The photosynthetic potential of canola embryos

Asokanthan, Ponita S.; Johnson, Robert W.; Griffith, Marilyn; Król, Marianna

doi:10.1111/j.1399-3054.1997.tb01008.x

Cited by 58 publications

(19 citation statements)

References 17 publications

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“…The low value for V TA (Table IV) may also be associated with the photosynthetic potential of B. napus embryos (76). One can assume that in chloroplasts TA activity is limited to the minimum needed for OPPP flux, because the TA reaction interferes with the regenerative phase of the photosynthetic reduction cycle and produces a futile cycle.…”

Section: Quantification Of the Split Of Carbon Flux Between Glycolysimentioning

confidence: 99%

A Flux Model of Glycolysis and the Oxidative Pentosephosphate Pathway in Developing Brassica napus Embryos

Schwender

Ohlrogge

Shachar‐Hill

2003

Journal of Biological Chemistry

240

252

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Computer simulation of the steady state distribution of isotopomers in intermediates of the glycolysis/OPPP network was used to fit metabolic flux parameters to the experimental data. The observed distribution of label in cytosolic and plastidic metabolites indicated that key intermediates of glycolysis and OPPP have similar labeling in these two compartments, suggesting rapid exchange of metabolites between these compartments compared with net fluxes into end products. Cycling between hexose phosphate and triose phosphate and reversible transketolase velocity were similar to net glycolytic flux, whereas reversible transaldolase velocity was minimal. Flux parameters were overdetermined by analyzing labeling in different metabolites and by using data from different labeling experiments, which increased the reliability of the findings. Net flux of glucose through the OPPP accounts for close to 10% of the total hexose influx into the embryo. Therefore, the reductant produced by the OPPP accounts for at most 44% of the NADPH and 22% of total reductant needed for fatty acid synthesis.Brassica napus (rapeseed, canola) is one of the world's major oilseed crops and is also a well studied model for oilseed metabolism (1-21). The main storage compounds in seeds of B. napus are oil (triacylglycerols) and storage proteins, which are derived from sugars and amino acids taken up from the surrounding endosperm liquid (11,12,22,23). Because of its high oil content and ease of genetic transformation, B. napus has also been a target for metabolic engineering of oil metabolism. However, some attempts to engineer plant oils have had limited success (for a review, see Ref. 24). In order to make advances in improving oil yield and quality, a more detailed understanding of metabolism during seed development is needed. In particular, a number of fundamental metabolic issues remain unresolved. These include the source(s) of reductant and ATP for fatty acid synthesis; the degree to which cytosolic, plastidial, and mitochondrial metabolic fluxes are integrated; and the chief metabolic and transport route(s) by which carbon flows from maternal sources to seed storage products.

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Section: Quantification Of the Split Of Carbon Flux Between Glycolysimentioning

confidence: 99%

A Flux Model of Glycolysis and the Oxidative Pentosephosphate Pathway in Developing Brassica napus Embryos

Schwender

Ohlrogge

Shachar‐Hill

2003

Journal of Biological Chemistry

240

252

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“…Chloroplasts in seeds are characterized by high rates of uncoupled electron transport, a high chlorophyll a to b ratio, and abundant proteins associated with photosystem II (Banerji and Rauf, 1979;Eastmond et al, 1996;Asokanthan et al, 1997). On the other hand, photosynthetic CO 2 fixation has been reported to be low in embryos of oilseed rape (Asokanthan et al, 1997), pea (Flinn, 1985), and soybean (Saito et al, 1989).…”

Section: Embryonic Photosynthesis Provides Energy Supply and Increasementioning

confidence: 99%

“…Their photosynthetic potential to fix carbon is rather low compared with leaves and pods (Harvey et al, 1976;Atkins and Flinn, 1978;Flinn, 1985;Eastmond et al, 1996;Asokanthan et al, 1997). Embryos are therefore predominantly heterotrophic.…”

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confidence: 99%

Energy Status and Its Control on Embryogenesis of Legumes. Embryo Photosynthesis Contributes to Oxygen Supply and Is Coupled to Biosynthetic Fluxes

