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Biomedical subjects

R M Leech

Publications and source records attributed to R M Leech.

At least 37 records · Page 2Linked to original sources
PubMed1980

Influence of Cell Age on Chlorophyll Formation in Light-grown and Etiolated Wheat Seedlings.

A method is described for relating the age of a cereal leaf cell to its distance from the leaf base. The rates of chlorophyll synthesis per plastid in the first leaf of light-grown and of greening etiolated seedlings of wheat (Triticum aestivum, var. Maris Dove) increase with cell age. Normally developing plastids of light-grown wheat take over 24 hours to reach the chlorophyll a/b ratio characteristic of mature wheat chloroplasts (4.5), but mature etioplasts need only 8 hours light to achieve this a/b ratio. Plastid greening potential depends only on cell age, whereas the chlorophyll a/b ratio is influenced both by cell age and by light.

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PubMed1979

Changes in Pool Sizes of Free Amino Acids and Amides in Leaves and Plastids of Zea mays during Leaf Development.

The concentrations of free amino acids and amides within isolated maize (Zea mays L.) plastids were determined and compared with concentrations in the leaf tissue. The concentrations were different for each individual amino acid and varied between 1 and 10 millimolar. At five different developmental stages concentrations in the plastids were greater than those in the intact leaf tissue. During development, from the proplastid stage to the mature chloroplast, the amount of each amino acid per plastid remained relatively constant, but there were decreases in concentrations of plastid amino acids resulting from the developmental increase in plastid volume. In proplastids, the free amino acids were present in greater concentrations than those previously found to inhibit partially amino acid-synthesizing enzymes located in chloroplasts. In the chloroplasts, the molarities of the free amino acids were within the range known to inhibit amino acid-synthesizing enzymes.

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PubMed1979

Chloroplast Division and DNA Synthesis in Light-grown Wheat Leaves.

Light-grown 7-day-old wheat seedlings (Triticum aestivum, var. Maris Dove) showed an increase of 200% in plastids per cell between 1.7 and 4.5 centimeters from the leaf base. This increase was the result of divisions of young chloroplasts at various stages of development, and was well separated in distance, and therefore in time from the region of cell division in the basal meristem. [(3)H]Thymidine was incorporated into plastid DNA throughout the zone of plastid division, but not above it.

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PubMed1977

Development of Photosystem I and Photosystem II Activities in Leaves of Light-grown Maize (Zea mays).

To compare chloroplast development in a normally grown plant with etiochloroplast development, green maize plants (Zea mays), grown under a diurnal light regime (16-hour day) were harvested 7 days after sowing and chloroplast biogenesis within the leaf tissue was examined. Determination of total chlorophyll content, ratio of chlorophyll a to chlorophyll b, and O(2)-evolving capacity were made for intact leaf tissue. Plastids at different stages of development were isolated and the electron-transporting capacities of photosystem I and photosystem II measured. Light saturation curves were produced for O(2)-evolving capacity of intact leaf tissue and for photosystem I and photosystem II activities of isolated plastids. Structural studies were also made on the developing plastids. The results indicate that the light-harvesting apparatus becomes increasingly efficient during plastid development due to an increase in the photosynthetic unit size. Photosystem I development is completed before that of photosystem II. Increases in O(2)-evolving capacity during plastid development can be correlated with increased thylakoid fusion. The pattern of photosynthetic membrane development in the light-grown maize plastids is similar to that found in greening etiochloroplasts.

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PubMed1976

Sequential changes in the lipids of developing proplastids isolated from green maize leaves.

Changes in lipid composition were followed as a proplastid develops into a chloroplast. Methods were devised for the isolation of developing proplastids from sections of five different ages from the same 7-day-old maize (Zea mays var. Kelvedon Glory) leaf. Electron micrographs illustrate the homogeneity of the five types of plastid suspension, minimal contamination with other cytoplasmic membranes, and the presence of morphologically intact plastids in the proportions 85% (youngest), 85%, 80%, 70% and 60% (oldest), respectively. Both bundle sheath and mesophyll plastids are well preserved in isolation. Plastid numbers were determined from calibration curves of the chlorophyll content of each type of suspension, and lipid values then expressed as nmoles/10(6) plastids. Monogalactosyl diglyceride (MGDG), digalactosyl diglyceride (DGDG), sulfoquinovosyl diglyceride, and phosphatidyl glycerol (PG) all increase during plastid development but the rate of increase is different for each lipid. The largest changes are in MGDG (6-fold) and DGDG (4-fold). Phosphatidyl choline shows a continuous decline during plastid development. Phosphatidyl inositol and phosphatidyl ethanolamine were found in all the suspensions in low concentrations (0.4-4.0% of total lipid): calculations showed their presence could not be accounted for by bacterial or mitochondrial contamination. The increase in PG parallels the chlorophyll changes during development and at maturity 1 molecule of PG is present per 3 molecules of chlorophyll. The results are discussed in the context of the molecular structure of the photosynthetic thylakoid membranes.

