Knockout of mitochondrial dicarboxylate carriers AtDIC1 and AtDIC3 affects germination, biomass accumulation, and carbon partitioning in Arabidopsis thaliana

KEY MESSAGE: AtDIC1 and AtDIC3 make distinct, partially overlapping contributions to mitochondrial metabolism, influencing germination performance, carbon allocation, and redox balance in Arabidopsis thaliana. The mitochondrial dicarboxylate carrier (DIC) is essential for the exchange of dicarboxylic acids across the inner mitochondrial membrane. Although three DIC homologues (AtDIC1-3) are present in Arabidopsis thaliana, the functional roles of AtDIC1 and AtDIC3 remain poorly understood. In this study, we characterized atdic1 and atdic3 single and double mutants to elucidate their physiological significance. Loss of either isoform delayed seed germination under both control and abiotic stress conditions. This delay was more pronounced in atdic1, particularly on sucrose-free medium, suggesting a requirement for AtDIC1 in the mobilization of endogenous carbon reserves. During vegetative development, the atdic3 mutant showed reduced primary root elongation under salt and osmotic stress, whereas atdic1 displayed normal or enhanced root growth under salinity, revealing distinct stress-response roles. Notably, all mutant lines exhibited increased shoot biomass compared to the wild type, supported by enhanced photosynthetic efficiency, improved chlorophyll re-oxidation, and a shift in carbon allocation toward starch at the expense of sucrose. Double mutant analysis showed no additive effects, suggesting that these isoforms do not act synergistically. Metabolomic and redox profiling demonstrated that atdic3 disruption specifically decreased the NADH/NAD+ ratio and increased NADPH/NADP+ levels, coupled with amino acid accumulation. Conversely, atdic1 mutants showed more substantial disruptions in sugar and ascorbate metabolism. Although AtDIC2 was upregulated in all mutant backgrounds, it provided only partial functional compensation. Our findings indicate that AtDIC1 and AtDIC3 fulfill distinct yet partially overlapping roles in coordinating mitochondrial metabolism with plant growth and environmental adaptation.