivated. Other heme-containing enzymes (peroxidases) and various enzymes of photosynthetic, photorespiratory or peroxide metabolism were not photoinactivated. After returning plants from a low to a physiological temperature (22°C), catalase activity recovered within 12 h through new synthesis. The leaf contents of H2O2 and organic peroxides were not affected by the photoinactivation of catalse. The content of malondialdehyde generally increased after exposure to a higher light intensity. High-light-induced increases of ascorbate, and particularly of glutathione, were more marked in catalase-deficient than in normal leaves. Photoinactivation of catalase was accompanied by severe inhibition of photosynthesis. Photoinhibition of photosynthesis was not related to the lack of catalase because photosynthesis was not impaired when catalase activity was kept low by growing the plants under non-photorespiratory conditions. Photoinhibition appeared to result from photodamage in primary photochemistry of photosystem II, as indicated by a decrease of the maximal variable fluorescence. Photoinhibition of photosynthesis and of catalase have in common that in both instances proteins are involved that are continuously inactivated in light and, therefore, particularly sensitive to stress conditions that prevent their replacement by repair synthesis.

Johanne de Boer

Papers

Comparison of the effects of cytokinins on enzyme development in different cell compartments of the shoot organs of rye seedlings