The Arabidopsis thaliana genome encodes twenty putative cyclic nucleotide-gated channel (CNGC) genes. Studies on A. thaliana CNGCs so far have revealed their ability to selectively transport cations that play a role in various stress responses and development, however, the regulation of plant CNGCs is not yet fully understood. Thus, in this study I have attempted to analyze the structure-function relationship of AtCNGCs, mainly by using suppressor mutants of the rare gain-of function mutant, cpr22. The A. thaliana mutant cpr22 resulted from an approximately 3kb deletion that fused the 5’ half and the 3’ half of two CNGC-encoding genes, AtCNGC11 and AtCNGC12, respectively. The expression of this chimeric CNGC, the AtCNGC11/12 gene confers easily detectable characteristics such as stunted morphology with curly leaves and hypersensitive response-like spontaneous lesion formation. Through a suppressor screen, twenty nine new alleles were identified in AtCNGC11/12. Since the cytosolic C-terminal region contains important regulatory domains, such as a cyclic-nucleotide binding domain, eleven cytosolic C-terminal mutants, S17, S35, S81, S83, S84, S100, S135, S136, S137, S140 and S144, were analyzed. A detailed analysis of two mutants, S100 (AtCNGC11/12:G459R) and S137 (AtCNGC11/12:R381H), suggested that G459 and R381 are important for basic channel function rather than channel regulation. Site-directed mutagenesis and fast protein liquid chromatography (FPLC) showed that these two amino acids influence both intra- and inter-subunit interactions that are involved in stabilizing the tertiary structure of the channel. In addition, calmodulin binding domain(s) (CaMBD) and cyclic nucleotide binding domain(s) (CNBD) of some of AtCNGCs were studied using computational modeling and biophysical analyses. The data indicated that AtCNGC12 has two CaMBDs in both N- and C- cytosolic termini, whereas AtCNGC11 has only one CaMBD located in the N-terminal region of the channel. In addition, a thermal shift assay suggested that AtCNGC12 has higher affinity to bind cAMP over cGMP. Taken together, the current study contributes to identify key residues for channel function and provides new insights into CaMBD and CNBD in plant CNGCs.

The Arabidopsis thaliana genome encodes twenty putative cyclic nucleotide-gated channel (CNGC) genes. Studies on A. thaliana CNGCs so far have revealed their ability to selectively transport cations that play a role in various stress responses and development, however, the regulation of plant CNGCs is not yet fully understood. Thus, in this study I have attempted to analyze the structure-function relationship of AtCNGCs, mainly by using suppressor mutants of the rare gain-of function mutant, cpr22. The A. thaliana mutant cpr22 resulted from an approximately 3kb deletion that fused the 5’ half and the 3’ half of two CNGC-encoding genes, AtCNGC11 and AtCNGC12, respectively. The expression of this chimeric CNGC, the AtCNGC11/12 gene confers easily detectable characteristics such as stunted morphology with curly leaves and hypersensitive response-like spontaneous lesion formation. Through a suppressor screen, twenty nine new alleles were identified in AtCNGC11/12. Since the cytosolic C-terminal region contains important regulatory domains, such as a cyclic-nucleotide binding domain, eleven cytosolic C-terminal mutants, S17, S35, S81, S83, S84, S100, S135, S136, S137, S140 and S144, were analyzed. A detailed analysis of two mutants, S100 (AtCNGC11/12:G459R) and S137 (AtCNGC11/12:R381H), suggested that G459 and R381 are important for basic channel function rather than channel regulation. Site-directed mutagenesis and fast protein liquid chromatography (FPLC) showed that these two amino acids influence both intra- and inter-subunit interactions that are involved in stabilizing the tertiary structure of the channel. In addition, calmodulin binding domain(s) (CaMBD) and cyclic nucleotide binding domain(s) (CNBD) of some of AtCNGCs were studied using computational modeling and biophysical analyses. The data indicated that AtCNGC12 has two CaMBDs in both N- and C- cytosolic termini, whereas AtCNGC11 has only one CaMBD located in the N-terminal region of the channel. In addition, a thermal shift assay suggested that AtCNGC12 has higher affinity to bind cAMP over cGMP. Taken together, the current study contributes to identify key residues for channel function and provides new insights into CaMBD and CNBD in plant CNGCs.