Rolletschek¹,

Weber²,

Borisjuk³

2003

Plant Physiology

107

105

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Legume seeds are heterotrophic and dependent on mitochondrial respiration. Due to the limited diffusional gas exchange, embryos grow in an environment of low oxygen. O 2 levels within embryo tissues were measured using microsensors and are lowest in early stages and during night, up to 0.4% of atmospheric O 2 concentration (1.1 m). Embryo respiration was more strongly inhibited by low O 2 during earlier than later stages. ATP content and adenylate energy charge were lowest in young embryos, whereas ethanol emission and alcohol dehydrogenase activity were high, indicating restricted ATP synthesis and fermentative metabolism. In vitro and in vivo experiments further revealed that embryo metabolism is O 2 limited. During maturation, ATP levels increased and fermentative metabolism disappeared. This indicates that embryos become adapted to the low O 2 and can adjust its energy state on a higher level. Embryos become green and photosynthetically active during differentiation. Photosynthetic O 2 production elevated the internal level up to approximately 50% of atmospheric O 2 concentration (135 m). Upon light conditions, embryos partitioned approximately 3-fold more [14 C]sucrose into starch. The light-dependent increase of starch synthesis was developmentally regulated. However, steady-state levels of nucleotides, free amino acids, sugars, and glycolytic intermediates did not change upon light or dark conditions. Maturing embryos responded to low O 2 supply by adjusting metabolic fluxes rather than the steady-state levels of metabolites. We conclude that embryogenic photosynthesis increases biosynthetic fluxes probably by providing O 2 and energy that is readily used for biosynthesis and respiration.Growing seeds are sink organs that import assimilates from the phloem, mainly as Suc and amino acids. Their photosynthetic potential to fix carbon is rather low compared with leaves and pods (Harvey et al., 1976;Atkins and Flinn, 1978;Flinn, 1985;Eastmond et al., 1996;Asokanthan et al., 1997). Embryos are therefore predominantly heterotrophic. Accordingly, their ATP supply is mainly covered by mitochondrial respiration, the regulation of which is therefore important for storage activity and maturation. Mitochondrial respiration is regulated at different levels like carbohydrate status, light, and temperature. Especially the availability of respiratory substrates is often regarded as a limiting factor. Because the overall sugar levels within embryo tissues are quite high at all stages, its availability should not limit mitochondrial respiration. During early seed growth (prestorage phase), embryos contain high levels of hexoses due to high invertase activity (Weber et al., 1995a). Later on, invertase decreases and mainly Suc is imported. The switch from hexoses to Suc is accompanied by cell differentiation and storage product synthesis (Borisjuk et al., 1995;Weber et al., 1998). The analysis of the spatial distribution of both Glc and Suc concentrations in growing broad bean (Vicia faba) cotyledons detected specific patter...

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“…Browse and Slack (1985) examined plastids from green (linseed) and nongreen (safflower) seeds and concluded that light could provide cofactors for FAS in green plastids but was unlikely to contribute to the carbon economy of seeds. Asokanthan et al (1997) studied plastid structure in Brassica napus embryos as well as measured O 2 evolution and CO 2 incorporation rates. Their conclusion was that embryo chloroplasts are shade-like, photoheterotrophic, and use light to produce some NADPH and ATP for FAS.…”

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confidence: 99%

The Capacity of Green Oilseeds to Utilize Photosynthesis to Drive Biosynthetic Processes

2004

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Seeds of many plant species are green during embryogenesis. To directly assess the influence of light on the physiological status of green oilseeds in planta, Brassica napus and soybean (Glycine max) seeds were rapidly dissected from plants growing in the light or dark. The activation state of malate dehydrogenase, which reflects reduced thioredoxin and NADP/NADPH ratios, was found to be as high in seeds exposed to light as in leaves and to decrease in the dark. Rubisco was highly activated (carbamylated) in both light and dark, most likely reflecting high seed CO 2 concentrations. Activities of Rubisco and phosphoribulokinase were sufficient to account for significant refixation of CO 2 produced during B. napus oil biosynthesis. To determine the influence of light on oil synthesis in planta, siliques on intact plants in full sunlight or detached siliques fed 3 H 2 O were partly covered with aluminum foil. Seeds from light and dark sections were analyzed, and fatty acid accumulation was found to be higher in seeds exposed to light than seeds from dark sections. The spectrum of light filtering through silique walls and the pigment composition of developing B. napus embryos were determined. In addition to a low chlorophyll a/b ratio, the carotenoid pigments of seeds can provide additional capture of the green light that filters through siliques. Together, these results demonstrate that even the low level of light reaching seeds plays a substantial role in activating light-regulated enzymes, increasing fatty acid synthesis, and potentially powering refixation of CO 2 .Oilseeds provide a major source of calories for human consumption and are an increasingly significant source of renewable industrial materials. Interest in engineering enhanced seed oil quantity and quality has prompted efforts to better understand the biosynthesis of oil and other storage products of seeds. Several of the major oilseed crops (e.g. soybean [Glycine max], rapeseed, cotton, and linseed) produce seeds that are green during their development, and this fact has prompted questions regarding the contributions of seed photosynthesis to oilseed metabolism.In leaf chloroplasts, fatty acid synthesis (FAS) is light dependent and utilizes ATP and reducing power generated by photosynthesis (Browse et al., 1986;Roughan and Ohlrogge, 1996). By contrast, plastids isolated from heterotrophic tissues, such as pea roots and cauliflower floral buds, require externally supplied ATP and/or reducing power for FAS (Mö hlmann et al., 1994;Xue et al., 1997). Similarly, plastids isolated from developing chlorophylless seeds of safflower and castor bean are heterotrophic in nature and require added ATP or reducing equivalents (Browse and Slack, 1985;Smith et al., 1992). The situation is more complicated in photosynthetic oilseeds that are green during development. Although these seeds are predominantly sink tissues, they contain chloroplasts with the thylakoid structures and enzymes of typical photosynthetic machinery. However, rather than producing carbon for ex...

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The photosynthetic potential of canola embryos

Cited by 58 publications

References 17 publications

A Flux Model of Glycolysis and the Oxidative Pentosephosphate Pathway in Developing Brassica napus Embryos

A Flux Model of Glycolysis and the Oxidative Pentosephosphate Pathway in Developing Brassica napus Embryos

Energy Status and Its Control on Embryogenesis of Legumes. Embryo Photosynthesis Contributes to Oxygen Supply and Is Coupled to Biosynthetic Fluxes

The Capacity of Green Oilseeds to Utilize Photosynthesis to Drive Biosynthetic Processes

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