Davies, Dewi Rhys (1974)
Carbohydrate metabolism in sugar cane chloroplasts.
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Some tropical plants (Hatch-Slack-Kortschak or HSK plants), for example sugar canes, have an unusual leaf morphology in that there are single layers of cells surrounding the vascular bundles. This layer is in turn surrounded by nesophyll cells. The chloroplast in these two types of cell exhibit a structural dimorphism. The mesophyll chloroplasts appear to be 'normal' plant chloroplasts whereas the bundle sheath chloroplasts have no grana and the strona contains large starch grains when the plant is photosynthesizing and this starch is metabolized when the plant is placed in the dark.
Methods for the isolation and separation of these dimorphic chloroplasts have been developed and the carbohydrate metabolism in the two types of organelle investigated. The enzyme activity associated with the soluble protein fractions iron both mesophyll and bundle sheath chloroplasts have been examined. Significant activities of a-amyiase, sucrose synthetase" sucrose phosphate synthetase UDP-D-glucose pyrophosphorylase, ADP-D-glucose trans-glucosylase and pyrophosphatase were associated with both types of organelle whereas those enzymes thought to be concerned in the regulation of starch metabolism, i.e. ADP-D-glucose pyrophosphorylase and starch phosphorylase were localised mainly in the bundle sheath chloroplasts. These results suggest that whereas both types of organelles are able to synthesize sucrose from hexose phosphates, the bundle sheath chloroplasts are specialized for starch storage. The properties and regulation of some of these enzymes have also been examined. The unusual process of CO2 fixation and the general metabolism of starch and sucrose in these plants is discussed.
The galactolipid contents of the mesophyll and bundle sheath chloroplasts were analysed and it was found that the monogalactosyl diglyceride: digalactosyl diglyceride ratio in bundle sheath chloroplasts was lower than in mesophyll chloroplasts. This suggests that the stroma lamellae are rich in digalactosyl diglyceride.
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Institution: University of London, Royal Holloway College (United Kingdom).