A Mutant of Arabidopsis Thaliana Deficient in Chloroplast Dicarboxylate Transport Activity
Somerville, Shauna Christine
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https://hdl.handle.net/2142/71571
Description
Title
A Mutant of Arabidopsis Thaliana Deficient in Chloroplast Dicarboxylate Transport Activity
Author(s)
Somerville, Shauna Christine
Issue Date
1981
Department of Study
Agronomy
Discipline
Agronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Plant Physiology
Abstract
The photorespiratory pathway in C3 plants is comprised of reactions occurring in three organelles, chloroplasts, peroxisomes, and mitochondria. This pathway therefore provides a unique opportunity for studying transport systems and their role in mediating interactions between organelles. This study is concerned with the characterization and physiological analysis of a mutant of Arabidopsis thaliana (L.) Heyn. defective in dicarboxylate transport across the chloroplast envelope. This mutant was recovered in a screen for photorespiratory mutants and exhibits the growth requirement for high CO(,2) (1% CO(,2), balance air) common to such mutants. This growth requirement is inherited as a simple, recessive, nuclear mutation and the locus defined by this mutation has been designated dct (dicarboxylate transport). Evidence used to support the conclusion that the altered photorespiratory phenotype in the mutant was due to a defect in chloroplast dicarboxylate transport was obtained using both indirect and direct measures of transport. Chloroplast reactions dependent upon transport for a supply of substrates were assayed. Glutamate synthase and glutamate-oxaloacetate aminotransferase were assayed on intact chloroplasts and found missing in the mutant. Transport activity was also measured directly using the silicone oil filter centrifugation technique. Malate, 2-oxoglutarate, glutamate and aspartate uptake were severely reduced or missing in chloroplasts from the mutant.
The loss of dicarboxylate transport activity in the chloroplast envelope disrupted photorespiratory metabolism, apparently by limiting chloroplast glutamate synthase activity. The consequent reduction of glutamate supply restricted glycine formation and ammonia refixation in the photo-respiratory carbon and nitrogen cycles.
The mutant plants are capable of normal growth under nonphotorespiratory conditions. Thus the primary physiological function of the chloroplast dicarboxylate transporter appears to be its role in linking fluxes though the photorespiratory carbon and nitrogen cycles. The shuttling of reducing equivalents between the chloroplast and the cytoplasm via malate/oxaloacetate or malate/aspartate shuttle systems is a function of minor importance.
The mutant has also been useful in delineating the specificity of the dicarboxylate transporter. Glutamine transport is normal in the mutant suggesting this compound is not carried on the same transporter as malate, aspartate, glutamate or 2-oxoglutarate. This result contradicts reports based on kinetic studies of spinach and pea chloroplasts.
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