Engineering plants for elevated CO(2): a relationship between starch degradation and sugar sensing

Area of Science:

• Plant physiology
• Biochemistry
• Molecular biology

Background:

• Plants face limitations in utilizing elevated atmospheric carbon dioxide (CO2) for photosynthesis.
• End-product synthesis limitations and sugar sensing mechanisms are hypothesized to regulate plant responses to increased CO2.
• Engineering sucrose-phosphate synthase (SPS) activity is one strategy to enhance plant productivity.

Purpose of the Study:

• To investigate the impact of enhanced sucrose-phosphate synthase (SPS) activity on plant growth and carbon metabolism under elevated CO2.
• To explore the role of starch degradation and hexokinase in plant responses to increased CO2.
• To test the hypothesis that starch degradation is sensed by hexokinase via a novel maltose-dependent pathway.

Main Methods:

• Genetic engineering to increase sucrose-phosphate synthase (SPS) activity in plants.
• Measurement of plant yield, photosynthetic rates, and carbon export.
• Analysis of metabolic pathways involved in starch to sucrose conversion.

Main Results:

• A two- to three-fold increase in SPS activity improved plant yield, but a 10-fold increase did not.
• Increased SPS activity enhanced carbon export from chloroplasts during the day but reduced it at night.
• Elevated CO2 and altered SPS activity did not directly increase overall photosynthesis rates.
• A newly discovered pathway for starch to sucrose conversion involving maltose and requiring hexokinase activity was identified.

Conclusions:

• SPS activity is a critical factor, but only up to a certain threshold, in determining plant yield under elevated CO2.
• Starch degradation and its sensing via hexokinase may play a significant role in regulating plant carbon metabolism and response to increased CO2.
• The identified maltose-dependent pathway highlights a novel regulatory mechanism in plant carbon partitioning.