Inhibition of de Novo Pyrimidine Synthesis in Growing Potato Tubers Leads to a Compensatory Stimulation of the Pyrimidine Salvage Pathway and a Subsequent Increase in Biosynthetic Performance

Abstract

Pyrimidine nucleotides are of general importance for many aspects of cell function, but their role in the regulation of biosynthetic processes is still unclear. In this study, we investigate the influence of a decreased expression of UMP synthase (UMPS), a key enzyme in the pathway of de novo pyrimidine synthesis, on biosynthetic processes in growing potato (Solanum tuberosum) tubers. Transgenic plants were generated expressing UMPS in the antisense orientation under the control of the tuber-specific patatin promoter. Lines were selected with markedly decreased expression of UMPS in the tubers. Decreased expression of UMPS restricted the use of externally supplied orotate for de novo pyrimidine synthesis in tuber tissue, whereas the uridine-salvaging pathway was stimulated. This shift in the pathways of UMP synthesis was accompanied by increased levels of tuber uridine nucleotides, increased fluxes of [ 14 C]sucrose to starch and cell wall synthesis, and increased amounts of starch and cell wall components in the tubers, whereas there were no changes in uridine nucleotide levels in leaves. Decreased expression of UMPS in tubers led to an increase in transcript levels of carbamoylphosphate synthase, uridine kinase, and uracil phosphoribosyltransferase, the latter two encoding enzymes in the pyrimidine salvage pathways. Thus, the results show that antisense inhibition of the de novo pathway of pyrimidine synthesis leads to a compensatory stimulation of the less energyconsuming salvage pathways, probably via increased expression and activity of uridine kinase and uracil phosphoribosyltransferase. This results in increased uridine nucleotide pool levels in tubers and improved biosynthetic performance.

TheArabidopsis onset of leaf death5Mutation of Quinolinate Synthase Affects Nicotinamide Adenine Dinucleotide Biosynthesis and Causes Early Ageing

“…Such a mechanism was previously observed for the enzymes of the salvage pathway of pyrimidine nucleotides following the antisense inhibition of an enzyme of the de novo pathway of pyrimidine biosynthesis (Geigenberger et al, 2005). The salvage of nicotinamide, released from NAD, starts with deamidation by nicotinamidases to nicotinate (Wang and Pichersky, 2007) and subsequent conversion by nicotinate phosphoribosyltransferase (NaPRT).…”

“…Next to that, the salvage pathway was shown to be activated and necessary for response to osmotic stress in Arabidopsis (Wang and Pichersky, 2007). The compensation for reduced de novo synthesis of NAD by up-regulation of the salvage pathway of pyridine nucleotides is a mechanism that has also been observed for pyrimidine biosynthesis (Geigenberger et al, 2005).…”

Uridine-Ribohydrolase Is a Key Regulator in the Uridine Degradation Pathway of Arabidopsis

“…Additionally, nucleotides are components of secondary metabolites like caffeine, cAMP, cGMP, and cytokinins (Boldt and Zrenner, 2003;Stasolla et al, 2003;Schmidt et al, 2004;Geigenberger et al, 2005). By reviewing the recently published work on nucleotide metabolism, it becomes obvious that many facets of this important biochemical aspect of plant metabolism are still poorly understood.…”

Decreasing the Mitochondrial Synthesis of Malate in Potato Tubers Does Not Affect Plastidial Starch Synthesis, Suggesting That the Physiological Regulation of ADPglucose Pyrophosphorylase Is Context Dependent

“…Of these reactions, although some of the control of starch synthesis resides in the plastidial phosphoglucomutase reaction (Fernie et al, 2001b), the AGPase reaction harbors the highest proportion of control within the linear pathway (Sweetlove et al, 1999;Geigenberger et al, 1999Geigenberger et al, , 2004. In addition, considerable control resides in both the Glc-6-P phosphate antiporter (Zhang et al, 2008) and the amyloplastidial adenylate transporter (Tjaden et al, 1998;Zhang et al, 2008) as well as in reactions external to the pathways, such as the amyloplastidial adenylate kinase (Regierer et al, 2002), cytosolic UMP synthase (Geigenberger et al, 2005), and mitochondrial NAD-malic enzyme (Jenner et al, 2001).…”