The Involvement of Secondary Messengers in Plant Receptor-Like Kinase Signal Transduction

In plant signaling networks, plasmalemma localized leucine-rich repeat receptor-like kinases (LRR-RLKs) are involved in signal perception and initiation of cytosolic signal transduction cascades that alter cell function. Little is currently known about the molecular mechanisms involved in the translation of ligand binding to any LRR-RLK into the initiation of a cytosolic signal. This dissertation presents studies with BRI1, PEPR1, and FLS2; LRR-RLKs involved with hormone perception and plant immunity. The ligands for these receptors are the steroid hormone brassinosteroid (BR), an endogenous plant elicitor peptide ‘Peps’, and the bacteria flagellin (flg) protein, respectively. Binding of BR, Peps, and flg22 to their cognate receptors induced cytosolic Ca2+ elevation, which was responsible for ligand-dependent gene expression and phenotypes. The BRI1 and PEPR1 receptors share a cytosolic-localized guanylyl cyclase (GC) domain. A cGMP activated Ca2+ conducting ion channel cyclic nucleotide gated channel (CNGC) 2 was involved in their ligand-dependent Ca2+ elevation. BR can induce secondary messenger cGMP elevation in vivo, and the treatments that either promote or prevent cytosolic cGMP generation mimicked or prevented the ligand-induced signaling cascades and gene expression. Ligand binding to the PEPR1 and FLS2 receptors initiated cytosolic Ca2+ signaling as well as NO and H2O2 generation, and pathogen immunization. Both PEPR1 and FLS2 receptors are required to reach the maximal level of signaling and phenotype responses induced by either of the activating ligands. Therefore, there is interdependence between receptor PEPR1 and FLS2, and both of these receptors are required to initiate defense immune responses. Flg is different from BR and Peps, intracellular Ca2+ release into the cytosol may contribute to flg dependent Ca2+ signaling. Secondary messenger inositol triphosphate (IP3) and IP6 (produced from IP3 phosphorylation) generation during plant cell signaling could lead to release of intracellular Ca2+ stores in plant cells. Experimental evidence suggesting that G proteins, and inositol phosphate elevations may be involved in translating perception of flagellin into release of intracellular Ca2+ stores, contributing to the cytosolic Ca2+ elevation that is a critical signal leading to immune responses in plant cells. Our results imply that secondary messengers are involved in the signal transduction of these receptors