| Summary: | 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.
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