A Role for the Plastidial GPT2 Translocator in the Modulation of Lignin Biosynthesis

Engineering plants with reduced lignin content can result in pleiotropic growth defects. In stems of Arabidopsis plants with reduced expression of hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT), the plastidial glucose 6-phosphate: phosphate co-transporter GPT2 is highly overexpressed, and this coincides with reduced lignin levels and extensive transcriptional and metabolic reprogramming. To explore the potential relationship between GPT2 expression and lignin accumulation, GPT2 transcript levels were evaluated in a suite of Arabidopsis thaliana and Medicago truncatula lignin-defective lines. We also examined lignin levels and composition, and transcriptomic and metabolic profiles in GPT2 loss-of-function, GPT2 overexpression, and wild-type Arabidopsis plants. Loss of GPT2 had no effect on lignin, but its overexpression caused a decrease in stem lignin levels due to reduced accumulation of both guaiacyl and syringyl lignins and their associated monolignol pools. HCT transcript levels were diminished in 35S-GPT2 lines, indicating a potential transcriptional regulatory connection between lignin biosynthesis and GPT2. Based on our transcriptomic and metabolomic analyses, we suggest that GPT2 operates to balance the flux between the biosynthesis of lignin and light-protective phenylpropanoid derivatives.