Radioisotope ratios discriminate between competing pathways of cell wall polysaccharide and RNA biosynthesis in living plant cells

Plant J. 2007 Oct;52(2):252-62. doi: 10.1111/j.1365-313X.2007.03225.x. Epub 2007 Aug 31.


Cell wall polysaccharides are synthesized from sugar-nucleotides, e.g. uridine 5'-diphosphoglucose (UDP-Glc), but the metabolic pathways that produce sugar-nucleotides in plants remain controversial. To help distinguish between potentially 'competing' pathways, we have developed a novel dual-radiolabelling strategy that generates a remarkably wide range of 3H:14C ratios among the various proposed precursors. Arabidopsis cell cultures were fed traces of D-[1-(3)H]galactose and a 14C-labelled hexose (e.g. D-[U-14C]fructose) in the presence of an approximately 10(4)-fold excess of non-radioactive carbon source. Six interconvertible 'core intermediates', galactose 1-phosphate <--> UDP-galactose <--> UDP-glucose <--> glucose 1-phosphate <--> glucose 6-phosphate <--> fructose 6-phosphate, showed a large decrease in 3H:14C ratio along this pathway from left to right. The isotope ratio of a polysaccharide-bound sugar residue indicates from which of the six core intermediates its sugar-nucleotide donor substrate stemmed. Polymer-bound galacturonate, xylose, arabinose and apiose residues (all produced via UDP-glucuronate) stemmed from UDP-glucose, not glucose 6-phosphate; therefore, UDP-glucuronate arose predominantly by the action of UDP-glucose dehydrogenase rather than through the postulated competing pathway leading from glucose 6-phosphate via myo-inositol. The data also indicate that UDP-galacturonate was not formed by a hypothetical UDP-galactose dehydrogenase. Polymer-bound mannose and fucose residues stemmed from fructose 6-phosphate, not glucose 1-phosphate; therefore GDP-mannose (guanosine 5'-diphosphomannose) arose predominantly by a pathway involving phosphomannose isomerase (via mannose phosphates) rather than through a postulated competing pathway involving GDP-glucose epimerization. Curiously, the ribose residues of RNA did not stem directly from hexose 6-phosphates, but predominantly from UDP-glucose; an alternative to the textbook pentose-phosphate pathway therefore predominates in plants.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / cytology*
  • Carbon / metabolism*
  • Carbon Radioisotopes
  • Cell Survival
  • Cell Wall / chemistry*
  • Cells, Cultured
  • Isotope Labeling
  • Phosphatidylinositols / chemistry
  • Phosphatidylinositols / metabolism
  • Polysaccharides / biosynthesis*
  • Polysaccharides / chemistry
  • RNA, Messenger / metabolism
  • RNA, Plant / biosynthesis*
  • Sucrose / chemistry
  • Sucrose / metabolism
  • Tritium / metabolism*


  • Carbon Radioisotopes
  • Phosphatidylinositols
  • Polysaccharides
  • RNA, Messenger
  • RNA, Plant
  • Tritium
  • Sucrose
  • Carbon