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Summary: Agrobacterium tumefaciens constructs an ecological niche in its host plant by transferring the T‐DNA from its Ti plasmid into the host genome and by diverting the host metabolism. We combined transcriptomics and genetics for understanding the A. tumefaciens lifestyle when it colonizes Arabidopsis thaliana tumors. Transcriptomics highlighted: a transition from a motile to sessile behavior that mobilizes some master regulators (Hfq, CtrA, DivK and PleD); a remodeling of some cell surface components (O‐antigen, succinoglucan, curdlan, att genes, putative fasciclin) and functions associated with plant defense (Ef‐Tu and flagellin pathogen‐associated molecular pattern‐response and glycerol‐3‐phosphate and nitric oxide signaling); and an exploitation of a wide variety of host resources, including opines, amino acids, sugars, organic acids, phosphate, phosphorylated compounds, and iron. In addition, construction of transgenic A. thaliana lines expressing a lactonase enzyme showed that Ti plasmid transfer could escape host‐mediated quorum‐quenching. Finally, construction of knock‐out mutants in A. tumefaciens showed that expression of some At plasmid genes seemed more costly than the selective advantage they would have conferred in tumor colonization. We provide the first overview of A. tumefaciens lifestyle in a plant tumor and reveal novel signaling and trophic interplays for investigating host–pathogen interactions. [ABSTRACT FROM AUTHOR]

In the tumor-inducing (Ti) Agrobacterium tumefaciens, quorum sensing activates the horizontal transfer of the virulent Ti plasmid. In pure culture, this process can be impaired by the A. tumefaciens BlcC lactonase, whose expression is induced by gamma-aminobutyrate ( GABA). It was therefore hypothesized that host GABA content might modulate quorum sensing and virulence gene dissemination during A. tumefaciens infection., We examined GABA metabolism and transport in Arabidopsis thaliana tumors combining transcriptomic, metabolomic and histological approaches. In addition, using genetically modified plants and bacteria, we evaluated the impact of plant host GABA content on Ti plasmid dissemination., The results showed that GABA and free proline, which acts as an antagonist of GABA uptake in A. tumefaciens, accumulated in wild-type tumors relative to uninfected plant tissues. Moreover, comparisons of tumors induced on Col-0 and her1 plants showed that the increase in the plant GABA : proline ratio was associated with both the upregulated expression of the blcC gene and the decreased dissemination of Ti plasmid in tumor-colonizing A. tumefaciens populations., This work demonstrates experimentally that the variation in the GABA content in plant tumors can interfere with the dissemination of A. tumefaciens Ti plasmids, and therefore highlights plant GABA content as an important trait in the struggle against pathogenic bacteria. [ABSTRACT FROM AUTHOR]