Control of intracellular ion homeostasis is pivotal to plant salt tolerance. Plants have developed a number of mechanisms to keep ions at appropriate concentrations. Both transporters and channels on the plasma membrane play important roles in this function. Plant cyclic nucleotide-gated channels (CNGCs) in the plasma membrane are non-selective monovalent and divalent cation channels. So far, most studies on plant CNGCs have been conducted on heterologous systems. In planta, reverse genetic studies revealed the role of different CNGCs in cation uptake, transport and homeostasis. However, there is little information available about the functional characteristics of plant CNGCs. Among the 20 members of this protein family in Arabidopsis, only AtCNGC2 has been functionally identified as an ion channel; therefore, more functional characterization needs to be done on other members of this protein family. Several CNGCs were suggested to be involved in K+, Ca2+ and Na+ uptake and transport, but available information is scarce. This study investigated the relationship between CNGC10 and ion transport in Arabidopsis, with a particular emphasis on the involvement of CNGC10 in salt tolerance. Arabidopsis thaliana wild type (WT) and two AtCNGC10 antisense lines (A2 and A3) were used to characterise the impact of different level of salt stress on (i) root growth, ion concentration in tissues, ion fluxes across the root surface and intracellular ion concentration and pH at the seedling stage, and (ii) photosynthesis and ion concentration in tissues at the flowering stage. Plants of both antisense lines had higher K+ and lower Ca2+ and Mg2+ concentrations in shoots than WT plants when grown in non-salt control 1/4 Hoagland solution. Altered K+, Ca2+ and Mg2+ internal concentrations in AtCNGC10 antisense lines compared with WT plants under non-salt conditions indicated disturbed long distance ion transport, especially xylem loading/retrieval and/or phloem loading. The results of ion fluxes across the root surface also suggested that AtCNGC10 might be involved in transport of K+, Ca2+ and Mg2+ in tissue. Under sudden salt exposure, higher Na+ efflux and smaller K+ efflux in both antisense lines suggested that AtCNGC10 channels are involved in Na+ and K+ transport. The shoots of AtCNGC10 antisense lines A2 and A3 contained higher Na+ concentrations and significantly higher Na+/K+ ratios compared to WT, resulting in impaired photosynthesis and increased salt sensitivity in A2 and A3 than in WT plants. In contrast, seedlings of both antisense lines exposed to salt stress had lower shoot Na+/K+ ratios and longer roots than WT seedlings, indicating that A2 and A3 were more salt-tolerant than WT in the seedling stage, likely because growth is less dependent on photosynthesis in the seedling than in the flowering stage. These results suggested CNGC gene might play a different role during different developmental stages and in various plant organs.

Cyclic nucleotide-gated channels (CNGCs) are non-selective cation channels that were first identified in vertebrate photosensory and olfactory neurons. Although the physiological roles and biophysical properties of animal CNGCs have been well studied, much less is known about these channels in plants. The Arabidopsis genome encodes twenty putative CNGC subunits that are postulated to form channel complexes that mediate various physiological processes involving abiotic and biotic stress responses, ion homeostasis and development. The identification of Arabidopsis autoimmune CNGC mutants, such as defense no death class (dnd1 and dnd2), and the constitutive expressor of pathogenesis related genes 22 (cpr22) implicate AtCNGC2, 4, 11 and 12 in plant immunity. Here, I present a comprehensive study of the molecular mechanisms involved in CNGC-mediated signaling pathways with emphasis on pathogen defense. Previously, a forward genetics approach aimed to identify suppressor mutants of the rare gain-of-function autoimmune mutant, cpr22, identified key residues that are important for CNGC subunit interactions and channel function. First, I present a structure-function analysis of one of these suppressor mutants (S58) that revealed a key residue in the cyclic nucleotide binding domain involved in the stable regulation of CNGCs. Second, I present a new suppressor screen using AtCNGC2 T-DNA knockout mutants that specifically aimed to identify novel downstream components of CNGC-mediated pathogen defense signaling. In this screen, I successfully isolated and characterized the novel Arabidopsis mutant, repressor of defense no death 1 (rdd1), and expanded this study to demonstrate its involvement in AtCNGC2 and AtCNGC4-mediated signal transduction. Additionally, I demonstrated for the first time, the physical interaction of AtCNGC2 and AtCNGC4 subunits in planta. The findings presented in this thesis broaden our current knowledge of CNGCs in plants, and provide a new foundation for future elucidation of the structure-function relationships and signal transduction mediated by these channels.

Cyclic nucleotide-gated channels (CNGCs) are non-selective cation channels that were first identified in vertebrate photosensory and olfactory neurons. Although the physiological roles and biophysical properties of animal CNGCs have been well studied, much less is known about these channels in plants. The Arabidopsis genome encodes twenty putative CNGC subunits that are postulated to form channel complexes that mediate various physiological processes involving abiotic and biotic stress responses, ion homeostasis and development. The identification of Arabidopsis autoimmune CNGC mutants, such as defense no death class (dnd1 and dnd2), and the constitutive expressor of pathogenesis related genes 22 (cpr22) implicate AtCNGC2, 4, 11 and 12 in plant immunity. Here, I present a comprehensive study of the molecular mechanisms involved in CNGC-mediated signaling pathways with emphasis on pathogen defense. Previously, a forward genetics approach aimed to identify suppressor mutants of the rare gain-of-function autoimmune mutant, cpr22, identified key residues that are important for CNGC subunit interactions and channel function. First, I present a structure-function analysis of one of these suppressor mutants (S58) that revealed a key residue in the cyclic nucleotide binding domain involved in the stable regulation of CNGCs. Second, I present a new suppressor screen using AtCNGC2 T-DNA knockout mutants that specifically aimed to identify novel downstream components of CNGC-mediated pathogen defense signaling. In this screen, I successfully isolated and characterized the novel Arabidopsis mutant, repressor of defense no death 1 (rdd1), and expanded this study to demonstrate its involvement in AtCNGC2 and AtCNGC4-mediated signal transduction. Additionally, I demonstrated for the first time, the physical interaction of AtCNGC2 and AtCNGC4 subunits in planta. The findings presented in this thesis broaden our current knowledge of CNGCs in plants, and provide a new foundation for future elucidation of the structure-function relationships and signal transduction mediated by these channels.