Metabolism of L-threonate, an ascorbate degradation product, requires a protein with L-threonate metabolizing domains inArabidopsis

L-Threonate is one of the major degradation products of ascorbate in plants. While bacteria can utilize L-threonate as a sole carbon source by converting it to dihydroxyacetone phosphate, a glycolysis intermediate, through a three- or four-step metabolic pathway, the corresponding processes in plants remain uncharacterized. Remarkably, anArabidopsisgene encodes a unique protein containing domains homologous to all three enzymes involved in the bacterial three-step pathway. We designated this protein as L-threonate metabolizing domains (LTD) and investigated its functional role in plant L-threonate metabolism. Despite extensive efforts, recombinant expression of LTD was unsuccessful, likely due to its large protein size. Therefore, a reverse genetic approach was employed, usingltdknockoutArabidopsislines to explore LTD function. Under continuous dark conditions, where ascorbate degradation is facilitated,LTDtranscription was significantly upregulated, leading to increased L-threonate dehydrogenase (LtnD) activity. Knockout lines of LTD exhibited no detectable LtnD activity under both light and dark conditions, alongside elevated levels of L-threonate compared to wild-type plants. These results indicate that LTD is essential for L-threonate metabolism inArabidopsis. TheLTDgene is highly conserved among land plants but is absent in green algae, providing a hypothesis that the rise in ascorbate concentrations during plant evolution necessitated a more active metabolism of ascorbate degradation products.