Engineering plants for elevated CO2:: A relationship between starch degradation and sugar sensing

In the future, plants will have additional CO2 for photosynthesis. However, plants do not take maximal advantage of this additional CO2 and it has been hypothesized that end product synthesis limitations and sugar sensing mechanisms are important in regulating plant responses to increasing CO2. Attempts to increase end product synthesis capacity by engineering increased sucrose-phosphate synthase activity have been generally, but not universally, successful. It was found that plants benefited from a two- to three-fold increase in SPS activity but a 10-fold increase did not increase yield. Despite the success in increasing yield, increasing SPS did not increase photosynthesis. However, carbon export from chloroplasts was increased during the day and reduced at night (when starch provides carbon for sucrose synthesis. We develop here a hypothesis that starch degradation is closely sensed by hexokinase because a newly discovered pathway required for starch to sucrose conversion that involves maltose is one of few metabolic pathways that requires hexokinase activity.