The chloroplastic 2‐oxoglutarate/malate transporter has dual function as the malate valve and in carbon/nitrogen metabolism

Abstract

SUMMARYTransport of dicarboxylates across the chloroplast envelope plays an important role in transferring carbon skeletons to the nitrogen assimilation pathway and exporting reducing equivalent to the cytosol to prevent photo-inhibition (the malate valve). It was previously shown that the Arabidopsis plastidic 2-oxoglutarate/ malate transporter (AtpOMT1) and the general dicarboxylate transporter (AtpDCT1) play crucial roles at the interface between carbon and nitrogen metabolism. However, based on the in vitro transport properties of the recombinant transporters, it was hypothesized that AtpOMT1 might play a dual role, also functioning as an oxaloacetate/malate transporter, which is a crucial but currently unidentified component of the chloroplast malate valve. Here, we test this hypothesis using Arabidopsis T-DNA insertional mutants of AtpOMT1. Transport studies revealed a dramatically reduced rate of oxaloacetate uptake into chloroplasts isolated from the knockout plant. CO 2 -dependent O 2 evolution assays showed that cytosolic oxaloacetate is efficiently transported into chloroplasts mainly by AtpOMT1, and supported the absence of additional oxaloacetate transporters. These findings strongly indicate that the high-affinity oxaloacetate transporter in Arabidopsis chloroplasts is AtpOMT1. Further, the knockout plants showed enhanced photo-inhibition under high light due to greater accumulation of reducing equivalents in the stroma, indicating malfunction of the malate valve in the knockout plants. The knockout mutant showed a phenotype consistent with reductions in 2-oxoglutarate transport, glutamine synthetase/glutamate synthase activity, subsequent amino acid biosynthesis and photorespiration. Our results demonstrate that AtpOMT1 acts bi-functionally as an oxaloacetate/malate transporter in the malate valve and as a 2-oxoglutarate/malate transporter mediating carbon/nitrogen metabolism.

Systems Analysis of a Maize Leaf Developmental Gradient Redefines the Current C4 Model and Provides Candidates for Regulation

“…Taken together, these results suggested a revised model of the C 4 cycle in maize ( Figure 5D): After PEP is carboxylated to OAA, it is moved to the chloroplast, either in exchange with malate through DiT1 (Kinoshita et al, 2011) or in exchange with Asp through DiT2 (Renne et al, 2003), which are produced by malate dehydrogenase and AspAT, respectively, in the chloroplast. The major AspAT in maize is predicted to be chloroplast localized.…”

Intracellular Redox Compartmentation and ROS-Related Communication in Regulation and Signaling

“…These systems allow the indirect transfer of redox equivalents across membranes at much more rapid rates than can occur through the direct transport of pyridine nucleotides, which are relatively large and highly charged metabolites. Dicarboxylate transporters can exchange redox equivalents as malate and oxaloacetate (Kinoshita et al, 2011). Such shuttles have long been considered to play key roles in several areas of redox homeostasis.…”

Interactions of C4 Subtype Metabolic Activities and Transport in Maize Are Revealed through the Characterization of DCT2 Mutants

“…In Arabidopsis thaliana, dicarboxylate transporters OMT1 and DCT1 were shown to be essential for photorespiration (Kinoshita et al, 2011); At-DCT2 (the ortholog of Zm-DCT2), however, is not expressed in photosynthetic leaf tissue in Arabidopsis (Renné et al, 2003). To examine the potential involvement of DCT2 in photorespiration in maize, we measured photorespiratory pool sizes and quantified isotopic labeling from 13 CO 2 in dct2-1 and wild-type plants (Supplemental Figure 5).…”