Biochemical Studies on Development of Mitochondria in Pea Cotyledons during the Early Stage of Germination: Effects of Antibiotics on the Development

Population heterogeneity of higher-plant mitochondria in structure and function

Mitochondria of rapidly developing mungbean seedlings were fractionated into four populations: two density classes, each from a 1500S and a 150S pellet. Each of the four populations exhibited cytochrome c oxidase (COX) activity and contained mitochondrial DNA and cardiolipin; plastid and glyoxysome content were found to be relatively low. Five mitochondrial membrane proteins, COXII/III, ATPase alpha/beta and porin, and a matrix enzyme, manganese superoxide dismutase (MnSOD), were detected by immunoblots in all four populations. Another matrix enzyme, pyruvate dehydrogenase was detected only in the two respiratory-competent 1500S populations. The two 150S populations contained a previously unidentified organelle that lacked demonstrable respiratory capability. This organelle, which we have tentatively referred to as "slow-sedimenting (ss-) mitochondrion", was small in size (below light-optics resolution, 70-300nm, majority < or =200nm) and possessed a peculiar looking boundary membrane, ribosomes, and an occasional prominent electron-dense spot. Characteristically, ss-mitochondria were almost always in contact with a filament-aligned membrane-like structure of varying length. Cristae structure, while undetected in small ss-mitochondria, appeared in larger individuals. Typical mitochondria were found in the denser 1500S population, while the lighter 1500S population consisted of 300-800 nm mitochondria exhibiting a varying degree of size-dependent inner membrane folding. Using electron microscopy (EM) immunolocalization and serial sectioning, we have identified in situ organelles resembling in size and in fine structure the ss-mitochondria, which also exhibit a size-dependent folding of the inner membrane. These results suggest that small ss-mitochondria may undergo a progressive development in situ. Taken together, our findings demonstrate the existence of a pattern of structure-function-coordinated gross heterogeneity among mitochondria. This pattern of mitochondrial heterogeneity, characterized both in isolated mitochondria and in situ, implies that small ss-mitochondria may represent a type of "nascent mitochondria" derived from a yet unidentified mitochondria-propagation mode operating during rapid seedling growth. Mitochondrial division by binary fission, characterized by the appearance of dumbbell-shaped intermediates, was also detected.

Isolation-Inflicted Injury to Mitochondria from Fresh Pollen Gradually Overcome by an Active Strengthening during Germination

Activities of segments of the electron transport pathway of mitochondria isolated from pollen of Typha latifolia L. during the course of germination in vitro were compared with those of mitochondria in intact grains. For this purpose, suitable inhibitors and artificial substrates were selected for their ability to penetrate through the exine, intine, and plasmalemma. In contrast to their counterparts in vivo, mitochondria isolated during the initial stages of germination exhibited low rates of electron transport, resulting from loss of NAD(+) and displacement of cytochrome c from its site of action. The phosphorylative capacity was also impaired. Great caution must be exercised therefore, before interpreting results obtained with isolated mitochondria.The gradually acquired resistance of mitochondria to injury during isolation as germination proceeds was shown to depend on an energy-requiring process and not solely on a rearrangement at the membrane level, or imbibitional differences. De novo syntheses of proteins or fatty acids were not required for the strengthening of mitochondria since cycloheximide, chloramphenicol, and cerulenin did not prevent this change. The nature of the energy-requiring process remains obscure. It is probable that strengthening of mitochondrial membranes during seed germination has been misinterpreted due to similar effects of isolational injury.

