Sugars and starch are the major respiratory substrates in plants, providing both the energy and metabolic precursors needed for growth. We are interested in understanding how the synthesis and degradation of these carbohydrates are regulated to meet the varying requirements of the plant (and humans) in response to changing environmental and developmental demands.
Part of our work focuses on the signal metabolite fructose 2,6-bisphosphate (Fru-2,6-P2) which is an important regulator of carbohydrate metabolism in all eukaryotes. In green plants Fru-2,6-P2contributes to control of photosynthetic carbon assimilation by inhibiting the first step in the pathway of sucrose synthesis in the cytosol. Changes in the amount of Fru-2,6-P2allow the rate of sucrose production to be coordinated with the rate of carbon dioxide assimilation, and also control the partitioning of carbon between photosynthetic products, sucrose and starch.
Exploiting mutant plant lines lacking Fru-2,6-P2, we found that Fru-2,6-P2-deficient plants produce more sucrose and less starch than regular plants, but surprisingly they have normal rates of photosynthesis under steady light conditions. This behaviour demonstrates the flexibility of plant metabolism to adjust to changes in the relative availability of different resources. However, if the lack of Fru-2,6-P2has no appreciable effect on the overall rates of photosynthesis or growth, why do plants continue to produce and degrade this compound?
Plants lacking Fru-2,6-P2take longer to accumulate photosynthetic intermediates, thereby extending the length of the lag phase. This effect is trivial under constant daytime conditions, however, a compromised ability to respond to changes in light intensity becomes more important in conditions in which photosynthesis has to adjust to more frequent environmental variations. Indeed, Fru-2,6-P2-deficient plants grow more slowly than standard plants when exposed to controlled continuously fluctuating light or temperature. Similarly, plants lacking Fru-2,6-P2produce 20–30% fewer seeds than standard plants when grown in natural day/night cycles at ambient temperature, implying a decrease in Darwinian fitness.
Our studies highlight the important role of Fru-2,6-P2 in allowing photosynthesis to respond effectively to rapid changes in the conditions faced by the plant, and indicate that “regulatory” properties may be involved in the speed at which metabolism adjusts to external conditions. In continuously varying natural environments the capacity to respond rapidly in order to optimise the metabolic activities of the system may be critical. These findings suggest that the physiological roles of metabolic regulators should be examined in fluctuating environments rather than the constant conditions used in most current studies.