Disruption of both chloroplastic and cytosolic FBPase genes results in a dwarf phenotype and important starch and metabolite changes in Arabidopsis thaliana

Lack of chloroplastic FBPase induces a dramatic reduction in plant development, while loss of the cytosolic enzyme increases the starch content without affecting the phenotype. Inactivation of both enzymes causes a wide range of metabolite changes.In this study, evidence is provided for the role of fructose-1,6-bisphosphatases (FBPases) in plant development and carbohydrate synthesis and distribution by analysing two Arabidopsis thaliana T-DNA knockout mutant lines, cyfbp and cfbp1, and one double mutant cyfbp cfbp1 which affect each FBPase isoform, cytosolic and chloroplastic, respectively. cyFBP is involved in sucrose synthesis, whilst cFBP1 is a key enzyme in the Calvin-Benson cycle. In addition to the smaller rosette size and lower rate of photosynthesis, the lack of cFBP1 in the mutants cfbp1 and cyfbp cfbp1 leads to a lower content of soluble sugars, less starch accumulation, and a greater superoxide dismutase (SOD) activity. The mutants also had some developmental alterations, including stomatal opening defects and increased numbers of root vascular layers. Complementation also confirmed that the mutant phenotypes were caused by disruption of the cFBP1 gene. cyfbp mutant plants without cyFBP showed a higher starch content in the chloroplasts, but this did not greatly affect the phenotype. Notably, the sucrose content in cyfbp was close to that found in the wild type. The cyfbp cfbp1 double mutant displayed features of both parental lines but had the cfbp1 phenotype. All the mutants accumulated fructose-1,6-bisphosphate and triose-phosphate during the light period. These results prove that while the lack of cFBP1 induces important changes in a wide range of metabolites such as amino acids, sugars, and organic acids, the lack of cyFBP activity in Arabidopsis essentially provokes a carbon metabolism imbalance which does not compromise the viability of the double mutant cyfbp cfbp1.