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Biomedical subjects

W M Laetsch

Publications and source records attributed to W M Laetsch.

9 recordsLinked to original sources
PubMed1974

Structure and function of developing barley plastids.

Five different regions of the first foliage leaf of etiolated barley seedlings were studied with respect to leaf growth, plastid growth and replication, differentiation of etioplasts, and conversion of etioplasts into chloroplasts upon illumination. Ultrastructural changes of the plastids were correlated with chlorophyll synthesis and development of photosynthetic activity as measured by (14)CO(2) incorporation and O(2) evolution. The first foliage leaf has greater linear growth over a longer period of time in the dark than in the light. Only the bottom two regions (4 and 5) are still growing in the 5-day etiolated leaf. Region 4 grows by cell elongation, and region 5 grows by both cell division and elongation. Plastids in all five regions of the leaf are capable of enlarging when exposed to light. This is true both for the intact plant and for excised sections. Plastid replication occurs predominantly in the younger regions of the leaf (regions 3, 4, and 5). The amount of chlorophyll synthesized by different regions in the intact plant is significantly higher (3-40 times) than that made by excised sections. Ultrastructural changes occurring in each region when excised sections are illuminated were classified into five stages involving increased membrane synthesis and appression into grana, and these changes were correlated with the first appearance of photosynthetic activity. The earliest detectable photosynthetic activity occurs in region 1 after 2 hours of illumination when chloroplasts show only a few overlaps in the thylakoids. Plastids in younger regions of the leaf require up to 24 hours of light to form grana and develop photosynthetic activity. Plastids in each region of the leaf are in different stages of development when photosynthesis is initiated, indicating that development of photosynthetic activity is not strictly correlated with a certain stage of plastid development. Membrane appression is not indicative of photosynthetic activity since overlaps are formed in the dark, but it was always present when photosynthetic activity was detectable. Likewise, there does not appear to be any strict correlation between the presence of chlorophyll and membrane appression. These results show that the particular structural and functional correlations that can be made depend to a large degree on age of the tissue.

Journal Article↗
PubMed1974

Formation of C-Labeled Alanine from Pyruvate during Short Term Photosynthesis in a C(4) Plant.

Large amounts of alanine are produced in the first few seconds of photosynthesis in Portulaca oleracea L. The normal precursor-product relationship (phosphoglyceric acid --> pyruvate --> alanine) does not appear to operate in this species since labeling in pyruvate precedes that in phosphoglyceric acid. Pulse-chase experiments show that the alanine is rapidly metabolized. After a 6-second pulse of (14)CO(2), the percentage of (11)C in alanine drops more than 30% in the first 10 seconds of a (12)CO(2) chase period. The percentage of (14)C in the other early-labeled photosynthetic products, aspartate and malate, also decreases during the (12)CO(2) chase. The decrease of label in these compounds is concomitant with an increase in the labeling of sucrose and alanine, which in this case is formed via phosphoglyceric acid. Randomization of label within alanine increases gradually throughout the 2-minute chase.Alpha-keto acids accounts for up to 20% of the total (14)CO(2) incorporated by young leaf tissue. Pyruvate alone accounts for at least 65% of the radioactivity in the keto acid fraction in both young and mature leaves. Other early-labeled keto acids are oxaloacetate and, in young tissue only, hydroxyphenylpyruvate. We propose that, in Portulaca, pyruvate is one of the earliest-labeled photosynthetic products and serves as the immediate precursor of alanine.

Journal Article↗
PubMed1972

Induction and regulation of chloroplast replication in mature tobacco leaf tissue.

Chloroplast replication was induced in mature tobacco leaf tissue (Nicotiana tabacum L.) by culturing leaf discs on a sterile medium composed of salts and sucrose. Chloroplast replicaton is greatly enhanced by the addition of kinetin to this medium. Kinetin also enhances cell enlargement, but cell division does not occur. Chloroplast replication is nonsynchronous and proceeds most rapidly when the cell enlargement rate decreases. Chloroplast replication is light-dependent, but cell enlargement occurs in both light and dark. Chloroplast replication resumes when discs cultured in the dark are returned to the light. It appears that chloroplast replication is related to cell expansion. The possibility of inducing synchronous replication of chloroplasts in tobacco cells is discussed.

Journal Article↗
PubMed1968

Control of senescence in rumex leaf discs by gibberellic Acid.

The kinetics of chlorophyll and protein decomposition and the effect of gibberellic acid (GA) were examined in senescing leaf discs of Rumex crispus and R. obtusifolius. Loss of Rumex total chlorophyll proceeds at a slow rate for about 2 days followed by a period of rapid logarithmic decline. Chlorophyll b is lost at a slightly faster rate than chlorophyll a during senescence in discs as well as in situ. GA causes a complete cessation of net chlorophyll and protein degradation for several days in Rumex, in contrast to the incomplete senescence inhibition generally observed with cytokinins. GA is fully effective even when added at the middle of the logarithmic phase of chlorophyll loss. Senescence inhibition by GA is apparently gradually reversed upon GA removal. The cytokinins, kinetin and 6-benzylaminopurine, were also effective in Rumex leaf discs, indicating that the senescence retarding effect was not restricted to the gibberellins.

Journal Article↗
PubMed1967

Regulation of senescence in bean leaf discs by light and chemical growth regulators.

The senescence of excised discs of primary leaves of Phaseolus vulgaris, L., var. Red Kidney was followed by measuring the net breakdown of protein and chlorophyll. The chemical growth regulators indoleacetic acid, 2,4-dichlorophenoxy-acetic acid, gibberellic acid, kinetin, and 6-benzylaminopurine were relatively ineffective in retarding senescence in this tissue. White light, on the other hand, was very effective in senescence retardation. The response to light did not have the characteristics of a low energy (phytochrome) response and was blocked by concentrations of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea which inhibited photosynthesis in the leaf discs. The light-induced retardation of senescence was concluded to be dependent on photosynthesis.

Journal Article↗
PubMed1965

Kinetin-induced chloroplast maturation in cultures of tobacco tissue.

Cultured tobacco tissue possesses proplastids capable of differentiating into mature chloroplasts. Kinetin (6-furfurylaminopurine) is a specific requirement for this differentiation; the absence of this compound results in a blockage of the formation of grana. The possibility that kinetin exerts a direct effect upon chloroplast differentiation is considered.

Cell Differentiation↗