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Genetic analysis of receptor-like protein SNC2-mediated plant resistance in Arabidopsis Ding, Yuli

Abstract

Plant immunity is usually governed by two types of immune receptors: 1) pattern recognition receptors (PRRs) recognize the conserved molecular features of pathogens (pathogen-associated molecular patterns, PAMPs) and trigger PTI (PAMP-triggered immunity) and 2) nucleotide-binding and leucine-rich repeats-containing proteins (NLRs) serve as intracellular immune receptors to recognize the presence of relatively diverse pathogen effectors and trigger ETI (effector-triggered immunity). The Arabidopsis thaliana mutant snc2-1D (suppressor of npr1-1, constitutive 2) contains a gain-of-function mutation in a receptor-like protein (RLP) and displays a dwarf morphology. Here I report the characterization of bda4-1D (bian da 4-1D), which was identified as a complete suppressor of snc2-1D dwarf morphology. Positional cloning showed bda4-1D contains a gain-of-function mutation in Non-Expressor of Pathogenesis-Related Proteins 4 (renamed npr4-4D). Functional analysis indicated NPR4, as well as its close homolog NPR3 (Non-Expressor of Pathogenesis-Related Proteins 3), function as transcriptional repressors. They function downstream of SNC2, independent of NPR1 (Non-Expressor of Pathogenesis-Related Proteins 1). In addition, salicylic acid (SA) was shown to inhibit the transcriptional activities of NPR3/4 and promote the expression of key immune regulators. The npr4-4D mutation leads to constitutive repression of SA-induced immune responses, indicating that the mutant protein can no longer respond to SA. On the other hand, the equivalent mutation in NPR1 also abolishes its ability to bind SA and renders reduced SA-induced defence gene expression. My results demonstrated that both NPR1 and NPR3/NPR4 are bona fide SA receptors, but play opposite roles in transcriptional regulation of SA-induced defence gene expression. In the independent eds5-3 snc2-1D npr1-1 suppressor screen, I report the identification and characterization of four more bda mutants, bda3-1D, bda5-1, bda6 and bda7. Cloning of BDA6 and BDA7 showed that they encode FMO1 and ALD1 respectively, which are involved in biosynthesis of N-Hydroxypipecolic Acid (NHP) and pipecolic acid. My results indicate that enzymes involved in Lysine metabolism are also important for signaling in SNC2-mediated immune pathway. Overall, the studies I completed in my Ph.D. thesis expand our knowledge in understanding of the signaling pathways downstream of SNC2 as well as the general regulatory mechanisms of SA receptors in plant innate immunity.

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