Microbodies in Higher Plants

To investigate if pollen possesses glyoxysomal function, we analyzed the activities of isocitrate lyase and malate synthase genes. Because the activities of these enzymes were exceedingly low in pollen extracts, we constructed fusion genes encoding j3-glucuronidase (CUS) that are regulated by isocitrate lyase or malate synthase promoters from Brassica napus L. to increase the sensitivity of our assays. Expression of the fusion genes in transgenic tobacco was qualitatively similar to that of the endogenous genes; CUS activity was low in dry seeds, maximal in seedlings, and very low or undetectable in leaves, indicating that the promoters are regulated correctly. We showed that isocitrate lyase and malate synthase genes are active at specific stages of pollen development and that their activities are not enhanced during pollen germination in transgenic tobacco. We also confirmed that the endogenous genes are active by showing that the corresponding mRNAs could be detected in pollen at specific stages of development. The activation of the isocitrate lyase and malate synthase genes suggests that glyoxysomal function is induced during pollen development.Glyoxysomes are specialized peroxisomes that play a pivotal role in the postgerminative growth of oilseed plants (Beevers, 1979;Huang et al., 1983;Trelease and Doman, 1984). These organelles contain the glyoxylate cycle and /3-oxidation enzymes that catalyze the net conversion of fatty acids into succinate and, thus, account for the ability of plants to utilize lipids as a carbon source for carbohydrate synthesis. The carbohydrates generated as a result of these reactions serve as nutrients for seedlings until they become photosynthetically active.Glyoxysomal function is induced at several stages of the plant life cycle when peroxisomes with different metabolic roles are converted into glyoxysomes. The induction of glyoxysomal function is generally monitored using two key glyoxylate cycle enzymes, isocitrate lyase and malate synthase, which are required for glyoxysomal function and are associated exclusively with glyoxysomes (reviewed by Olsen and

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confidence: 99%

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confidence: 99%

Isocitrate Lyase and Malate Synthase Genes from Brassica napus L. Are Active in Pollen

Zhang

Laudencia‐Chingcuanco

Comai

et al. 1994

“…The inducibility of peroxisomal glycolate dehydrogenase contrasts with the constitutive synthesis of hydroxypyruvate reductase (13,16), another peroxisomal enzyme (8). Light exposure has no effect on hydroxypyruvate reductase levels (13,16) suggesting that in contrast to higher plants (1,10,17), in Euglena the reversible and irreversible portions of the glycolate pathway are independently regulated.…”

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confidence: 99%

Nutritional Regulation of Organelle Biogenesis in Euglena

Horrum

Schwartzbach²

1981

Exposure of dark grown resting Euglena to ethanol produced a transient increase in the specific activity of the glyoxysomal enzyme malate synthase. Enzyme specific activity increased during the first 24 hours of ethanol treatment and then declined. Light exposure or malate addition failed to increase enzyme specific activity. The increase and decrease in enzyme specific activity represented changes in the amount of active enzyme. In both wild type cells and the plastidless mutant W3BUL, enzyme levels were always higher in the dark than in the light.The specific activity of the peroxisomal enzyme glycolate dehydrogenase began to increase 24 hours after dark grown resting Eugkena were exposed to light. Ethanol, but not malate, prevented the increase and promoted a decrease in glycolate dehydrogenase levels. Cycloheximide produced a decline in enzyme levels similar to the decline produced by ethanol addition. Glycolate dehydrogenase was present in the plastidless mutant W3BUL indicating that it is coded in the nucleus and synthesized on cytoplasmic ribosomes. Streptomycin, a specific inhibitor of chloroplast protein synthesis and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, an inhibitor of photosynthetic CO2 fixation, inhibited the photoinduction of glycolate dehydrogenase while having no effect on the photoinduction of NADP dependent glyceraldehyde-3-phosphate dehydrogenase, another light induced, nuclear coded, cytoplasmically synthesized enzyme. Taken together, these results suggest that microbodies are continuaDly synthesized in resting Euglena and their enzyme complement is determined through substrate induction of glyoxysomal and peroxisomal enzymes.

“…The fat-storing, potentially photosynthetic cotyledon cells of mustard and similar dicots produce two functional types of microbodies, the glyoxysome which is involved in storage fat mobilization, and the (leaf) peroxisome which is involved in photorespiratory glycolate metabolism (3,8). The ontogenetic relationship between glyoxysomes and peroxisomes in this type of cells has not yet been investigated by a direct biochemical approach, mainly because so far there is no method available for a physical separation of these two microbody types.…”

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confidence: 99%

“…The ontogenetic relationship between glyoxysomes and peroxisomes in this type of cells has not yet been investigated by a direct biochemical approach, mainly because so far there is no method available for a physical separation of these two microbody types. In general, microbodies from various sources band at a buoyant density close to 1.25 kg I' on isopycnic sucrose density gradients (3,8). This unusually high density compared to other organelles results from the fact that the microbody matrix space is freely accessible to sucrose so that the organelle's density approaches the over-all density of its matrix proteins on a sucrose gradient (8,19).…”

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confidence: 99%

Density of Microbodies on Sucrose Gradients during Phytochrome-mediated Glyoxysome Peroxisome Transformation in Cotyledons of Mustard Seedlings

Hong¹,

Schöpfer²

1980

The microbodies extracted from the cotyledons of mustard seedlings (Sinapis alba L.) form two bands (at 1.18 kilograms per liter together with the mitochondria, and at 1.24 kilograms per liter) on conventional isopycnic sedimentation density gradients. The artifactual co-banding of part of the microbodies with the mitochondria can be prevented by using flotation gradients. Using this procedure, a systematic investigation revealed no effect of seedlng age and irradiation (far red or white Light) on the density of the microbody population (1.242 ± 0.002 kilograms per liter). Thus, although light, through phytochrome, induces conspicuous changes in their enzyme composition the microbodies appear as a homogeneous population of constant density on a sucrose gradient.