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Mitochondria are required for a diverse array of cellular functions and processes. ATAD3 (ATPase family AAA domain containing protein 3) proteins are newly discovered mitochondrial membrane proteins in Arabidopsis thaliana. Homologous to ATAD3A in metazoans, Co-Immunoprecipitation/Mass spectrometry and genomic analysis identified a four ATAD3A homologues in A. thaliana. The four A. thaliana proteins are referred to as ATAD3A1 (At3g03060), ATAD3A2 (At5g16930), ATAD3B1 (At2g18330), and ATAD3B2 (At4g36580). Studies in metazoans indicate that ATAD3A localizes to Mitochondria-ER contact sites and is involved in a variety of processes required for proper mitochondrial function, but ATAD3A proteins are poorly defined in plants. ATAD3A is a mitochondrial membrane protein with unique topology. It comprises an N-terminal DUF (Domain of unknown function) domain that contains two transmembrane sequences the are inserted or interact with both the inner and outer mitochondrial membranes, two coiled-coil domains thought to help in oligomerization, and a region that is exposed to the cytosol, proposed to interact with the ER. It has a C-terminal AAA domain exposed to the mitochondrial matrix. ATAD3 proteins in A. thaliana have undergone two gene-duplication events, resulting in two clades, both of which are required for plant viability. I created artificial microRNA to knockdown expression of ATAD3A1 in the atad3b1 mutant background to assess the growth and mitochondrial phenotypes and found these plants displayed delayed and deficient growth and deformed mitochondria. I utilized Bi-Molecular Complementation Fluorescence and Laser-Scanning Confocal Microscopy to assess oligomeric patterns of A. thaliana ATAD3 proteins in vivo and discovered that ATAD3 proteins hetero-oligomerize with each other. I also created multiple constructs encoding ATAD3A1 fusion proteins to elucidate the amino acid sequence required to target ATAD3A1 to the mitochondria, and ATAD3A1 fusions with TurboID to identify protein-protein interactions using proximity-based labeling.

Understanding how plants respond to stress is of importance, considering the increasing need to feed a growing population and supply its energy. Plants have complex systems for detecting, and responding to stresses. One stress-responsive system involves myo-inositol (Ins). Ins is a precursor for cell wall components, inositol trisphosphate (Ins(1,4,5)P3) and phosphatidylinositol phosphate signaling molecules, and an alternate ascorbic acid (AsA) synthesis pathway. The enzyme, myo-inositol oxygenase (MIOX) is encoded by four genes in Arabidopsis and catalyzes the first step of Ins catabolism producing D-glucuronic acid (DGlcA). This research focuses on MIOX metabolism of Ins during plant growth and stress responses. I have examined miox mutants for alterations in metabolism and signaling. MIOX2 and MIOX4 expression patterns correlate with miox mutant root growth in varying nutrient conditions, and changes in flowering time. In miox2 mutants, I found an increase in Ins in most tissues, which was accompanied by cold- and abscisic (ABA)- sensitivity; however, miox4 mutants are ABA- insensitive, and have a small increase of Ins in flowers. MIOX2:GFP fusion protein accumulates in the cytoplasm and MIOX4:GFP accumulates in the cytoplasm and nucleus. Overexpresser MIOX4+ plants provide a model system to examine how directing carbon from Ins into DGlcA impacts Ins levels and Ins signaling. I have examined MIOX4+ plants for alterations in MIOX4 RNA and protein, and measured Ins by gas chromatography (GC). My results indicate that MIOX4+ tissues are impacted differently by the MIOX4 transgene, with decreases in Ins after seed imbibition, and increased Ins levels later in development. Ins depletion in seedlings was correlated with a decrease in Ins(1,4,5)P3. To determine the impact of reducing Ins and Ins(1,4,5)P3 in MIOX4+ seedlings, I examined processes known to involve Ins(1,4,5)P3 signaling. MIOX4+ seed have increased seed dormancy, NaCl-sensitivity, and ABA-insensitivity. These results suggest MIOX affects Ins signaling in response to ABA. Together, these data indicate that transcriptional control of MIOX2 and MIOX4 results in distinct roles in plant growth, and that MIOX2 and MIOX4 function in metabolic and signaling processes critical for growth, nutrient sensing, and stress responses. / Ph. D.

