[Intracellular localization and function of hydrolytic enzymes in tobacco]

The senescence syndrome in plants: an overview of phytogerontology

The deteriorative processes leading to the death of a population, individual or part of an individual can be genetically programmed or induced by environmental perturbations, physical damage, pests and diseases. Senescence in multicellular plants is typically a phenomenon resulting from cell differentiation and loss of totipotency. Recycling of nutrients released from senescent cells, abscission layer formation, containment of pathogens and dispersal of progeny are crucial aspects of senescence management. Senescence-related autocatalytic changes induced by substances generally thought to regulate senescence may not mirror the sequence of changes occurring naturally through correlative processes in the intact plant. The chloroplast has a key role in reversing senescence-related degradation of other organelles. Conventional symptoms of senescence used in plant sciences have obscured common theories of senescence regulation for all types of organism.

Nucleic acid and protein synthesis in discs cut from mature leaves of Nicotiana tabacum L. and cultured on nutrient agar with and without kinetin

The effect of kinetin on aspects of the metabolism of discs cut from mature leaves of Nicotiana tabacum and cultured in the light on agar containing mineral salts and sucrose was studied. In the first few days of culture there was a rapid decline in chlorophyll content. Discs treated with kinetin in the light began to resynthesise chlorophyll after 3-4 days and this was correlated with chloroplast replication. Kinetin promoted chloroplast replication but was not always essential. An increase in fresh weight also occurred, due mainly to cell expansion. Nitrate reductase activity increased rapidly during the first few hours after placing discs on the culture medium but kinetin had no effect on this reponse. Subsequently there were dramatic increases in RNA and protein content which were largely independent of kinetin. Gel electrophoresis showed that cytoplasmic and chloroplast ribosomal RNA and a large amount of soluble RNA were synthesised during culture of the discs. These results are discussed in relation to the role of kinetin in delaying leaf sensescence.

Anatomical and ultrastructural changes in aleurone and myrosin cells of Sinapis alba during germination

The cells of the embryo of Sinapis alba L. include either aleurone or myrosin grains and all cells contain oil bodies. Aleurone grains and oil bodies are degraded during germination. The myrosin grains of each myrosin cell, on the other hand, gradually turn into one big vacuole containing the myrosin. Probably very little, if any, new myrosin is formed in the cotyledons and hypocotyl of the seedling after germination. No difference was found between aleurone and myrosin cells in the development of organelles. The cells of provascular bundles of the mature embryo contain different amounts of aleurone grains in different stages of development, and their organelles are more developed than those of all other cells.

Isolation and properties of lysosomes from dark-grown potato shoots

A method is described for the isolation of lysosomal fractions from dark-grown potato shoots using a single stage separation on a Ficoll gradient. Peaks of acid hydrolase activity consisting of acid phosphatase, phosphodiesterase, ribonuclease, carboxylic esterase and β-glycerophosphatase were well separated from peaks of mitochondrial and glyoxysomal enzymes. A heavy lysosomal fraction with particle diameters from 0.1 to 1.6 μ and density of 1.10 g cm(-3) containing relatively low hydrolase activity was distinguishable from a light fraction with diameters 0.025 to 0.6 μ and density of 1.07 g cm(-3) with a higher level of hydrolase activity. Both fractions appeared heterogeneous by electron microscopy, but the fine structure of the membranes of both heavy and light lysosomes was similar. The heavy lysosomal fraction was rich in autophagic vacuoles (secondary lysosomes) containing organelles and amorphous cytoplasmic material. Both fractions were rich in ribonucleic acid.Freezing and thawing, high speed blending and ultrasonication either singly or in combination solubilised a maximum of ca. 30% of the acid phosphatase from crude lysosomal fractions derived from dark-grown potato shoots. Treatment with Triton X-100 and deoxycholate released appreciably more enzyme activity but acetone and carbon tetrachloride failed to solubilise any acid phosphatase. Only detergent treatments gave marked overrecovery of enzyme and indicated structure-linked latency. Liberation of enzyme from lysosomes varied with pH and was almost complete at both extremes of pH. Crude snake venom was rapid and effective in solubilising acid phosphatase from lysosomal preparations, purified phospholipase A was less effective and phospholipases C and D had negligible effects. Phospholipase and venom mediated release of acid phosphatase was accompanied by the coincident release of an acid end-product. Gel filtration of acid phosphatase liberated from heavy and light lysosomal fractions by snake venom digestion revealed that each of these fractions was characterised by the presence of distinct molecular forms of the enzyme. The nature of the association of acid phosphatase with potato shoot lysosomes is discussed.

