Photosynthetic metabolism is severely impaired on the parallel reduction of plastidial and cytosolic isoforms of phosphoglucomutase

The aim of this work was to examine the role of phosphoglucomutase (PGM) activities (EC 5.4.2.2) in photosynthetic carbon partitioning. We have previously described the generation of transgenic potato (Solanum tuberosum cv. Desiree) lines expressing either the StcPGM or the StpPGM genes, encoding cytosolic and plastidial isoforms of PGM, in the antisense orientation under the control of the 35S promoter. These studies suggested that the cytosolic isoform of this enzyme might be redundant in the leaf and that another activity may be able to fulfil this role. Here we investigate the effect on photosynthetic metabolism of repressing both isoforms of PGM. This manipulation resulted in a marked reduction in biomass in addition to a restricted photosynthesis, an alteration in carbon partitioning and mild changes in the levels of metabolic intermediates. The results are consistent with the hypothesis that the plastidial PGM can substitute for its cytosolic counterpart during photosynthetic sucrose synthesis.