The Role of Chromatin Associated Proteins in Plant Innate Immunity and Jasmonic Acid Signaling

Abstract

Pathogen-associated molecular pattern (PAMP) recognition occurs by plasma membrane located receptors that induce among other processes nuclear gene expression. The plant FLS2-BAK1 receptor complex binds the bacterial PAMP, flg22 and induces a series of defense responses. The resulting signal transduction events occur through the activation of two MAPK signaling cascades, which trigger a rapid and strong activation of MPK3, MPK4 and MPK6. Cellular responses to pathogens are regulated by the activated MAPKs, which lead to the eventual phosphorylation of cytoplasmic and nuclear substrates. These MAPK substrates in turn respond to phosphorylation by reprogramming the expression of defense genes. A large scale phosphoproteomics screen of nuclear proteins in wild type and mpk mutant plants in response to flg22 revealed several novel putative targets of MAP kinases. This thesis is aimed at identifying the role of two of these chromatin associated proteins in plant immunity and their signaling mechanisms. The chromatin associated proteins we chose to study here are LITTLE NUCLEI/CROWDED NUCLEI (LINC/CRWN), LINC1 and the AT-HOOK MOTIF CONTANING NUCLEAR LOCALIZED 13 (AHL13) proteins. We demonstrate that these two chromatin associated proteins play a positive regulatory role in jasmonic acid signaling and immunity. Knock out mutants for both genes exhibit impairment in early and late innate immune reposes to both PAMP and hemibiotrophic pathogen strains. We also demonstrate that these mutants are compromised in regulating the expression of genes involved in jasmonic acid (JA) signaling and responses and genes involved in the biosynthesis both the indole and aliphatic glucosinolate (GS) pathways. Moreover, Pst DC3000 hrcC triggers JA and JAIle accumulation in these mutants, whereas salicylic acid (SA) levels are unchanged. We were also able to identify and validate two novel MAPK targeted phosphosites in AHL13 that affect the protein stability of AHL13 and we establish its role as a MPK6 substrate that affects jasmonic acid biosynthesis and PTI responses. Together this work identifies two novel signaling components involved in the regulation of jasmonic acid homeostasis and immunity.