Carbon assimilation and metabolism in potato leaves deficient in plastidial phosphoglucomutase

Abstract

We have previously described the generation of transgenic potato ( Solanum tuberosum L. cv. Desiree) lines expressing the S. tuberosum plastidial phosphoglucomutase ( StpPGM) gene in the antisense orientation under the control of the 35S promoter and characterised heterotrophic metabolism in these lines [E. Tauberger et al. (2000) Plant J 23:43-53]. The aim of the current work was to examine the role of plastidial phosphoglucomutase (pPGM, EC 5.4.2.2) in photosynthetic carbon partitioning. Here we characterise the metabolism of leaves of the same lines and show that reducing the activity of this enzyme has profound effects on carbon partitioning, characterised by a strong (up to 50%) reduction in the rate of starch accumulation accompanied by a minor reduction in the rate of sucrose accumulation. Gas-exchange and (14)CO(2)-feeding experiments revealed that the transgenic lines exhibited a decreased rate of photosynthesis and a corresponding reduced assimilation of radiolabel into starch, even in lines exhibiting only a minor decrease in pPGM activity. In illuminated leaves, decreasing the amount of pPGM resulted in decreased amounts of triose-phosphates, hexose-phosphates and inorganic phosphate without changes in the level of 3-phosphoglycerate. Most importantly, the deduced ratio of phosphoesters to inorganic phosphate increased, indicating the likelihood that photosynthesis was phosphate-limited in these lines. Determination of a more complete metabolic profile of leaf material from these lines revealed a large number of changes in the levels of amino and organic acids, consistent with an inhibition of triose-phosphate export from the chloroplast, but little change in the energy status of the transformants. We discuss the implications of these changes with respect to both consequences of inhibiting starch synthesis and of inhibiting photosynthesis, and conclude that a high activity of pPGM is required both to prevent phosphate limitation of photosynthesis and for co-ordination of plastidially and cytosolically compartmented photosynthetic metabolism.

Most of ADP·glucose linked to starch biosynthesis occurs outside the chloroplast in source leaves

“…EC 2.7.7.27) is the only enzyme catalyzing the synthesis of ADPG, and it acts as the major limiting step of the gluconeogenic process (8)(9)(10). This popular view, consistent with the idea that the chloroplast is a complete photosynthetic unit that can produce starch from the atmospheric CO 2 (11,12), appears to be supported by a wide variety of mutant and transgenic lines in which enzymes and transport machineries have been modulated (13)(14)(15)(16)(17). However, there is a growing volume of evidence that points out inconsistencies with such mechanism and previews the likelihood of alternative gluconeogenic pathway(s) in which the ADPG linked to starch biosynthesis is produced in the cytosol (18)(19)(20).…”

Arabidopsis Responds to Alternaria alternata Volatiles by Triggering Plastid Phosphoglucose Isomerase-Independent Mechanisms

“…GAP and 3PGA were measured as described by Vogt et al (1998) and Lytovchenko et al (2002), respectively. The amino acid contents were determined as described by Loiret et al (2009).…”

Alterations in Cytosolic Glucose-Phosphate Metabolism Affect Structural Features and Biochemical Properties of Starch-Related Heteroglycans

“…For this reason, we have studied the glycans in leaves from cPGMoverexpressing lines from potato. Properties of the heteroglycans were compared with those isolated from wild-type plants and also from transgenic lines that, due to the expression of the transgene in the antisense orientation, possess decreased levels of the cPGM Lytovchenko et al, 2002a).…”