Effect of senescence and hormone treatment on the activity of a β-1,3-glucan hydrolase in Nicotiana glutinosa leaves

A high level of activity of a β-1,3-glucan hydrolase is present in leaves of Nicotiana glutinosa and the enzyme is also present in the roots, midribs, petioles and stems. By comparison, very low levels of β-1,4-glucan hydrolase are found throughout the plant. The activity of the β-1,3-glucan hydrolase in leaves aged on the plant was found to increase 14-fold during the course of leaf senescence and to reach a maximum in yellow-green leaves. Detached leaves and leaf discs floated on water in the dark showed similar patterns of change.The increase in β-1,3-glucan hydrolase activity during senescence is apparently not due to the loss of an inhibitor from young green leaves or to the formation of an enzyme activator in yellow leaves. The enzyme in yellow leaves was electrophoretically indistinguishable from that in green leaves. The hydrolase is not firmly attached to the cell walls and is not present in the particulate fraction sedimenting at 105400xg for 60 min. Within the leaf cell it is therefore likely to be located either in the cytoplasm or in an easily disrupted structure such as a vacuole.The relationship of the hydrolase to leaf senescence was investigated by examining the effect of plant hormones on the changes in level of hydrolase, protein and chlorophyll in leaf discs during senescence. IAA (10 μM) and GA3 (50 μM) did not alter the normal patterns of change, whilst Kin (50 μM) delayed the loss of protein and chlorophyll and also delayed and decreased the rise in hydrolase activity. In contrast, ABA (190 μM) which increased the rate of loss of protein and chlorophyll, also caused a decrease in the rate and extent of the rise in hydrolase.Possible functions of the hydrolase in the leaf are discussed.

Subcellular localization and possible functions of acid β-glycerophosphatases and naphthol esterases in plant cells

The subcellular localization of esterases and acid phosphatases in described for cells from roots of Vicia faba. The possible mode of transport of the esterases from a cytoplasmic site of synthesis to the cell wall is discussed.

Ultrastructural localization of acid phosphatase in cultured cells ofDaucus carota

Acid phosphatase localization has been studied, using the lead salt method, in suspension-cultured cells of the wild carrot. Enzyme activity in most of the cells was restricted to the walls and vacuoles. However, in some senescent cells activity was also seen in the nucleus, at one face of the dictyosomes, and in nearby dictyosome-derived vesciles.The activity in the walls was closely associated with the central portion of the wall which ultimately disintegrates in auxin-containing media. However, the large vesicles which accumulate in this portion of the wall as it breaks down never showed acid phosphatase activity, nor did the multivesicular bodies which appear to transfer vesicular material into the wall space. Although multivesicular bodies in plant cells resemble the multivesicular lysosomes of animal cells, no evidence could be obtained in this study for the presence in such bodies of hydrolytic enzymes.

[Isolation and characterization of sphaerosomes and glyoxisomes from tobacco endosperm]

1. Sphaerosomes (oil droplets) extracted and isolated from tobacco endosperm contain over 90% of the total endosperm lipids. About 99% of the dry matter of these organelles consist of reserve lipids. 2. Upon germination 90% of the lipid reserves are mobilized within 96 hours. The hydrolysis of the neutral fat is achieved by lipolytic activity present in sphaerosomes. This activity could be demonstrated in sphaerosomes of resting seeds. 3. A large fraction of the total activities of proteases, acetyl esterase, phosphatase, RNase and DNase present in the endosperm extract is localized in sphaerosomes. These enzyme activities are present in resting seeds. Protease, acetyl esterase and phosphatase activities increase temporarily in the course of germination. 4. The sphaerosomes of tobacco endosperm are interpreted as lysosomes in which not only the accumulation and mobilization of the reserve lipids take place, but also the breakdown of autophagocytized cytoplasmic material. The morphological and functional relations between sphaerosomes and other types of vacuoles are discussed. 5. Enzymes of the glyoxylic acid cycle present in the endosperm extract from germinating seeds can be sedimented together with mitochondrial enzymes. Upon centrifugation of the mitochondrial fraction in density gradients of sucrose mitochondria can be separated from glyoxisomes. 6. In addition to isocitrate lyase and malate synthetase catalase is associated with glyoxisomes. The former enzymes are present almost exclusively in the fraction of glyoxisomes whereas about 90% of the latter occur in the soluble fraction. 7. The function of glyoxisomes in the conversion of fatty acids into sugar is discussed.

