David P. Hackett

Recent Studies on Plant Mitochondria

Publisher Summary One of the major recent advances in cellular architectonics, the spatial and structural organization of reactions and processes, is the elucidation of the role of mitochondria in cell metabolism. Plant cell is not simply a bag of haphazardly arranged enzymes. The cell particulates possess a high degree of structural and functional organization, which is, under certain conditions, very labile. The fundamental processes carried out by the mitochondria appear to be the terminal transfer of electrons, the coupling of energy-trapping mechanisms (phosphorylations) to oxidations, and the Krebs cycle reactions and the numerous ancillary processes that eventually funnel through the cycle. In spite of the remarkable autonomy of the mitochondria, the overall activities of the cell are the result of intimate interactions among the various cellular components. Glycolysis involves the plastids, the soluble fraction, and possibly the nucleus as well as the mitochondria1 enzymes. Certain enzymes involved in the Krebs cycle (e.g., malic dehydrogenase) and in hydrogen transfer (e.g., cytochrome reductase) are not confined entirely to the mitochondria. The mitochondria interact with the nucleus in phosphorylation, with the chloroplasts in photosynthesis, and with the microsomes in protein synthesis. In close relation to the outer cell boundary, the mitochondria may participate actively in the movement of substances into the cell or in the growth of the cell wall.

Terminal oxidases and growth in plant tissues. I. The terminal oxidase mediating growth of Avena coleoptile and Pisum stem sections

Abstract 1. 1. The effects of oxygen pressure, carbon monoxide, cyanide, and several copper-enzyme poisons on the elongation of isolated sections of Pisum stems and Avena coleoptiles were studied in an effort to characterize the terminal oxidase mediating this growth process. 2. 2. Growth of Pisum sections is independent of the partial pressure of oxygen between 5 and 220% of an atmosphere, whereas the growth of Avena sections is markedly inhibited when the oxygen pressure falls below 10% of an atmosphere. 3. 3. Prolonged exposure to moderate positive pressures of nitrogen superimposed upon 1 atm. of air has little effect on growth. 4. 4. Twenty-four-hour exposure to positive pressures of carbon monoxide superimposed upon 1 atm. of air markedly inhibits the growth of the isolated sections. A 5:1 ratio of carbon monoxide to oxygen causes 70% inhibition of elongation of Avena and 50% inhibition of Pisum ; at a 20:1 ratio the corresponding values are 86% and approximately 99%. Upon return to air, the sections grow rapidly and are able in large measure to “catch up” to the controls. 5. 5. The carbon monoxide inhibition of growth is essentially completely reversed by light at all carbon monoxide to oxygen ratios. 6. 6. Growth may also be inhibited by cyanide, but is insensitive to copper-enzyme poisons. 7. 7. It is concluded that the enzyme inhibited by carbon monoxide is cytochrome oxidase, and that the growth of Avena and Pisum sections is mediated entirely by this enzyme.

Terminal oxidases and growth in plant tissues. II. The terminal oxidase mediating water uptake by potato tissue

Abstract 1. 1. Auxin-induced water uptake by slices of potato tissue is shown to depend on oxygen tension, reaching its maximum only above 15% of an atmosphere of oxygen. 2. 2. The water uptake is decreased somewhat by positive pressures of nitrogen, helium, or argon superimposed upon air. 3. 3. Allowing for these effects it could be shown that positive pressures of carbon monoxide markedly inhibit water uptake, 2 atm. of carbon monoxide superimposed upon air causing an 80% inhibition. Upon return to air the tissues fully recover their power to take up water. 4. 4. The inhibition caused by carbon monoxide is largely reversed by light (70%), and reasons are given for considering the light reversal to be essentially complete. 5. 5. Copper-enzyme poisons produce no inhibition. 6. 6. It is deduced therefore that the water uptake process in potato tissue is controlled by cytochrome oxidase.