Mechanism of the increase in cytochrome c oxidase activity in pea cotyledons during seed hydration. The presence of free cytochrome-c-oxidase subunits in dry cotyledons and their probable assembly into the holoenzyme during seed hydration

The increase in cytochrome c oxidase activity in pea cotyledons during seed hydration was investigated with two kinds of antibodies against the enzymes purified from mitochondria of pea shoot and sweet potato root. The antibody against the pea enzyme recognized only the largest of the enzyme subunits (subunit I), while that against the sweet potato one did other three of the pea enzyme subunits: the second largest (subunit II) and the two smallest (subunits IV and V). There was no increase in the amount of protein immunoprecipitated with the anti-(pea enzyme) antibody in a crude fraction of cotyledon membranes during seed hydration, despite the marked increase in cytochrome c oxidase activity. Neither cycloheximide nor chloramphenicol had any effect on the activity or the amount. The crude membrane fraction from dry cotyledons contained an enzymatically inert protein precipitated with the anti-(pea enzyme) antibody. The inert protein could be separated from the active enzyme protein through solubilization of the protein from the membranes with various concentrations of Triton X-100. It consisted of only one kind of polypeptide with the same molecular weight as subunit I and decreased with the concomitant increase in the active enzyme protein during seed hydration. Dry pea cotyledons also contained a soluble protein that was immunoprecipitated by antibody to the sweet potato enzyme. The soluble form of the immunoreactive protein decreased with the concomitant increase in active cytochrome c oxidase protein in the crude membrane fraction from the cotyledons during seed hydration. None of these changes during seed hydration were inhibited by cycloheximide or chloramphenicol. The soluble, immunoreactive protein contained only one kind of polypeptide, of which the molecular weight was identical to that of subunit IV. The mitochondrial inner membranes of dry cotyledons were separated into three fractions by sucrose density gradient centrifugation. The lightest was the richest of the three in the enzymatically inert protein immunoprecipitated with the anti-(pea enzyme)antibody; cytochrome c oxidase activity in this fraction increased immediately after the onset of seed hydration. These results indicate that a membrane-bound, free form of subunit I and a soluble, free form of subunit IV of cytochrome c oxidase exist in dry pea cotyledons. We propose that these free forms of subunits are assembled with the other subunits to form the active cytochrome c oxidase during seed hydration.

Biogenesis of Mitochondria in Imbibed Peanut Cotyledons : II. DEVELOPMENT OF LIGHT AND HEAVY MITOCHONDRIA

THERE ARE TWO TYPES OF MITOCHONDRIA PRESENT IN IMBIBED PEANUT COTYLEDONS: a light type (density 1.182 grams per cubic centimeter) and a heavy type (density 1.205 grams per cubic centimeter). The membrane fractions from these two types can be distinguished using sucrose density gradient analysis, and differences in membrane density between the light and heavy types are reflected in differences in their protein N and phospholipid P composition. With increasing time after imbibition, there is a substantial increase in the amount and activity of the light type of mitochondria due to their de novo synthesis. The membrane density of the light mitochondrial fraction declines over 5 days after the start of imbibition as the phospholipid P to protein N ratio increases. The heavy mitochondrial fraction declines during the first 3 days after the start of imbibition, and then it remains at a low, but constant, level thereafter. Even during the decline, however, there is synthesis of proteins comparable to that into light mitochondria. The mitochondrial biogenesis that has been observed in peanut cotyledons is of the light type, the function and physiological importance of the minor heavy type is not known.

Presence in Dry Pea Cotyledons of Soluble Succinate Dehydrogenase That Is Assembled into the Mitochondrial Inner Membrane during Seed Imbibition

SUCCINATE DEHYDROGENASE (SUCCINATE: phenazine methosulfate oxidoreductase, EC 1.3.99.1) activity in crude mitochondrial fraction from pea (var. Alaska) cotyledons increased during seed imbibition to reach a maximum after about 12 hours. The increase was not inhibited by either cycloheximide or d(-)threo-chloramphenicol. The postmicrosomal fraction from dry cotyledons, but not that from fully imbibed ones, contained a soluble form of succinate dehydrogenase. The soluble enzyme was partially purified by ammonium sulfate fractionation and diethylaminoethyl-cellulose and Sepharose 6B column chromatography. The enzyme showed no succinate-coenzyme Q oxidoreductase activity and had a molecular mass of about 100,000 daltons. The soluble enzyme seemed to differ only slightly from succinate dehydrogenase solubilized from the mitochondrial inner membrane from fully imbibed cotyledons by a detergent. It is proposed that the soluble succinate dehydrogenase is associated with an inert mitochondrial inner membrane in dry cotyledons to form an active one during seed imbibition.

