Cornichon (CNI) proteins are a conserved family of proteins among eukaryotes, from Erv14 in the yeast Saccharomyces cerevisiae to CNI homologs (CNIHs) in mammals and plants. Erv14 functions as a cargo receptor of coat protein complex II (COPII) for protein trafficking from the endoplasmic reticulum (ER) to the Golgi apparatus, en route to their final destinations. By interacting with specific cargo proteins, CNI proteins regulate key steps of embryo polarity in Drosophila, budding in yeast, and synaptic transmission in the mammalian brain. However, we have very limited understanding of plant CNIHs. Positive-strand RNA viruses assemble their viral replication complexes (VRCs) at specific host organelle membranes. With a better understanding of host factors involved in targeting viral replication proteins to the preferred organelles, we expect to block trafficking of viral replication proteins and thus, viral infection, by manipulating the required host proteins. Brome mosaic virus (BMV) is a model of positive-strand RNA viruses and its replication can be recapitulated in yeast. Importantly, BMV replication protein 1a is the only required viral protein to form VRCs at the perinuclear ER membrane in yeast. I demonstrate that Erv14 and COPII coat proteins are required for targeting BMV 1a to the perinuclear ER in yeast, suggesting a novel function of COPII vesicles in protein trafficking to the perinuclear ER membrane and in the BMV VRC formation. As for cellular functions, I show that plant CNIHs complement the defective distribution of BMV 1a in yeast mutant lacking Erv14. Taking advantage of Arabidopsis thaliana knockout mutants and knockdown of gene expression in Nicotiana benthamina, I also discover that CNIHs unexpectedly play crucial roles in pollen development, infection of a bacterial pathogen, and maintenance of ER tubules. I further confirm that CNI proteins are also required for maintaining ER tubules in yeast, suggesting a novel and conserved role in shaping ER morphology. Therefore, these findings indicate the functional diversity and redundancy of CNI proteins in key cellular processes and suggest a novel strategy to control plant pathogenic viruses and bacteria by manipulating plant CNIHs. / Ph. D. / Many cellular proteins play important roles in plant development but unfortunately are hijacked by plant viral, bacterial, and/or fungal pathogens for their infections. Cornichon (CNI) proteins are a conserved family of proteins and a great example that is involved in both plant development and plant pathogen infection. CNI protein was first described in a <i>Drosophila</i> mutant. Only 3% of mutant cells survived, but showed abnormal phenotype in abdominal segmentation with a similar shape of “pickle” during embryo development. Later on, this family of proteins was well studied in yeast and mammals but rarely studied in plants. Erv14, one of CNI proteins in yeast, is a cargo receptor of coat protein complex II (COPII) vesicles that participate in cellular early secretory pathway. COPII vesicles serve as cellular carriers to recruit cargo proteins from the endoplasmic reticulum (ER) membrane and depart for the Golgi apparatus, en route to their final destinations for proper cellular processes. In this dissertation, I have discovered that Erv14 and COPII components are unexpectedly involved in targeting a replication protein of a plant RNA virus to the perinuclear ER membrane, instead of the Golgi apparatus, suggesting a novel function of COPII in targeting proteins to the perinuclear ER. Erv14 has never been shown as involved in viral infection and thus, my work has identified a new host protein required for viral infection. I have further explored the cellular functions of CNI proteins in plants, and found that plant CNI proteins play significant roles in maintaining cellular ER network, supporting normal pollen development, and bacterial pathogen infection. Therefore, plant CNI proteins function similarly as Erv14 to recruit various cargo proteins into COPII vesicles en route to their final destinations for proper cellular processes. These cellullar processes may include, but are not limited to: ER morphology maintenance, pollen development, and plant immune response to pathogen infection. Furthermore, it is possible to develop a novel strategy to make plants resistant to plant viruses and/or bacteria by manipulating plant CNIHs.