Investigating bacterial growth/biofilm formation and the plant transcriptional landscape in the Arabidopsis-Pseudomonas syringae pv tomato interaction

Abstract

Abstract Arabidopsis thaliana exhibits a developmentally regulated disease resistance response known as Age-Related Resistance (ARR), a process that requires intercellular accumulation of salicylic acid (SA), which is thought to act as an intercellular antimicrobial and antibiofilm agent against the phytopathogen Pseudomonas syingae during infection of Arabidopsis leaves. During infection, P. syingae forms aggregates thought to consist of bacterial cells and extracellular polymers, such as alginate, extracellular DNA, lipids, and proteins. In this thesis the Arabidopsis thaliana-Pseudomonas syringae pv tomato (Pst) pathosystem was used to explore i) the contribution of alginate in protecting Pst during growth in vitro against the antimicrobial and antibiofilm effects of SA, and ii) various aspects of ARR including global gene expression analysis. At moderate SA concentrations, planktonic free-swimming bacterial cells of Pst ∆algU mucAB ∆algD, a mutant with reduced virulence, grew to lower densities than wild-type Pst and the alginate biosynthesis mutant Pst ∆algD in apoplast-mimicking minimal media, leading to the idea that AlgU regulates the T3SS and iron storage and uptake genes. This study also provides evidence that the Arabidopsis PAMP recognition mutants efr-1 are fls2 are partially ARR-defective and may contribute to ARR and the bak1-3 mutant may be required for ARR, suggesting PTI-related responses contribute to ARR. Lastly, comparative transcriptome analyses of susceptible young and ARR-competent mature plants responding to Pst, suggests that JA pathway genes are expressed in young but not mature plants. This work highlights the complex interaction between Arabidosis thaliana and Pseudomonas syringae pv. tomato.