Overexpression of pyrophosphatase leads to increased sucrose degradation and starch synthesis, increased activities of enzymes for sucrose-starch interconversions, and increased levels of nucleotides in growing potato tubers

Abstract

Overexpression of inorganic pyrophosphatase (PPase) from Escherichia coli in the cytosol of plants (ppa 1 plants) leads to a decrease of inorganic pyrophosphate (PPi; U. Sonnewald, 1992, Plant J 2: 571-581). The consequences for sucrose-starch interconversions have now been studied in growing potato (Solanum tuberosum L. cv. Desirée) tubers. Sucrose is degraded via sucrose synthase and UDP-glucose pyrophosphorylase in growing tubers, and it was expected that the low PPi in the ppa 1 transformants would restrict the mobilisation of sucrose and conversion to starch. Over-expression of PPase resulted in an accumulation of sucrose and UDP-glucose, and decreased concentrations of hexose phosphates and glycerate-3-phosphate in growing ppa 1 tubers. Unexpectedly, the rate of degradation of [14C] sucrose was increased by up to 30%, the rate of starch synthesis was increased, and the starch content was increased by 20-30% in ppa 1 tubers compared to wild-type tubers. Reasons for this unexpectedly efficient conversion of sucrose to starch in the ppa 1 tubers were investigated. (i) The transformed tubers contained increased activities of several enzymes required for sucrose-starch interconversions including two- to three-fold more sucrose synthase and 60% more ADP-glucose pyrophosphorylase. They also contained 30-100% increased activities of several glycolytic enzymes and amylase, increased protein, and unaltered or slightly decreased starch phosphorylase, acid invertase and mannosidase. (ii) The transformants contained higher pools of uridine nucleotides. As a result, although the UDP-glucose pool is increased two- to threefold, this does not lead to a decrease of UTP or UDP. (iii) The transformants contained twofold larger pools of ATP and ADP, and ADP-glucose was increased by up to threefold. In stored ppa 1 tubers, there were no changes in the activities of glycolytic enzymes, and nucleotides did not increase. It is concluded that in growing tubers PPi has a wider-significance than just being an energy donor for specific reactions in the cytosol. Increased rates of PPi hydrolysis also affect general aspects of cell activity including the levels of nucleotides and protein. Possible ways in which PPi hydrolysis could affect these processes are discussed.

H+‐pyrophosphatase IbVP1 promotes efficient iron use in sweet potato [Ipomoea batatas (L.) Lam.]

“…Pyrophosphatases generate energy and release free Pi, which affects plant carbon metabolism (Gaxiola et al ., 2012; Geigenberger et al ., 1998). The altered partitioning of photoassimilate in tobacco and potato expressing an Escherichia coli cytosolic inorganic pyrophosphatase gene (Sonnewald, 1992) and the suppression of sucrose synthesis in the cotyledons of avp Arabidopsis mutant (Ferjani et al ., 2011) support the significant role of PPases in sucrose metabolism.…”

Oil and Protein Accumulation in Developing Seeds Is Influenced by the Expression of a Cytosolic Pyrophosphatase in Arabidopsis

“…Constitutive expression leads to pleiotropic growth defects and increased accumulation of Suc, hexoses, nucleotide sugars, and transient starch in photosynthetic tissues. In potato (Solanum tuberosum) tubers, a nonphotosynthetic sink tissue, constitutive expression of heterologous PPiase activity increases the levels of soluble sugars including Suc, but in contrast to photosynthetic tissues, increased levels of nucleotide sugars are accompanied by increased rates of Suc degradation and starch accumulation (Geigenberger et al, 1998). The increased levels of hexoses and nucleotide sugars in Figure 7.…”

“…We acknowledge that the compositional phenotype of immature siliques or seed of the lo15571 mutant and GY1::At1g01050 events is somewhat reminiscent of the tuber phenotype of potato plants expressing heterologous PPiase enzymes in the cytosol (Geigenberger et al, 1998). However, it should be noted that we are altering the expression of an endogenous cytosolic pyrophosphate enzyme (instead of introducing a heterologous enzyme that may operate in an unregulated mode).…”

Phloem Import and Storage Metabolism Are Highly Coordinated by the Low Oxygen Concentrations within Developing Wheat Seeds

“…Sugars and glycolytic intermediates were measured enzymatically as in Geigenberger et al (1998). Total amino acids were measured as in Geigenberger et al (1998).…”

“…Sugars and glycolytic intermediates were measured enzymatically as in Geigenberger et al (1998). Total amino acids were measured as in Geigenberger et al (1998). Nucleotides, including UDP-Glc were analyzed by HPLC as described in Geigenberger et al (1997).…”