Pathways of starch and sucrose biosynthesis in developing tubers of potato (Solanum tuberosum L.) and seeds of faba bean (Vicia faba L.)

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Abstract
Tissue slices from developing potato tubers (Solanum tuberosum L.) and developing cotyledons of faba bean (Vicia faba L.) were incubated with specifically labelled [13C]glucose and [13C]ribose. Enriched[13C]glucose released from starch granules was analysed by nuclear magnetic resonance (NMR). Spectral analyses were also performed on sucrose purified by high-performance liquid chromatography. In both tissues a low degree of randomisation (< 11 % in potato and < 14% in Vicia) was observed between carbon positions 1 and 6 in glucose released from starch when material was incubated with [13C]glucose labelled in positions 6 and 1, respectively. Similarly, with [2-13C]glucose a low degree of randomisation was observed in position 5. These findings indicate that extensive transport of three-carbon compounds across the amyloplast membrane does not occur in storage organs of either species. This is in agreement with previously published data which indicates that sixcarbon compounds are transported into the plastids during active starch synthesis. When [1-13C]ribose was used as a substrate, 13C-NMR spectra of starch indicated the operation of a classical pentose-phosphate pathway. However, with [2-13C]glucose there was no preferential enrichment in either carbon positions 1 or 3 relative to 4 or 6 of sucrose and starch (glucose). This provides evidence that entry of glucose in this pathway may be restricted in vivo. In both faba bean and potato the distribution of isotope between glucosyl and fructosyl moieties of sucrose approximated 50%. The degree of randomisation within glucosyl and fructosyl moieties ranged between 11 and 19.5%, indicating extensive recycling of triose phosphates.