The plant pathogen Pseudomonas syringae pv. tomato strain DC3000 (DC3000) enters leaves to grow in the extracellular space between cells (the apoplast). The mechanisms by which DC3000 recognizes the apoplast environment and acquires nutrients needed for growth are not well understood. One factor that may be involved in acquisition of nutrients while DC3000 is growing in this environment is the DC3000 dctA1 gene, which encodes a C₄-dicarboxylate transporter. The role of the DctA1 transporter in acquisition of different dicarboxylates was investigated by comparing growth of a dctA1 mutant with wild type DC3000 in apoplast-like media with different carbon sources. The dctA1 mutant failed to grow when the dicarboxylates malate, fumarate, or succinate were the only carbon sources present. This indicates that DctA1 is required for uptake of these carbon sources in culture, and suggests that DctA1 plays an important role in acquisition of these nutrients when DC3000 is growing in the plant apoplast. DC3000 genome sequence analysis also revealed that adjacent to the dctA1 gene are genes that may play a role in recognition of the apoplast environment and encode a homologue of PhoP-PhoQ two component system, associated with virulence in many gram negative pathogens. Expression analysis of the phoP-phoQ operon showed that expression was higher in DC3000 cells grown in apoplast-like media than in cells grown in rich medium. These results suggest that PhoP and PhoQ may be required in the plant apoplast environment, and could be involved in inducing expression of virulence genes. Further analysis of the PhoP-PhoQ system in recognizing the apoplast environment and the DctA1 transporter in uptake of nutrients in the apoplastic space will lead to a better understanding of how DC3000 causes disease on plants.