Title

Delineation of Molecular Signal Transduction events facilitating pathogen defense responses in plants

Date of Completion

January 2010

Keywords

Biology, Botany|Agriculture, Plant Culture

Degree

Ph.D.

Abstract

Plant innate immunity has been a topic of interest for many biologists over the past decade. Accumulative data have identified key components of the innate immune cascade to which we have assigned roles. This dissertation permits us to uncover the ordered steps of these molecular components that contribute to pathogen defense that may parallel/overlap with abscisic acid (ABA) signaling. Ca2+ signals occur in the cytosol, as an early response to the perception of pathogen, in plant cells that are facilitated through cyclic nucleotide gated channels (CNGCs). We provide evidence that [Ca2+]cyt rise, nitric oxide (NO) and hydrogen peroxide (H2O2) generation along with a suite of other responses leads to the hypersensitive response (HR) when pathogen (with avirulence genes) and/or conserved microbe-associated molecular proteins (MAMPs) are detected. cAMP/cGMP-dependent Ca2+ evolution is required to initiate the suite of pathogen defense responses. CNGC2 (Ca2+ conducting channel) also functions as a mediator of 'basal' NO production during plant development and NO acts as a negative regulator during plant leaf senescence signaling. Molecular components (calmodulin-like protein 24 (CML24), CNGC2, and G-proteins) mediate Ca2+ signals necessary for NO/H 2O2 generation and other responses during pathogen defense and abiotic stress responses. CML24 transduce Ca2+ signals, which are essential for stomatal closure. G-proteins are not required to activate downstream MAMP lipopolysaccharide signaling events but can hasten HR. However, G-proteins are required for MAMP flagellin signaling. ^ Leucine rich repeat receptor-like kinases (LRR-RLKs) recognize components of pathogens, or endogenous plant peptides, which can initiate immune defense responses. Bacterial flagellin protein binds to the LRR-RLK FLS2. Endogenous plant peptides (Peps) bind to the LRR-RLK PEPR1. Ligand binding to both of these receptors initiates a cytosolic Ca2+ elevation that also leads to NO, H2O2 generation, and defense gene expression. We find that in Arabidopsis plants, with loss-of-function mutations in PEPR1 or FLS2, a defense signal induced by either of the ligands is compromised showing cross-sensitivity. Therefore, these pathogen defense-related receptor proteins require each other to initiate defense immune responses. We also find that Pep-dependent signaling requires CNGC2 for uncompromised immune responses. Both FLS2 and PEPR1 bind to a third LRR-RLK, BAK1. Our results imply that these receptor proteins interact in some as-yet-uncharacterized manner. ^