Fine-structural characterization of plant microbodies

Morphology and distribution of the relatively less well known organelles of plants have been studied with the electron microscope in tissues fixed in glutaraldehyde and postfixed in osmium tetroxide. An organelle comparable morphologically to the animal microbody and similar to the plant microbody isolated by MOLLENHAUER et al. (1966) has been encountered in a variety of plant species and tissues, and has been studied particularly in bean and radish roots, oat coleoptiles, and tobacco roots, stems and callus. The organelle has variable shape and is 0.5 to 1.5 μ in the greatest diameter. It has a single bounding membrane, a granular to fibrillar matrix of variable electron density, and an intimate association with one or two cisternae of rough endoplasmic reticulum (ER). Microbodies are easily the most common and generally distributed of the less well characterized organelles of plant cells. It seems very probable that they contain the enzymes characteristic of animal lysosomes (containing hydrolases) or animal microbodies (containing catalase and certain oxidases). Spherosomes are also possible sites of enzyme activity but are not as common or as widely distributed as microbodies. For this reason it appears likely that the particles designated as "plant lysosomes", "spherosomes", "peroxisomes", etc., in some of the cytochemical and biochemical studies on enzyme localization will prove to be microbodies.Variations in the morphology and ER associations of microbodies in tissues of bean and radish are described and discussed. "Crystal-containing bodies" (CCBs) are interpreted as a specialized type of microbody characteristic of metabolically less active cells. Stages in the formation of CCBs from microbodies of typical appearance are illustrated for Avena.The general occurrence of microbodies in meristematic and differentiating cells and their close association with the ER suggest that they may play active roles in cellular metabolism. The alterations in their morphology and numbers that are observed in certain differentiating cells suggest further that the enzyme complements and metabolic roles of microbodies might change during cellular differentiation. If so, microbodies could be the functional equivalent of both microbodies and lysosomes of animal cells.

Glutaraldehyde activation of nuclear acid phosphatase in cultured plant cells

Cultured tobacco cells exhibited a nuclear reaction in the cytochemical test for acid phosphatase after exposure to 2 percent glutaraldehyde. A slight preference for adenosine monophosphate over beta glycerophosphate as substrate was noted, but the enzymatic activity was not typically 5' nucleotidase. Other aldehydes tested did not elicit the same enzyme response. Nucleolar localization was distinct from that in the rest of the nucleus. Assay for spectrophotometric measurements of enzymatic activity in vitro was developed as a modification of the Burstone procedure with naphthol AS-B1 phosphate and diazo blue B.

[Enzymatic characterization of lysosome equivalents (spherosomes) in corn seedlings]

1. The following acid hydrolases are partially sedimentable from cell-free extracts of corn seedlings: protease (pH-optimum 4,2), phosphatase (pH-optimum 5,0 and 6,5), unspecific esterase (pH-optimum 5,2), RNase (pH-optimum 6,5), arylsulphatase-C (pH-optimum 5,0) and α- and β-amylase (pH-optimum 7,0 and 5,0 respectively). 2. After differential centrifugation of cell-free extracts the sedimentable hydrolases are recovered mainly in the mitochondrial and microsomal fraction. 3. It could be demonstrated that the acid hydrolases protease, phosphatase, RNase, and esterase of the mitochondrial fraction are contained in membrane-bound particles. 4. Isopycnic centrifugation of cell-free extracts in sucrose gradients revealed the presence of three particulate fractions carrying hydrolases. The heaviest fraction has a relative density of 1,138 g\sdcm(-3) and contains acid protease,-phosphatase,-RNase, and acid esterase. A lighter fraction (d=1,105 g\sdcm(-3)) contains the same acid hydrolases. The specifically lightest cell fraction (d=1,070 g\sdcm(-3)) contains the acid hydrolases glucose-6-phosphatase, arylsulphatase-C and small amounts of \ga- and \gb-amylase activity. This fraction also contains NADH-diaphorase activity. 5. By means of enzymatic characterization and staining with fluorochromes the structures carrying hydrolases were identified as two kinds of spherosomes and as fragments of the endoplasmatic reticulum. From these results it is concluded that the spherosomes represent organelles equivalent to the lysosomes of animal cells. 6. β-Glucuronidase, phospholipase-C, lipase, and arylsulphatase A and-B, all of with are typical enzymes of animal lysosomes, are completely absent in cell-free extracts of corn seedlings. 7. The isolated spherosomes do not exhibit indophenoloxydase activity; thus the histochemical demonstration of this enzyme in spherosomes must be considered to be an artifact. 8. Isolated lipid droplets, spherosomes, prospherosomes, and mitochondria incorporate H(-3)-acetate into lipids. Consequently they contain all the nessesary enzymes for lipid synthesis. The high activity of lipid synthesis in the spheroromes points to a possible conversion of these organelles into lipid bodies. 9. Two transaminases are partially bound to the mitochondria; the rest of the activity is probably bound to fragments of membranes less dense than an 18% sucrose solution.