Regulation of plant defense responses downstream of PAMP receptors

The primary layer of plant immunity is pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI). In PTI, when pattern recognition receptors (PRRs) recognize PAMPs, a highly regulated, rapid downstream signaling response such as activation of mitogen-activated protein (MAP) kinases, production of ROS and defense gene expression is initiated. My study involves the identification of MAP kinases activated in PTI and regulation of PTI responses by E3 ubiquitin ligases. In Arabidopsis thaliana, MPK3, MPK4, and MPK6 are activated upon PAMP treatment. However, previous studies suggest that there are more MAP kinases activated upon PAMP treatment. To identify the additional MAP kinases phosphorylated upon PAMP perception, transgenic plants expressing candidate MPKs with a ZZ-FLAG double tag (approx. 17kD) were generated in Col-0 background. Western blot analysis identified three MAP kinases, MPK1, MPK11 and MPK13 that are phosphorylated upon PAMP treatment. To identify E3 ligases involved in PTI, E3 ligases whose transcripts are up-regulated upon PAMP treatment were selected for our study. Transgenic lines overexpressing candidate E3 ligases were assayed for deficiencies in PTI. Overexpression of U6, one of the selected E3 ligases, lead to severe reduction of flg22-induced reactive oxygen species (ROS) production and increased susceptibility to Pseudomonas syringae pv. tomato (P.s.t.) DC3000 hrcC-. Furthermore, when U6 was overexpressed in BIK1 (a positive regulator of PTI)-HA transgenic plants, there was a decrease in BIK1-HA protein expression, leading to the hypothesis that BIK1 may be a potential target of U6. Overall, my thesis contributes to a better understanding of the signaling and negative regulation of PTI. Advancing our knowledge in plant immunity leads to the potential of its use in agriculture and plant protection.Science, Faculty ofBotany, Department ofGraduat