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

Abstract

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, an Arabidopsis gene 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, using ltd knockout Arabidopsis lines to explore LTD function. Under continuous dark conditions, where ascorbate degradation is facilitated, LTD transcription 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 in Arabidopsis. The LTD gene 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.