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.

Doctor of Philosophy / Department of Plant Pathology / Jianmin Zhou / Plants activate innate immune responses or innate immunity upon pathogen infection. There are two types of plant innate immunity: PAMP-triggered innate immunity (PTI) and effector-triggered innate immunity (ETI). The molecular basis for ETI has been well documented. However, the study on PTI and its interplay with pathogen virulence is in its infancy. My research focuses on the interplay between PTI and bacterial virulence in Pseudomonas-Arabidopsis interactions. NHO1, a gene required for nonhost resistance to Pseudomonas syringae, encodes for the 3-glycerol kinase in Arabidopsis genome. NHO1 functions, at least in part, by depriving glycerol from nonhost bacteria cells. NHO1 is induced by a well-known bacteria PAMP flg22. The induction of NHO1 correlates well with the resistance against Pseudomonas syringae pv. tabaci because a mutant strain of P. s. pv. tabaci deficient in NHO1 induction gains partial virulence on Arabidopsis plants. P. s. pv. tomato strain DC3000 induces transient NHO1 expression that is suppressed in a type III secrection system-dependent manner. Using protoplast assay, nine DC3000 effectors that are able to suppress NHO1 were identified. One of them, HopAI1, induces leaf chlorosis and helps nonpathogenic bacterial growth when expressed in Arabidopsis plants, suggesting that HopAI1 has virulence activity in planta. To study AvrB virulence activity in Arabidopsis plants, one mutant compromised in AvrB-specific RAR2.6 induction has been characterized in detail. rrb3 is more susceptible to a nonhost bacteria P. s. pv. tabaci strain 6505, a virulent bacteria P. s. pv. tomato strain DC3000 and an avirulent bacteria strain DC3000 (avrB). The mutant allele rrb3 carries a point mutation at the end of RAR1 CHORD II domain. RRB3 (RAR1), together with NDR1, is involved in the type II nonhost resistance to P. s. pv. tabaci but not in the type I nonhost resistance to P. s. pv. phaseolicola. RAR1 participates in basal resistance against DC3000 by antagonizing COI1 activity. AvrB targets RAR1 to trigger AvrB-dependent leaf chlorosis and enhanced bacterial growth. The AvrB-dependent enhanced bacterial growth but not leaf chlorosis requires COI1, suggesting that AvrB targets JA signaling pathway to promote parasitism.

Plant cells are mostly immobile, therefore it is crucial for them to distinguish a direction of the signals coming into the cell and on the other hand they have to precisely target their own signals. To achieve this communication, plant cells use endomembrane system and secretory vesicles, which are recruited to the specific membrane domains. This ability is important for the plant defense against pathogenic microorganisms and it even forms a part of the innate plant immunity. Two complexes, the exocyst and SNARE, play a prominent role in the process of polarized secretion. In this work, we focused on a possible interaction between these two complexes in preinvasive defense and particularly, we studied the exocyst subunit EXO70B2 and SNARE protein SYP121. We obtained double mutant plants of EXO70B2 and SYP121 by utilizing the reverse genetics approach. These mutant plants did not show any obvious phenotype under standard conditions in comparison with Wt plants. However, we observed marked defects of secretory pathway in double mutant exo70B2/syp121 after infection by pathogenic fungi Blumeria graminis f. sp. hordei. Using histochemical staining, we described problems with the deposition of defensive papilla and secretion of haustorial encasement. We prove that these defects are not connected with...