Fast sucrose production is critical for full photosynthetic efficiency

Photosynthesis converts light into chemical energy and is the prerequisite for life on earth. It is divided into the light reaction, which captures light energy and dark reaction, which fixes atmospheric CO2. The polysaccharide starch represents the primary end product of the photosynthesis and thus the main energy storage. In all photoautotrophic eukaryotes these reactions are localized in the chloroplast. During the night when photosynthesis is inactive, starch is degraded and exported as maltose from the chloroplast to fuel cellular processes. However, not maltose but sucrose is the main transport form of chemically bound energy to non-green, heterotopic plant tissues. Therefore a biochemical conversion in needed to synthesize sucrose, which takes place exclusively in the cytosol.

HFSP Long-Term Fellow Hans-Henning Kunz and colleagues
authored on Mon, 20 October 2014

In a collaborative research effort at the University of Cologne and the University of California, San Diego that was partially funded by HFSP, Arabidopsis thaliana mutant plants with dramatically decreased cytosolic (cPGi) Phosphoglucoisomerase enzyme activity were isolated and investigated. It was discovered that even an exclusively cytosolic process such as sucrose production can be limiting for photosynthesis. The decrease of cPGi enzyme activity caused a negative feedback on the chloroplast, lowering photosynthetic efficiency. Moreover,  an imbalance of plant carbohydrate metabolism with an inhibition of starch breakdown accompanied by lower plant biomass yield was observed. Intriguingly, leaf sucrose levels were only slightly lower than in the wild-type control which underpins the flexibility of plant carbon metabolism. However, flexibility of carbohydrate metabolism could not be confirmed for all plant tissues. When expanding the investigations to T-DNA insertion loss-of-function mutants, it was found that a complete loss of cPGI enzyme activity results in poor fertility and no homozygous mutants could be obtained. In other words, the aforementioned flexibility and genetic redundancy in this aspect of plant carbon metabolism is restricted to the leaf tissue.

With today’s goal to increase photosynthetic efficiency in plants or to design artificial photosynthetic devices, a systemic understanding of all processes involved in natural photosynthesis is mandatory. This also includes all downstream reactions that impact the primary pathway. The results gained from this research add to the understanding of plant carbon metabolism and can now be incorporated into the current photosynthesis model. Moreover, it is of highest importance for plant breeding to unravel all genetic components involved in plant reproduction. These data reveal that phosphoglucoisomerase activity represents a critical component during this developmental stage.

Reference

Loss of Cytosolic Phosphoglucose Isomerase Affects Carbohydrate Metabolism in Leaves and Is Essential for Fertility of Arabidopsis. Hans-Henning Kunz, Shirin Zamani-Nour, Rainer E. Häusler, Katja Ludewig, Julian I. Schroeder, Irina Malinova, Joerg Fettke, Ulf-Ingo Flügge and Markus Gierth.  Published online before print August 2014, doi: http:/​/​dx.​doi.​org/​10.​1104/​pp.​114.​241091 Plant Physiology October 2014 vol. 166 no. 2 753-765.

Pubmed link

Plant Physiology link