Reduced Cytosolic Fructose-1,6-Bisphosphatase Activity Leads to Loss of O2 Sensitivity in a Flaveria linearis Mutant

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Abstract
The mutant plant of Flaveria linearis characterized by Brown et al. (Plant Physiol. 81: 212-215) was studied to determine the cause of the reduced sensitivity to O(2). Analysis of CO(2) assimilation metabolites of freeze clamped leaves revealed that both 3-phosphoglycerate and ribulose 1,5-bisphosphate were high in the mutant plant relative to F. linearis with normal O(2) sensitivity. The k(cat) of ribulose-1,5-bisphosphate carboxylase (RuBPCase) was equal in all plant material tested (range 18-22 s(-1)) indicating that no tight binding inhibitor was present. The degree of RuBPCase carbamylation was reduced in the mutant plant relative to the wild-type plant. Since 3-phosphoglycerate was high in the mutant plant and photosynthesis did not exhibit properties associated with RuBPCase limitations, we believe that the decarbamylation of RuBPCase was a consequence of another lesion in photosynthesis. Fructose 1,6-bisphosphate and its precursors, such as the triose phosphates, were in high concentration in the mutant plant relative to the wild type. The concentrations of the product of the fructose 1,6-bisphosphatase reaction, fructose 6-phosphate, and its isomer, glucose 6-phosphate, were the same in both plants. We found that the mutant plant had up to 75% less cytosolic fructose 1,6-bisphosphatase activity than the wild type but comparable levels of stromal fructose 1,6-bisphosphatase. We conclude that the reduced fructose-1,6-bisphosphatase activity restricts the mutant plant's capacity for sucrose synthesis and this leads to reduced or reversed O(2) sensitivity.

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