Differentiation of Dicarboxylate Transporters in Mesophyll and Bundle Sheath Chloroplasts of Maize

Abstract

;In NADP-malic enzyme-type C 4 plants such as maize (Zea mays L.), efficient transport of oxaloacetate and malate across the inner envelope membranes of chloroplasts is indispensable. We isolated four maize cDNAs, ZmpOMT1 and ZmpDCT1 to 3, encoding orthologs of plastidic 2-oxoglutarate/malate and general dicarboxylate transporters, respectively. Their transcript levels were upregulated by light in a cell-specific manner; ZmpOMT1 and ZmpDCT1 were expressed in the mesophyll cell (MC) and ZmpDCT2 and 3 were expressed in the bundle sheath cell (BSC). The recombinant ZmpOMT1 protein expressed in yeast could transport malate and 2-oxoglutarate but not glutamate. By contrast, the recombinant ZmpDCT1 and 2 proteins transported 2-oxoglutarate and glutamate at similar affinities in exchange for malate. The recombinant proteins could also transport oxaloacetate at the same binding sites as those for the dicarboxylates. In particular, ZmpOMT1 transported oxaloacetate at a higher efficiency than malate or 2-oxoglutarate. We also compared the activities of oxaloacetate transport between MC and BSC chloroplasts from maize leaves. The K m value for oxaloacetate in MC chloroplasts was one order of magnitude lower than that in BSC chloroplasts, and was close to that determined with the recombinant ZmpOMT1 protein. Southern analysis revealed that maize has a single OMT gene. These findings suggest that ZmpOMT1 participates in the import of oxaloacetate into MC chloroplasts in exchange for stromal malate. In BSC chloroplasts, ZmpDCT2 and/or ZmpDCT3 were expected to import malate that is transported from MC.

Consequences of C4 Differentiation for Chloroplast Membrane Proteomes in Maize Mesophyll and Bundle Sheath Cells

“…TC282039 was closer to A. thaliana Dit1 (AT5G64290), TC282161 was closer to Dit2 (AT5G64280), and they preferentially accumulated in the M envelopes (BS/M ϭ 0.2) and BS envelopes (BS/M ϭ 1.7), respectively. This observation is consistent with previous observations of cell-specific transcript accumulation of Dit1,2 in sorghum (125) and maize (41). Consequently a two-translocator model for Dit1,2 was proposed (126) in which M-enriched Dit1 imports 2-oxoglutarate into chloroplast in exchange for malate, whereas BS enriched Dit2 imports malate and exports glutamate (for a review, see Ref.…”

“…However, no systematic comparison of envelope proteomes of differentiated BS and M maize chloroplasts has been carried out, but major differences can be expected in e.g. translocators of carbohydrates (34,41).…”

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

“…DICARBOXYLIC ACID TRANSPORTER1 (DCT1), DCT2, and 2-OXOGLUTARATE/MALATE TRANSPORTER1 (OMT1) (Taniguchi et al, 2004) are predicted to be chloroplast localized (Taniguchi et al, 2004;Majeran and van Wijk, 2009) and differentially expressed between the M and BS cells (Li et al, 2010;Chang et al, 2012;Tausta et al, 2014), but their metabolic function to date is unclear. The DCT2 protein and transcript are enriched in the BS of maize (Bräutigam et al, 2008;Li et al, 2010), and expression profiles across the developmental leaf gradient coincide with other photosynthetic genes (Li et al, 2010).…”

Exogenous Malate Application Inhibits the Photochemical Activity of Photosystem II in Rice Leaves

“…In C4 plants like maize, CO2 is primarily fixed to form malate in MC and then transferred into bundle sheath cells (BSC) by the C4 pathway to generate NADPH and release CO2, which is reduced by Calvin cycle. Because BSC in maize lack PSII photochemical activity, the necessary NADPH for CO2 reduction mainly comes from MC through malate transport (Taniguchi et al, 2004;Majeran et al, 2008). In the leaves of C3 plants, CO2 fixation and reduction are not compartmentalized as in C4 plants, because BSC in C3 plants do not photosynthesize.…”