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PubMed1975

The effect of adenosine 5'-triphosphate on the shibata shift and on associated structural changes in the conformation of the prolamellar body in isolated maize etioplasts.

Isolated maize (Zea mays var. kelvedon glory) etioplasts have been used to investigate the relationships between the spectral shifts and ultrastructural changes which occur during light-induced chloroplast development. After primary photoconversion, the Shibata shift was observed as a change from 680 to 670 nm in the chlorophyllide absorption maximum. When 1.5 nm ATP was added to the incubation medium the maximum was 675 nm even after 3.5 hours of illumination. Difference spectra for this effect indicate ATP inhibition of the Shibata shift. Two bands with maxima at 682 and 669 nm can be used to fit spectra of both ATP-treated and control etioplasts, the estimated proportions of chlorophyllide 682 being 36% and 6%, respectively. Quantitative analysis of electron micrographs of the etioplasts showed that the frequency of untransformed prolamellar bodies was also higher in the presence of ATP (73% untransformed compared to 22% in the absence of ATP). A similar correlation was observed when transformation was measured for two etioplast fractions which show the shift to different extents. These results imply that the Shibata shift and prolamellar body transformation are related events, both being inhibited by the presence of ATP. ATP may therefore have an important role in regulating the early stages of plastid development.

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PubMed1975

The Effect of ATP on the Photoconversion of Protochlorophyllide in Isolated Etioplasts of Zea mays.

The transformation of protochlorophyllide (PChle) into chlorophyllide (Chle) has been studied in isolated etioplasts from Zea mays. ATP (1.5mm) prevented the transformation of photoconvertible PChle 650 to PChle 630 in aged etioplasts. Curve analysis indicated that the ATP effect on photoconvertibility could be entirely accounted for by changes in the proportions of PChle 630 and PChle 650 and examination of the isolated pigments revealed that only unphytylated PChle could be activated for photoconversion by ATP. In etioplasts aged for 5 hours, ATP also stimulated photoconversion of PChle 630 into Chle 670. The process was temperature-sensitive and involved PChle 650 and Chle 680 as intermediates. AMP alone had no effect, but inhibited ATP retardation of PChle loss. ADP had a similar but lesser effect than ATP. The ADP response, but not the ATP response, was considerably enhanced in the presence of an ATP-generating system (phosphoenolpyruvate/pyruvate kinase). UTP, GTP, and CTP gave 40 to 50% of the ATP response with intact etioplasts. In envelope-free etioplasts, ATP gave the greatest response but the other nucleotides were now 80% as effective as ATP. After primary photoconversion, ATP stimulated resynthesis of PChle 650. It is proposed that ATP both gives the holochrome the ability to bind to the PChle molecule and enables additional association of the pigment-protein complex to form PChle 650.

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PubMed1974

The Galactolipid, Phospholipid, and Fatty Acid Composition of the Chloroplast Envelope Membranes of Vicia faba. L.

The galactolipid, phospholipid, and fatty acid composition of chloroplast envelope membrane fractions isolated from leaves of Vicia faba L. has been determined. The major lipids in this fraction are: monogalactosyldiglyceride, 29%; digalactosyldiglyceride, 32%; phosphatidylcholine, 30%; and phosphatidylglycerol 9%. The lipid composition of the chloroplast envelope membranes is qualitatively similar to that of the lamellar membranes isolated from the same plastids, but the proportion of each lipid present is very different. The total galactolipid to total phospholipid ratio was 1.6: 1 in the envelope and 11.1: 1 in the lamellae. The monogalactosyldiglyceride-digalactosyl-diglyceride ratio was 0.9: 1 in the envelope and 2.4: 1 in the lamellae. Both membranes lack phosphatidylethanolamine.Linolenic acid is the major fatty acid in the envelope lipids representing 63% of the total fatty acid, whereas in the lamellae it represents 83%. The same fatty acids are present in both the envelope and lamellar lipids except the trans-Delta(3)-hexadecenoic acid, which is confined to the lamellar lipids, particularly the phospholipid fraction.A quantitative comparison of the lipid and fatty acid compositions of the envelope with those of mitochondrial and microsomal fractions indicates that the chloroplast envelope has a composition intermediate between that of the chloroplast lamellae and these extrachloroplastic membranes.

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PubMed1974

Lipid biosynthesis in green leaves of developing maize.