Cyanide-insensitive Respiration in Pea Cotyledons

Mitochondria isolated by a zonal procedure from the cotyledons of germinating peas possessed a cyanide-resistant respiration. This respiration was virtually absent in mitochondria isolated during the first 24 hours of germination but thereafter increased gradually until the 6th or 7th day of seedling development. At this time between 15 and 20% of the succinate oxidation was not inhibited by cyanide. The activity of the cyanide-resistant respiration was also determined in the absence of cyanide. Relationships among mitochondrial structure, cyanide-resistant respiration, and seedling development are discussed.

Mitochondrial development and activity of binucleate and trinucleate pollen during germination in vitro

Bi-and trinucleate pollen generally differ in the extent of their mitochondrial development at anther dehiscence and in the rate of their attainment of maximum-phosphorylative capacity during germination in vitro, as judged from experiments with representatives of both groups.The typically trinucleate pollen of Aster tripolium L. immediately respired at a high rate, maintaining a high energy charge. Mitochondria attained maximum electron-transducing capacity within 2 min of incubation, while tube growth started within 3 min. In contrast, the binucleate pollen of Typha latifolia L. only gradually reached a relatively low rate of respiration, concomitant with a temporary decrease in energy charge, upon immersion in the germination medium. Development of the mitochondrial, electrontransducing system occurred in about 75 min, after which the first pollen tubes emerged. Starting from a poor differentiation, mitochondria became increasingly normal in appearance as germination proceeded.The binucleate pollen of Nicotiana alata Link et Otto and Tradescantia paludosa Anders. et Woods. showed intermediate characteristics: Nicotiana resembled Typha but mitochondria developed at a higher rate; Tradescantia germinated more rapidly and resembled the trinucleate pollen of Aster.Inhibitors of mitochondrial or cytoplasmic protein synthesis failed to affect the development of the mitochondrial, respiratory capacities during pollen germination. It is concluded that the duration of the lag period is determined by the level and rate of mitochondrial development and not by the division of the generative cell.

Transient changes during soybean imbibition

Air dry cotyledons of soybean (Glycine max Merr. var. Wayne) imbibe water rapidly for about 10 minutes followed by a slower, linear rate of uptake. Leakage of solutes out of the coytledon likewise shows an initial rapid period, followed by a slower, nearly linear rate after 5 to 10 minutes; both the rapid and the steady rate leakage are greater for initially drier seeds. Respiratory activity of cotyledons as measured by manometric techniques becomes apparent after about 10 minutes of imbibition while polarographic studies of ground particles suggest that O(2) comsumption begins almost immediately upon wetting. Initial wetting of the seed causes the release of adsorbed gases, and a series of changes in volume of the seed-water mixture are charted. The data are interpreted as indicating that extensive physical changes occur in the first few minutes of water entry, including a rearrangement of membranes changing them from a relatively porous to a less permeable condition, and a release of adsorbed gases which cause an inflation or swelling of the seed.

Phytochrome-mediated development of mitochondria in the cotyledons of mustard (Sinapis alba L.) seedlings

The development of mitochondria from promitochondria is regulated by phytochrome. This conclusion is based on four lines of evidence: 1. The activity of representative mitochondrial marker enzymes (fumarase, EC 4.2.1.2; succinate dehydrogenase, EC 1.3.99.1; cytochrome oxidase, EC 1.9.3.1) is increased by continuous far-red light and (in 2 of the 3 enzymes) by brief red pulses, the effect of which is reversible by brief far-red pulses. These effects do not merely represent a general growth or proliferation of mitochondria already present but specific responses of individual enzymes. Inhibitors of protein synthesis but not of RNA synthesis suppress the increase of these enzyme activities. 2. Continuous far-red light changes some structural properties of the mitochondrial membranes, detectable by an increased requirement of detergent (Triton X-100) for the solubilization of cytochrome oxidase and a more efficient retainment of the matrix enzyme fumarase during isolation of mitochondria. Continuous far-red light increases the apparent buoyant density of mitochondria on a sucrose density gradient. 3. Continuous far-red light has a strong effect on the morphology of the inner mitochondrial membrane system. Electron micrographs from dark-grown cotyledons show arrays of parallel, plate-like cristae while typical plant mitochondria with irregularly oriented sacculi are formed in the light. These responses indicate the involvement of mitochondria in cytophotomorphogenesis during the transition of the cotyledons from dissimilatory to assimilatory metabolism.

