Screening of Novel Active Salicylic Acid Analogs and Identification of a Bacterial Effector Targeting Key Proteins Involved in Salicylic Acid-Mediated Defense

Ian Palmer


The master regulator of salicylic acid (SA)-mediated plant defense, NPR1 (NONEXPRESSER OF PR GENES 1), and its paralogs NPR3 and NPR4 act as SA receptors. After the perception of a pathogen, plant cells produce SA in the chloroplast. In the presence of SA, NPR1 protein is reduced from oligomers to monomers, and translocated into the nucleus. There, NPR1 binds to TGA and WRKY transcription factors to induce expression of plant defense genes. EDS1 and PBS3 are two key proteins involved in SA biosynthesis. Previous research has shown that several plant pathogens produce SA hydroxylases. These pathogen-produced hydroxylases act to degrade SA, preventing their host plant’s cells from perceiving this important defense signal, rendering the host susceptible to infection. Additionally, bacterial pathogens deliver effectors into their host’s cells via the type three secretion system. These effectors target key defense proteins to subvert plant defense. Using a computational approach, a list of salicylic acid analogs has been created. Several of these analogs can induce SA- mediated defense and inhibit bacterial growth in Arabidopsis. These analogs, when sprayed on Arabidopsis, can induce the accumulation of the master regulator of plant defense NPR1. In a yeast two-hybrid system, these analogs can strengthen the interactions between NPR proteins. I demonstrate that these analogs can induce the expression of the defense marker gene PR1 and induce PR1’s accumulation. I hope to test in future assays whether these analogs avoid degradation by pathogenic SA hydroxylases. Additionally, I demonstrate that a bacterial effector secreted by Pseudomonas syringae pv. tomato DC3000, HopAA1-2, interacts with EDS1 and PBS3, causing a reduction in the amount of these two proteins when transiently expressed in tobacco. This interaction may be an attempt to subvert SA-mediated defense.