*Article* **Novel Salicylic Acid Analogs Induce a Potent Defense Response in Arabidopsis**

**Ian Arthur Palmer 1, Huan Chen 1,2, Jian Chen 1,2, Ming Chang 1,2, Min Li 1, Fengquan Liu 2,\* and Zheng Qing Fu 1,\***


Received: 11 June 2019; Accepted: 5 July 2019; Published: 8 July 2019

**Abstract:** 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, TCP, and WRKY transcription factors to induce expression of plant defense genes. A list of compounds structurally similar to SA was generated using ChemMine Tools and its Clustering Toolbox. Several of these analogs can induce SA-mediated defense and inhibit growth of *Pseudomonas syringae* 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 among NPR proteins. We demonstrated that these analogs can induce the expression of the defense marker gene *PR1*. Furthermore, we hypothesized that these SA analogs could be potent tools against the citrus greening pathogen *Candidatus liberibacter* spp. In fact, our results suggest that the SA analogs we tested using *Arabidopsis* may also be effective for inducing a defense response in citrus. Several SA analogs consistently strengthened the interactions between citrus NPR1 and NPR3 proteins in a yeast two-hybrid system. In future assays, we plan to test whether these analogs avoid degradation by SA hydroxylases from plant pathogens. In future assays, we plan to test whether these analogs avoid degradation by SA hydroxylases from plant pathogens.

**Keywords:** salicylic acid; analogs; NPR1; NPR3; NPR4; PR1; *Pseudomonas syringae*
