Investigating the role of Pseudomonas syringae pv. tomato biofilm formation during successful infections and the effect of PAMP-Triggered Immunity on biofilm formation in Arabidopsis

Abstract

Plants rely on innate immunity to perceive and respond to pathogenic microbes. Pathogenic microbes suppress and evade plant immune responses to obtain nutrients and multiply resulting in plant diseases and death. One battleground for the arms race between plants and microbial invaders is located in the leaf intercellular space, specifically between Pseudomonas bacteria and Arabidopsis. This thesis seeks to understand the virulence mechanisms that allow Pseudomonas bacteria to grow within the leaves of Arabidopsis and how the plant immune response reduces pathogen growth and reproduction. Some plant pathogens produce specific extracellular polysaccharides to potentially enhance pathogenicity during infection of plants. The objective of this thesis is to understand the importance of biofilms for Pseudomonas success and determine if Arabidopsis suppresses biofilm formation as part of the plant immune response. It was hypothesized that biofilm formation contributes to Pseudomonas success in planta and Arabidopsis suppresses biofilm formation during PAMP-Triggered Immunity (PTI) to reduce bacterial growth. Wild-type plants and defense mutants were infiltrated with flg22 or mock (water) treatments to induce or mock-induce PTI in plants, followed by observing GFP-expressing Pseudomonas via florescence microscopy to determine if biofilm-like aggregate formation was occurring. In vivo studies in this thesis indicate that biofilm-like aggregate formation contributes to bacterial success during Arabidopsis infection. Additionally, the phytohormone, salicylic acid (SA), accumulates in leaf intercellular spaces of resistant plants during PTI that suppresses biofilm formation, suggesting that SA acts as an anti-microbial and anti-biofilm agent that contributes to the suppression of pathogen growth during plant defense.