Investigation of the molecular basis of PAMP-induced resistance
The recognition of conserved microbial features termed pathogen-associated molecular patterns (PAMPs) by surface-localized pattern-recognition receptors (PRRs) constitutes the first layer of plant innate immunity. Although details of early immune signaling events are starting to be unveiled, the molecular mechanisms leading to restriction of pathogen growth are still poorly understood. To gain more insight into this process, two different approaches were employed. I used reverse genetics to study the involvement of three different secondary metabolites, namely camalexin, glucosinolates and callose, in PAMP-triggered immunity (PTI) against the phytopathogenic bacterium Pseudomonas syringae pv. tomato (Pto) DC3000. These are well known active defences Arabidopsis employs to restrict fungi and oomycetes invasion. Results showed that these compounds are dispensable for antibacterial resistance triggered by the bacterial PAMP flagellin (flg22). In addition, as an unbiased approach, I performed a novel genetic screen aimed at identifying molecular components required for induced resistance to Pto DC3000. For this, I developed a high-throughput assay for bacterial infection in Arabidopsis seedlings that enabled to select mutants impaired in flg22-induced resistance to Pto DC3000. The pir (PAMP-induced resistance) screen identified four loci whose mutation leads to a reproducible reduction of flg22-induced resistance. These genes have not been previously characterized for their role in immunity, and therefore can be considered as novel components of PTI. By employing a combination of reverse genetics, metabolomics and chemistry approaches, I obtained preliminary data suggesting that flavonoids act as cellular buffers and/or are employed as active defenses against bacteria. In addition, interference with the mevalonic acid biosynthetic pathway impairs antibacterial defenses, suggesting a role in immunity. Additional tests are underway to better assess the contribution of these PIR genes to PTI. Therefore, through the pir screen, I have identified several novel loci required for plant immunity that will increase our knowledge of the plant immune system.
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