The complete role of inositol signaling in plants and humans is still elusive. The plant Arabidopsis thaliana contains fifteen predicted inositol polyphosphate 5- phosphatases (5PTases, E.C. 3.1.3.36) that have the potential to remove a 5-phosphate from various inositol second messenger substrates. To examine the substrate specificity of one of these Arabidopsis thaliana 5PTases (At5PTases), recombinant At5PTase1 was obtained from a Drosophila melanogaster expression system and analyzed biochemically. This analysis revealed that At5PTase1 has the ability to catalyze the hydrolysis of four potential inositol second messenger substrates. To determine whether At5PTase1 can be used to alter the signal transduction pathway of the major drought-sensing hormone abscisic acid (ABA), plants ectopically expressing At5PTase1 under the control of a constitutive promoter were characterized. This characterization revealed that plants ectopically expressing At5PTase1 had an altered response to ABA. These plants have stomata that are insensitive to ABA, and have lower basal and ABA-induced inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃] levels. In addition, At5PTase1 mRNA and protein levels are transiently regulated by ABA. These data strongly suggest that At5PTase1 can act as a signal terminator of ABA signal transduction. Like the Arabidopsis At5PTase1, a human 5PTase, Ocrl, has the ability to catalyze the hydrolysis of a 5-phosphate from several inositol-containing substrates. The loss of functional Ocrl protein results in a rare genetic disorder known as Lowe oculocerebrorenal syndrome. To gather information concerning the specificity determinants of the Ocrl protein, a structure-function analysis of Ocrl was conducted using a vibrational technique, difference Fourier transform infrared (FT-IR) spectroscopy. Upon the introduction of Ins(1,4,5)P₃ substrate, structural changes in carboxylic acid and histidine residues were observed. The net result of changes in these residues indicates that upon Ins(1,4,5)P₃ introduction, a carboxylic acid-containing residue is protonated, and a histidine residue is deprotonated. This interpretation supports the idea that the deprotonation of the histidine residue is concomitant with the coordination of a divalent cation upon Ins(1,4,5)P₃ introduction. This work allows for the proposal of a new model for the role of the active site histidine of OCRL. / Ph. D.

myo-Inositol synthesis and catabolism are crucial in many multicellular eukaryotes for production of phosphatidylinositol and inositol phosphate signaling molecules. myo-inositol monophosphatase (IMP) is a major enzyme required for the synthesis of myo-inositol and breakdown of inositol (1,4,5)-trisphosphate (InsP3), a potent second messenger involved in many biological activities. Arabidopsis contains a single canonical IMP gene, which was previously shown in our lab to encode a bifuntional enzyme with both IMP and L-galactose 1-phosphatase activity. Analysis of metabolite levels in imp mutants showed only slight modifications with less myo-inositol and ascorbate accumulation in these mutants. This result suggests the presence of other functional IMP enzymes in plants. Two other genes in Arabidopsis encode chloroplast proteins, which we have classified as IMP-like (IMPL), because of their greater homology to the prokaryotic IMPs such as the SuhB, and CysQ proteins. Prokaryotic IMP enzymes are known to dephosphorylate D-Inositol 1-P (D-Ins 1-P) and other substrates in vitro, however their in vivo substrates are not characterized. A recent study revealed the ability of IMPL2 to complement a bacterial histidinol 1-phosphate phosphatase mutant defective in histidine synthesis, which suggested an important role for IMPL2 in amino acid synthesis. The research presented here focuses on the characterization of IMPL functional roles in plant growth and development. To accomplish this I performed kinetic comparisons of the Arabidopsis recombinant IMPL1 and IMPL2 enzymes with various inositol phosphate substrates and with L-histidinol 1-phosphate, respectively. The data supports that IMPL2 gene encodes an active histidinol 1-phosphate phosphatase enzyme in contrast to the IMPL1 enzyme which has the ability to hydrolyze D-Ins 1-P substrate and may be involved in the recycling of inositol from the second messenger, InsP3. Analysis of metabolite levels in impl2 mutant plants reveals that impl2 mutant growth is impacted by alterations in the histidine biosynthesis pathway. Together these data solidify the catalytic role of IMPL2 in histidine synthesis in plants and highlight its importance in plant growth and development. / Ph. D.

Most plant pathogens are specialists where only few plant species are susceptible, while all other plants are resistant. Unraveling the mechanisms behind this can thus provide valuable information for breeding or engineering crops with durable disease resistance. A group of Pseudomonas syringae strains with different host ranges while still closely related were thus chosen for comparative study. We confirmed their close phylogenetic relationship. We found evidence supporting that these strains recombined during evolution. The Arabidopsis thaliana and tomato pathogen P. syringae pv. tomato (Pto) DC3000 was found to be an atypical tomato strain, distinct from the typical Pto strains commonly isolated in the field that do not cause disease in A. thaliana, such as Pto T1. Comparing A. thaliana defense responses to DC3000 and T1, we found that T1 is eliciting stronger responses than DC3000. T1 is likely lacking Type III effector genes necessary to suppress plant defense. To test this, we sequenced the genomes of strains that cause and do not cause disease in A. thaliana. Comparative genomics revealed candidate effector genes responsible for this host range difference. Effector genes conserved in strains pathogenic in A. thaliana were expressed in T1 to test whether they would allow T1 to growth better in A. thaliana. Surprisingly, most of them reduced T1 growth. One of the effectors, HopM1, was of particular interest because it is disrupted in typical Pto strains. Although HopM1 has known virulence function in A. thaliana, HopM1 reduced T1 growth in both A. thaliana and tomato. HopM1 also increased the number of bacterial specks but reduced their average size in tomato. Our data suggest that HopM1 can trigger defenses in these plants. Additionally, transgenic detritivore Pseudomonas fluorescens that can secrete HopM1 shows dramatically increased growth in planta. The importance of genetic background of the pathogen for the functions of individual effectors is discussed. T1 cannot be manipulated to become an A. thaliana pathogen by deleting or adding individual genes. We now have a list of genes that can be studied in the future for the molecular basis of host range determination. / Ph. D.