Loss of Cytosolic Phosphoglucose Isomerase Affects Carbohydrate Metabolism in Leaves and Is Essential for Fertility of Arabidopsis

Carbohydrate metabolism in plants is tightly linked to photosynthesis and is essential for energy and carbon skeleton supply of the entire organism. Thus, the hexose phosphate pools of the cytosol and the chloroplast represent important metabolic resources that are maintained through action of phosphoglucose isomerase (PGI) and phosphoglucose mutase interconverting glucose 6-phosphate, fructose 6-phosphate, and glucose 1-phosphate. Here, we investigated the impact of disrupted cytosolic PGI (cPGI) function on plant viability and metabolism. Overexpressing an artificial microRNA targeted against cPGI (amiR-cpgi) resulted in adult plants with vegetative tissue essentially free of cPGI activity. These plants displayed diminished growth compared with the wild type and accumulated excess starch in chloroplasts but maintained low sucrose content in leaves at the end of the night. Moreover, amiR-cpgi plants exhibited increased nonphotochemical chlorophyll a quenching during photosynthesis. In contrast to amiR-cpgi plants, viable transfer DNA insertion mutants disrupted in cPGI function could only be identified as heterozygous individuals. However, homozygous transfer DNA insertion mutants could be isolated among plants ectopically expressing cPGI. Intriguingly, these plants were only fertile when expression was driven by the ubiquitin10 promoter but sterile when the seed-specific unknown seed protein promoter or the Cauliflower mosaic virus 35S promoter were employed. These data show that metabolism is apparently able to compensate for missing cPGI activity in adult amiR-cpgi plants and indicate an essential function for cPGI in plant reproduction. Moreover, our data suggest a feedback regulation in amiR-cpgi plants that fine-tunes cytosolic sucrose metabolism with plastidic starch turnover.