Biochemical Properties of Mitochondrial Membrane from Dry Pea Seeds and Changes in the Properties during Imbibition

An attempt to isolate intact mitochondria from dry pea seeds (Pisum sativum var. Alaska) ended in failure. Cytochrome oxidase in crude mitochondrial fraction from dry seeds was separated into three fractions by sucrose density gradient centrifugation. Two of the fractions contained malate dehydrogenase, whereas the other did not. Equilibrium centrifugation of mitochondrial membrane on sucrose gradients revealed that the membrane from the fraction without malate dehydrogenase was lighter than that from the others. Differences were observed in relative content of phospholipid to protein and in polypeptide composition analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis among the membranes from three fractions and imbibed cotyledons. Membrane from the fraction without malate dehydrogenase was rich in phospholipid and lacking in polypeptides with relatively high molecular weights as compared with that from others. During imbibition, the fraction without malate dehydrogenase and one of the other two disappeared rapidly after a lag phase lasting for at least 1 hour. Concomitantly, active and stable mitochondria increased in the cotyledons. The results were interpreted to indicate that there were at least three types of mitochondria in dry seeds, the membranes of which differed in their biochemical properties, and that the mitochondria became active and stable through assembly of protein into the membranes during imbibition.

Hypochlorite and tissue sterilization

Trace amounts of sodium hypochlorite that remain on the surface of seeds (Lycopersicon esculentum Mill.) after sterilization interfere with subsequent uptake and incorporation of leucine into protein when the seeds are used in metabolic studies. The hypochlorite can be washed away with 0.01 N HCl but not by washing several times with water.

Pitfalls in using sodium hypochlorite as a seed disinfectant in C incorporation studies

Seeds sterilized with sodium hypochlorite (NaOCl) retained sufficient amounts to interfere with studies of amino acid metabolism of the sterilized seeds during germination. Repeated washing in water did not remove NaOCl completely. However, soaking the seeds for 10 min in 0.01 n HCl removed NaOCl completely, without reducing germinability.Residual NaOCl reacted with the amino acids and reduced their concentrations in the incubation media. This reaction resulted in high production of CO(2) and low uptake of amino acids by the seeds. Decarboxylation of the amino acids occurred in the incubation medium outside the seed, was independent of the presence of seeds in the reaction, and therefore was not related to amino acid metabolism by the seeds. Effects of NaOCl on uptake, incorporation, and CO(2) production from indoleacetic acid were similar to those of the amino acids studied.

Effects of cycloheximide, D-threo-chloramphenicol, erythromycin and actinomycin D on De-novo synthesis of cytoplasmic and mitochondrial proteins in the cotyledons of germinating pea seeds

Inhibitors of, and radioactive substrates for, protein synthesis were introduced into germinating pea (Pisum sativum L.) seeds, and protein synthesis was allowed to proceed in vivo. Subsequent analyses of subcellular fractions showed the following: Cycloheximide strongly inhibited the incorporation of [(14)C]leucine into both mitochondrial and cytoplasmic proteins. D-Threo-chloramphenicol and erythromycin did not affect cytoplasmic protein synthesis, but partially inhibited mitochondrial protein synthesis. These results suggest that most of the new mitochondrial proteins were originally synthesized in the cytoplasm. Actinomycin D did not appreciably affect the initial incorporation of [(14)C]leucine into either mitochondrial or cytoplasmic proteins, suggesting that information (mRNA) concerning the initially synthesized proteins may be present in the quiescent seeds. The lack of appreciable incorporation of [(3)H]thymidine into mitochondrial DNA supported our previons report that mitochondria may not be synthesized de novo in pea cotyledons.