Successive leaf sections from the base to the tip of rapidly developing leaves of 7-day-old maize (Zea mays var. Kelvedon Glory), grown in the light, utilized acetate for fatty acid biosynthesis in a very divergent manner. Basal regions of the leaf containing proplastids synthesized insignificant proportions of unsaturated fatty acids and appreciable proportions of fatty acids with 20 or more carbon atoms. An increase in the light intensity during incubations with acetate-1-(14)C resulted in very little enhancement of either total or polyunsaturated fatty acid biosynthesis in this tissue.When the distal leaf sections, containing mesophyll chloroplasts with well developed grana and bundle sheath chloroplasts without grana, were incubated with acetate at 150 ft-c and 30 C, approximately 30% of the newly synthesized fatty acids were unsaturated (mainly 18: 1 and 18: 2). At 2800 ft-c and 20 C, 60% of the fatty acids were unsaturated and the total synthesis of fatty acids increased 4-fold. No detectable amount of fatty acids with 20 or more carbon atoms were synthesized in this morphologically mature tissue, and the proportions of newly synthesized fatty acids more closely resembled the endogenous fatty acids in the immature tissue.Only 4% of the newly synthesized fatty acids were 18: 3 but most of this was incorporated into monogalactolipid. In the distal sections, 20 to 25% of the newly synthesized fatty acids in monogalactolipid were 18: 3 compared with the endogenous proportions of 85%. The differences in the composition of the newly synthesized fatty acids and the endogenous fatty acids appear to be related very largely to the low rate of 18: 3 biosynthesis from 18: 1 and 18: 2. Phosphatidyl choline and phosphatidyl ethanolamine, with lower proportions of 18: 3, contained radioactive fatty acids which resembled the endogenous composition more closely.Phosphatidyl choline was quantitatively the most important acyl lipid synthesized under both light conditions. In addition, there was considerable stimulation of acetate incorporation into phosphatidyl glycerol and monogalactolipid, especially in the morphologically most mature regions of the tissue at the higher light intensity.

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PubMed1973

Plastid differentiation, acyl lipid, and Fatty Acid changes in developing green maize leaves.

Plastid differentiation, acyl lipid, and fatty acid composition have been followed in successive 2-cm sections from the base (youngest tissue) to the tip (oldest tissue) of green Zea mays (maize) leaves grown under a normal diurnal light regime. Although the youngest cells (0-4 cm from the leaf base) had only proplastids with one or two grana, they contained chlorophylls a and b, monogalactosyldiglyceride, digalactosyldiglyceride, sulfolipid, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. In the more mature sections, the plastids increased in size 5-fold, and differentiation into mesophyll and bundle-shealth chloroplasts had occurred. Concomitantly, the levels of all the lipids increased with the exception of phosphatidylcholine and phosphatidylethanolamine which decreased. With increasing cell maturity, the percentage of linolenic acid increased in all the individual acyl lipids, but palmitic acid remained constant in phosphatidylcholine, phosphatidylethanolamine, and sulfolipid. The Delta(3t)-hexadecenoic acid was only detectable in the phosphatidylglycerol of the most mature maize tissue.

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PubMed1972

Amino Acid Biosynthesis by Isolated Chloroplasts during Photosynthesis.

The pool sizes of the common amino acids in purified intact chloroplasts from Vicia faba L. were measured (nanomoles per milligram chlorophyll). The three amino acids present in the highest concentrations were glutamate, aspartate, and threonine. Alanine, serine, and glycine were each present at levels between 15 and 20 nanomoles per milligram chlorophyll and 13 other amino acids were detectable at levels below 10.Only aspartate, alanine, glycine, serine, threonine, and lysine became labeled during photosynthetic (14)CO(2) fixation by isolated chloroplasts: the label in aspartate represented over 60% of the total (14)C found in the amino acids. Glutamate dehydrogenase, glutamate-oxaloacetate, and glutamate-pyruvate aminotransferases were present in the chloroplasts, but no other transferase activities from glutamate could be detected. The chloroplasts were able to synthesize a total of 17 other protein amino acids from either alanine or aspartate, but no synthesis of leucine by aminotransferase reactions could be detected. The synthesis of aspartate was studied in more detail. The enzyme systems required for the generation of oxaloacetate from triose phosphate were virtually absent from the chloroplasts but present in the leaf cytoplasmic fraction. Addition of either a leaf "cytoplasmic" fraction or an oxaloacetate generating system resulted in an increased proportion of the total (14)C fixed being found in the amino acid fraction during photosynthetic (14)CO(2) fixation.It is suggested that the supply of oxaloacetate from the cytoplasm is one of the important factors controlling the synthesis of amino acids by the chloroplast.

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