Metabolic Flux Analysis and the Characterization of dct2 in Maize

At the heart of plant productivity is the balance of carbon assimilation through photosynthesis and its partitioning including steps that respire carbon as well those that produce protein, oil, carbohydrates or other compounds. Measuring dynamic plant activity such as the fluxes through cellular metabolism requires isotopic tracers that can assess atom transitions without disturbing pathway operation. In the case of C4 photosynthesis in grasses, the movement of metabolites is coordinated through two specialized leaf cell types, mesophyll and bundle sheath to concentrate CO2 around Rubisco. Despite the importance of this process our quantitative understanding of the involved steps remains limited. In maize, ZmDCT2 has been proposed to function as a plastid-localized malate transporter and is preferentially expressed in BS cells. Here, the role of ZmDCT2 in maize leaves was characterized using Ac-tagged mutant alleles. Our results indicate that ZmDCT2 enables the transport of malate into the BS chloroplast.  Isotopic labeling experiments show that the loss of ZmDCT2 resulted in markedly different metabolic network operation and dramatically reduced biomass production. In the absence of a functioning malate shuttle, dct2 mutants survive through the enhanced use of the PEPCK carbon shuttle pathway that in wild-type maize accounts for approximately 25% of the photosynthetic activity. The results emphasize the importance of malate transport during C4 photosynthesis, define the role of a primary malate transporter in BS cells, and support a model for carbon exchange between BS and M cells in maize.