Downregulation of pyrophosphate: d-fructose-6-phosphate 1-phosphotransferase activity in sugarcane culms enhances sucrose accumulation due to elevated hexose-phosphate levels |
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Authors: | Margaretha J van der Merwe Jan-Hendrik Groenewald Mark Stitt Jens Kossmann Frederik C Botha |
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Institution: | (1) Institute of Plant Biotechnology, University of Stellenbosch, Merriman Avenue, Stellenbosch, 7602, South Africa;(2) Present address: PlantBio, Postnet Suite 201, P/Bag X6, Cascades, 3202, South Africa;(3) SRDC, PO Box 12050, George Street, Brisbane, QLD, 4003, Australia;(4) Max Planck Institute of Molecular Plant Physiology, Am Muehlenberg 1, Golm, 14476, Germany |
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Abstract: | Analyses of transgenic sugarcane clones with 45–95% reduced cytosolic pyrophosphate: d-fructose-6-phosphate 1-phosphotransferase (PFP, EC 2.7.1.90) activity displayed no visual phenotypical change, but significant
changes were evident in in vivo metabolite levels and fluxes during internode development. In three independent transgenic
lines, sucrose concentrations increased between three- and sixfold in immature internodes, compared to the levels in the wildtype
control. There was an eightfold increase in the hexose-phosphate:triose-phosphate ratio in immature internodes, a significant
restriction in the triose phosphate to hexose phosphate cycle and significant increase in sucrose cycling as monitored by
13C nuclear magnetic resonance. This suggests that an increase in the hexose-phosphate concentrations resulting from a restriction
in the conversion of hexose phosphates to triose phosphates drive sucrose synthesis in the young internodes. These effects
became less pronounced as the tissue matured. Decreased expression of PFP also resulted in an increase of the ATP/ADP and
UTP/UDP ratios, and an increase of the total uridine nucleotide and, at a later stage, the total adenine nucleotide pool,
revealing strong interactions between PPi metabolism and general energy metabolism. Finally, decreased PFP leads to a reduction
of PPi levels in older internodes indicating that in these developmental stages PFP acts in the gluconeogenic direction. The
lowered PPi levels might also contribute to the absence of increases in sucrose contents in the more mature tissues of transgenic
sugarcane with reduced PFP activity. |
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