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 CO(2) and (13)CO(2) 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 O(2) partial pressure, respectively), and in the dark. Under approximately current ambient O(2) partial pressure (18.4 kPa pO(2)), the gdch knockdown plants showed an expected photorespiratory-deficient phenotype, with lower leaf net CO(2) assimilation rates (A) than the wild-type. Additionally, under these conditions, the gdch knockdown plants had greater leaf net discrimination against (13)CO(2) (Δ(o)) than the wild-type. This difference in Δ(o) was in part due to lower (13)C photorespiratory fractionation (f) ascribed to alternative decarboxylation of photorespiratory intermediates. Furthermore, the leaf dark respiration rate (R(d)) was enhanced and the (13)CO(2) composition of respired CO(2) (δ(13)C(Rd)) showed a tendency to be more depleted in the gdch knockdown plants. These changes in R(d) and δ(13)C(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 (13)CO(2) exchange, particularly the (13)C decarboxylation fractionation associated with photorespiration.