Knockdown of glycine decarboxylase complex alters photorespiratory carbon isotope fractionation in Oryza sativa leaves

The influence of reduced glycine decarboxylase complex (GDC) activity on leaf atmosphere CO₂ and ¹³CO₂ exchange was tested in transgenic Oryza sativa with the GDC H-subunit knocked down in leaf mesophyll cells. Leaf measurements on transgenic gdch knockdown and wild-type plants were carried out in the light under photorespiratory and low photorespiratory conditions (i.e. 18.4 kPa and 1.84 kPa atmospheric O₂ partial pressure, respectively), and in the dark. Under approximately current ambient O₂ partial pressure (18.4 kPa pO₂), the gdch knockdown plants showed an expected photorespiratory-deficient phenotype, with lower leaf net CO₂ assimilation rates (A) than the wild-type. Additionally, under these conditions, the gdch knockdown plants had greater leaf net discrimination against ¹³CO₂ (Δ_o) than the wild-type. This difference in Δ_o was in part due to lower ¹³C photorespiratory fractionation (f) ascribed to alternative decarboxylation of photorespiratory intermediates. Furthermore, the leaf dark respiration rate (R_d) was enhanced and the ¹³CO₂ composition of respired CO₂ (δ¹³C_Rd) showed a tendency to be more depleted in the gdch knockdown plants. These changes in R_d and δ¹³C_Rd were due to the amount and carbon isotopic composition of substrates available for dark respiration. These results demonstrate that impairment of the photorespiratory pathway affects leaf ¹³CO₂ exchange, particularly the ¹³C decarboxylation fractionation associated with photorespiration.