Open Collections

Liu, Yang

University of British Columbia

Plant roots associate with diverse microbial communities that include pathogens and commensals. Plants innate immune systems monitor microbiota by recognizing conserved microbe-associated molecular patterns (MAMPs). Despite containing identical MAMPs as pathogens, commensals persist in the rhizosphere indicating that they must suppress or evade host immunity. I found that rhizosphere acidification via gluconic acid biosynthesis is necessary for immunity suppression in an opportunistic pathogen Pseudomonas aeruginosa PAO1. However, a Pseudomonas simiae WCS417 ∆pqqF mutant can still suppress flg22-induced immunity, suggesting a distinct mechanism of host immunity suppression by WCS417. To identify mutants that were unable to suppress flg22-stimulated immunity, we performed a forward genetic screen in EMS mutagenized WCS417. We identified an ornithine carbamoyltransferase argF mutant that lost immunity suppression and rhizosphere acidification. Although argF is auxotrophic, it is required for host immunity suppression and rhizosphere acidification by preventing accumulation of ornithine, an alkaline precursor to arginine biosynthesis. This work shows that beneficial Pseudomonas suppress host immunity via rhizosphere acidification through two synergistic mechanisms, one through organic acid biosynthesis and the other by conversion of alkaline ornithine to amino acids. In addition to modulation of host immunity, commensals are known to protect plants from pathogens. Pseudomonas fluorescens WCS365 protects plants from a phylogenetically closely related opportunistic pathogen Pseudomonas sp. N2C3 in the rhizosphere. To uncover mechanisms of protection, I used two collections of WCS365 biofilm formation transposon insertion mutants including 1) increased attachment mutants (iam) and 2) surface attachment defective (sad) mutants, which have increased and decreased biofilm formation respectively on abiotic surfaces. As bacteria must form a biofilm to successfully colonize their host, I tested whether these mutants failed to form biofilm in planta by testing whether they could protect against the N2C3 pathogen on plant roots. I found that only a subset of “iam sad” mutants lost protection against the N2C3 pathogen implying distinct mechanisms of biofilm formation in vitro versus in planta. All unprotective strains showed rhizosphere fitness defects suggesting that biofilm formation is necessary for bacterial colonization and protection. Collectively, my work enhances our understanding of how beneficial bacteria suppress host immunity and protect plants from pathogens.

Text

2024-09-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/85382

Microbiology and Immunology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/