Figure 4

Scheme of four possible pathways for Ser arising from GL. (1) Ser was synthesized directly from GL-derived 3PG. The carbon backbone in GL was not disrupted and Ser had the same MDV as GL. (2) Ser was synthesized directly from Calvin cycle-derived 3PG, half of which would have an unlabeled carbon atom at position 1 deriving from atmospheric CO₂. The ¹³C enrichment of Ser1 should be diluted regardless of the labeling pattern of GL. (3) Ser was synthesized from GOC through photorespiration. The carbon backbone in GL was recombined during the Calvin cycle and Ser1 was derived from RuBP2. (4) Ser was synthesized from GOC via the GOC cycle, as reported by Zheng et al. In this case, Ser1 originated solely from GL2 and the ¹³C enrichment should be zero when [1, 3-¹³C]GL was used as substrate.