Potato plants exhibiting combined antisense repression of cytosolic and plastidial isoforms of phosphoglucomutase surprisingly approximate wild type with respect to the rate of starch synthesis

Abstract

Here, we describe results observed on the simultaneous down-regulation of both isoforms of phosphoglucomutase (EC 5.4.2.2; PGM) within potato ( Solanum tuberosum L.) plants. Transgenic plants were created that were inhibited in both activities using the antisense approach. Surprisingly, the metabolic phenotype displayed by these transgenics neither resembles either of the phenotypes observed when only one of the isoforms is inhibited nor displays additive effects of these but is rather more reminiscent of the wild type phenotype. The double transgenic lines display a partial or total recovery in both starch accumulation and starch biosynthetic flux and exhibit wild type levels of intermediary metabolites. Given the dramatic reduction of PGM activity in these lines, and the lack of pleiotropic effects on other enzymes that mediate the sucrose to starch transition, these data strongly suggest a conditional bypass of the normal starch biosynthetic pathway. Based on these data, we present and discuss three models that can be postulated in which the reactions catalysed by the isoforms of PGM can be bypassed. Evaluation of the possible candidates that could mediate this pathway suggests that the uptake of glucose-1-phosphate is the most likely mechanism of alleviating the inhibition of starch synthesis imposed by independent inhibition of either isoform.