Intramolecular ¹³C pattern in hexoses from autotrophic and heterotrophic C₃ plant tissues

The stable carbon isotope ¹³C is used as a universal tracer in plant eco-physiology and studies of carbon exchange between vegetation and atmosphere. Photosynthesis fractionates against ¹³CO₂ so that source sugars (photosynthates) are on average ¹³C depleted by 20% compared with atmospheric CO₂. The carbon isotope distribution within sugars has been shown to be heterogeneous, with relatively ¹³C-enriched and ¹³C-depleted C-atom positions. The ¹³C pattern within sugars is the cornerstone of ¹³C distribution in plants, because all metabolites inherit the ¹³C abundance in their specific precursor C-atom positions. However, the intramolecular isotope pattern in source leaf glucose and the isotope fractionation associated with key enzymes involved in sugar interconversions are currently unknown. To gain insight into these, we have analyzed the intramolecular isotope composition in source leaf transient starch, grain storage starch, and root storage sucrose and measured the site-specific isotope fractionation associated with the invertase (EC 3.2.1.26) and glucose isomerase (EC 5.3.1.5) reactions. When these data are integrated into a simple steady-state model of plant isotopic fluxes, the enzyme-dependent fractionations satisfactorily predict the observed intramolecular patterns. These results demonstrate that glucose and sucrose metabolism is the primary determinant of the ¹³C abundance in source and sink tissue and is, therefore, of fundamental importance to the interpretation of plant isotopic signals.