Genome-wide identification of tomato xylem sap fitness factors for plant-pathogenic Ralstonia

Authors: GEORGOULIS, STRATTON - University Of California, Davis; SHALVARJIAN, KATIE - University Of California; Helmann, Tyler; HAMILTON, CORRI - University Of Wisconsin; CARLSON, HANS - Lawrence Berkeley National Laboratory; DEUTSCHBAUER, ADAM - Lawrence Berkeley National Laboratory; LOWE-POWER, TIFFANY - University Of California, Davis

Submitted to: mSystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2021
Publication Date: 11/2/2021
Citation: Georgoulis, S., Shalvarjian, K., Helmann, T.C., Hamilton, C., Carlson, H., Deutschbauer, A., Lowe-Power, T. 2021. Genome-wide identification of tomato xylem sap fitness factors for plant-pathogenic Ralstonia. mSystems. 6(6):e01229-21. https://doi.org/10.1128/mSystems.01229-21.
DOI: https://doi.org/10.1128/mSystems.01229-21

Interpretive Summary: Bacterial wilt pathogens are a diverse group of plant pathogens. Our goal is to identify core traits and lineage-specific traits that contribute to wilt pathogen growth, survival, and transmission. Here, we created and validated a genetic tool to quantify pathogenic success of three Ralstonia strains. Importantly, we identified genes that are essential for growth in xylem sap from tomato plants, a common and agriculturally important host. We determined that growth in xylem sap requires Ralstonia to remodel its envelope, likely due to preformed plant defenses within xylem sap. This information will be used to better understand the cause and prevention of bacterial wilt disease.

Technical Abstract: Plant pathogenic Ralstonia spp. colonize plant xylem and cause wilt diseases on a broad range of host plants. To identify genes that promote growth of diverse Ralstonia strains in xylem sap from tomato plants, we performed genome-scale genetic screens (random barcoded transposon mutant sequencing screens; RB-TnSeq) in three strains spanning the genetic, geographical, and physiological range of plant pathogenic Ralstonia: R. solanacearum IBSBF1503, R. pseudosolanacearum GMI1000, and R. syzygii PSI07. Contrasting mutant fitness phenotypes in culture media versus in xylem sap suggest that Ralstonia strains are adapted to ex vivo xylem sap and that culture media impose foreign selective pressures. Although wild-type Ralstonia grew in sap and in rich medium with similar doubling times and to a similar carrying capacity, more genes were essential for growth in sap than in rich medium. Each strain required many genes associated with envelope remodeling and repair processes for full fitness in xylem sap. These genes were associated with peptidoglycan peptide formation (murI), secretion of periplasmic proteins (tatC), LPS biosynthesis, periplasmic protein folding (dsbA), and synthesis of osmoregulated periplasmic glucans (mdoGH). Mutants in four genes had strong, sap-specific fitness defects in all strain backgrounds: murI, thiC, purU, and a lipoprotein (RSc2007). Many amino acid biosynthesis genes were required for fitness in both minimal medium and xylem sap. Mutants with insertions in multiple virulence regulators had gains-of-fitness in culture media and neutral fitness in sap. Our genome-scale genetic screen identified Ralstonia fitness factors that promote growth in xylem sap, an ecologically relevant condition.