Mitogen-activated protein kinases (MAPKs) belong to the most conserved signaling pathways and are found in all eukaryotes, including humans where they play important roles in various diseases and cancer. Stimulation of this signal transduction pathway by microbe-associated molecular patterns (MAMP) results in a multitude of events to regulate innate immune responses in Arabidopsis thaliana stimulating large-scale changes in gene expression. Starting from a phosphoproteomic screen in Arabidopsis thaliana wild type and mpk3, mpk4 and mpk6 mutants following microbe-associated molecular pattern (MAMP) treatment, several novel chromatin-associated proteins were identified that are differentially phosphorylated by stress-induced protein kinases. Arabidopsis Ras GTPase-activating protein SH3-domain-binding protein (AtG3BP-1) is a downstream putative substrate of the MAMP-stimulated MAPK pathway that is phosphorylated by MPK3, 4 and 6 in in vitro kinase assays. AtG3BP1 belongs to a highly conserved family of RNA-binding proteins in eukaryotes that link kinase receptormediated signaling to RNA metabolism. Here, we report the characterization of the Arabidopsis homolog of human G3BP1 in plant innate immunity. AtG3BP1 negatively regulates plant immunity and defense immune responses. Atg3bp1 mutant lines show constitutive stomata closure, expression of a number of key defense marker genes, and accumulate salicylic acid but not jasmonic acid. Furthermore, Atg3bp1 plants exhibit enhanced resistance to the biotrophic pathogen Pseudomonas syringae pv. tomato. Pathogen resistance was mediated by stomatal and apoplastic immunity in Atg3bp1. More generally, our data reinforce that AtG3BP1 is a key mediator of plant defense responses and transient expression of AtG3BP1 delivered striking disease resistance in the absence of yield penalty, highlighting a potential application of this gene in crop protection.

<p>Salicylic acid (SA) is a central plant defense signal. It is not only required for closing the stomata upon infection to prevent pathogens from entering into the plant apoplast, but also mediates defense responses activated by pathogen-originated microbe-associated molecular patterns (MAMPs) and effectors in the infected tissues. In addition, SA is a necessary and sufficient signal for systemic acquired resistance (SAR). In <italic>Arabidopsis</italic> <italic>thaliana</italic>, SA level increases in response to pathogen attack, which is essential for activating defense responses. This SA accumulation involves transcriptional activation of several genes including <italic>ICS1</italic> (<italic>ISOCHORISMATE</italic> <italic>SYNTHASE</italic> <italic>1</italic>), <italic>EDS5</italic> (<italic>ENHANCED</italic> <italic>DISEASE</italic> <italic>SUSCEPTIBILITY</italic> <italic>5</italic>), <italic>EDS1</italic> (<italic>ENHANCED</italic> <italic>DISEASE</italic> <italic>SUSCEPTIBILITY</italic> <italic>1</italic>), <italic>PAD4</italic> (<italic>PHYTOALEXIN-DEFICIENT</italic> <italic>4</italic>) and <italic>PBS3</italic> (<italic>avrPphB</italic> <italic>SUSCEPTIBLE</italic> <italic>3</italic>). However, it is not well understood how pathogenic signals induce these SA accumulation genes. Interestingly, our time-course transcriptome analysis showed that these five genes share a similar pathogen-induced expression pattern, suggesting the existence of common transcription factors (TFs). Through yeast-one-hybrid screening, a TF NTL9 was identified for its interactions with the promoters of the SA accumulation genes. Preferentially expressed in guard cells, NTL9 activates the expression of SA accumulation genes in guard cells. The <italic>ntl9</italic> mutant is defective in pathogen-induced stomatal closure mediated by a well-characterized MAMP, flg22. Consistent with the stomatal closure defect, the <italic>ntl9</italic> mutant exhibits elevated susceptibility to surface-inoculated pathogens. The stomatal closure defect of the <italic>ntl9</italic> mutant can be rescued by exogenous application of SA, demonstrating that NTL9 acts upstream of SA in stomatal closure response. These results suggest that NTL9-mediated activation of SA accumulation genes is essential for MAMP-triggered stomatal closure.</p><p>While plants induce SA to activate defense responses, pathogens can also produce virulence factors to counteract the effects of SA. Coronatine is one such virulence factor produced by <italic>Pseudomonas</italic> <italic>syringae</italic>. Coronatine is known to promote opening of stomata for bacterial entry, bacterial growth in the apoplast, systemic susceptibility and development of disease symptoms such as chlorosis. In the process of examining the mechanisms underlying coronatine-mediated virulence, three homologous TFs, ANAC019, ANAC055 and ANAC072, were found to be activated by coronatine directly through the TF, MYC2. Genetic characterization of these three TF mutants revealed that these TFs mediate multiple virulence effects of coronatine by inhibiting SA accumulation. To exert this inhibitory effect, these TFs repress <italic>ICS1</italic> and activate <italic>BSMT1</italic>, genes involved in SA biosynthesis and inactivation modification, respectively. Thus, a signaling cascade downstream of coronatine was illustrated to dampen SA-mediated defense responses through differential transcriptional regulation of genes related to SA level.</p><p>Taken together, my dissertation studies revealed novel transcriptional regulation of SA production and demonstrated that this transcriptional regulation is a vital point not only for plant defense activation but also for pathogen manipulation to counteract defense responses. Further studies on the interplay of this transcriptional regulation by different TFs would broaden our understanding about the dynamics of plant-pathogen interaction.</p> / Dissertation