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 to cause of the reduced sensitivity to O2. Analysis of CO2 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 O2 sensitivity. The kcat 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 concentrations 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-bisphosphate. We conclude that the reduced fructose-1,6-bisphosphatase activity restricts the mutant plant''s capacity for source synthesis and this leads to reduced or reversed O2 sensitivity.