Agrobacterium tumefaciens is a niche-constructing biotroph that exploits host plant metabolites.We combined metabolomics, transposon-sequencing (Tn-seq), transcriptomics, and reverse genetics to characterize A. tumefaciens pathways involved in the exploitation of resources from the Solanum lycopersicum host plant.Metabolomics of healthy stems and plant tumors revealed the common (e.g. sucrose, glutamate) and enriched (e.g. opines, c-aminobutyric acid (GABA), c-hydroxybutyric acid (GHB), pyruvate) metabolites that A. tumefaciens could use as nutrients. Tn-seq and transcriptomics pinpointed the genes that are crucial and/or upregulated when the pathogen grew on either sucrose (pgi, kdgA, pycA, cisY) or GHB (blcAB, pckA, eno, gpsA) as a carbon source. While sucrose assimilation involved the Entner-Doudoroff and tricarboxylic acid (TCA) pathways, GHB degradation required the blc genes, TCA cycle, and gluconeogenesis. The tumorenriched metabolite pyruvate is at the node connecting these pathways. Using reverse genetics, we showed that the blc, pckA, and pycA loci were important for aggressiveness (tumor weight), proliferation (bacterial charge), and/or fitness (competition between the constructed mutants and wild-type) of A. tumefaciens in plant tumors.This work highlighted how a biotroph mobilizes its central metabolism for exploiting a wide diversity of resources in a plant host. It further shows the complementarity of functional genome-wide scans by transcriptomics and Tn-seq to decipher the lifestyle of a plant pathogen.