| Popis: |
The aim of the work described in this thesis was to investigate the role and importance of the glucose transporter in the chloroplast envelope. I believed that further characterisation would contribute to our knowledge of transitory starch degradation at night, and to the control of carbon partitioning. I have supplied tracer quantities of [U-14C]glucose and [U-14C]glycerol in the dark to leaves selected from a diverse range of species, and demonstrated that the net flux in the cytosol, during starch degradation in the dark, is glycolytic, so that a gluconegoenic flux from triose-phosphates to sucrose does not occur. This provides evidence that a flucose transporter, rather than the phosphate translocator, is active in the chloroplast envelope for the synthesis of sucrose, and suggests that glucose transport across the chloroplast envelope is found throughout the plant kingdom. I have demonstrated, using the technique of silicon oil centrifugation, that chloroplasts of wild-type Arabidopsis thaliana and tobacco take up glucose but do not take up maltose from the external medium, that glucose transport in Arabidopsis is unaffected by ATP. I have also shown, by pre-labelling starch with 14CO2, that wild-type Arabidoposis chloroplasts export glucose, but not maltose, during starch degradation in the dark. These results imply that maltose is not an important final product of starch degradation in these species. I used transgenic tobacco plants, with altered levels of the triose-phosphate translocater and little impact on carbon fixation, to investigate the role of hexose transport in carbon partitioning. Careful measurements of phosphate and glucose uptake in chloroplasts of antisense and wild-type plants revealed that there was no alteration in glucose uptake, and that the endogenous wild type capacity may be sufficiently high to account for a compensatory flux in glucose transport. I hypothesised that a mutant of Arabidopsis (sex1), with chloroplasts deficient in glucose transport and accumulating starch, did so through increased starch synthesis at night. To support this, I found no differences in the maximal catalytic activities of amylases, starch phosphorylase, phosphoglucomutase, glucokinase and phosphofructokinase (ATP) in chloroplasts isolated from dark-acclimated leaves of sex1 and the wild type. |
