Control of carbohydrate metabolism in a starchless mutant of Arbidopsis thaliana

The biochemical regulation of photosynthate partitioning was investigated in a starchless mutant (TC7) of Arabidopsts thaliana (L.) Henyh, that was deficient in chloroplast phosphoglucomutase (Caspar et al. 1985. Plant Physiol. 79: 11–17). Plants were raised at 20°C with a 20 h light and 4 h dark period, so that the growth rates of the mutant and wild type were similar. Two or 3 isoforms of phosphoglucomutase were separated by ion-exchange chromatography using mutant and wild type leaf preparations, respectively. Initial rate kinetics of all isoforms were similar. Light-saturated photosynthetic oxygen evolution rates of the mutant and wild type were 224 and 302 nmol g^-1 chlorophyll h^-1, respectively. Starch, sucrose and hexose concentrations were unchanged in wild type leaves after a dark to light transition, whereas sucrose and hexose increased in mutant leaves. Hexose-phosphates accumulated in both genotypes in the light, although the steady-state leaf concentrations of glucose 6-phosphate were 3-fold higher in mutant than in wild type samples. Fructose 2,6-bisphosphate and glucose 1,6-bisphosphate were lower in the mutant than in the wild type at the end of the dark period when mutant leaves were depleted of carbohydrates. Levels of UTP were lower in the mutant than in the wild type, possibly indicating that growth conditions had induced phosphate limited photosynthesis. These results are discussed in relation to the regulation of photosynthetic carbon metabolism.