microRNAs (miRNAs) are non-coding RNAs with short sequences that negatively regulate gene expression at the post-transcriptional levels by either binding to mRNAs for degradation, or by inhibiting protein translation. miRNAs are highly evolutionarily conserved, from lower mosses to higher flowering plants, and have been shown to play an important role in plants by regulating growth and development, developmental timing, hormone signaling, organogenesis, and response to environmental stresses. Based on the conservation of mature miRNA sequences, computational methods have been used to predict thousands of miRNAs in numerous plant species, such as soybean, maize, cotton, potato, rice, apple, and switchgrass. In this project, using tobacco as my model organism, I first employed genome survey sequence analysis to identify conserved miRNAs in tobacco and then investigated their expression profiles during different growth conditions. My results identified 259 potentially conserved miRNAs in tobacco, belonging to 65 miRNA families. In addition, I also discovered antisense miRNAs as well as miRNA clusters in tobacco and predicted putative target genes for the newly identified tobacco miRNAs. Eleven of these miRNAs are highly expressed in young tobacco seedlings as well as under different environmental stress conditions. The expression profiles of tobacco miRNAs were significantly affected by Titanium Dioxide nanoparticles, salt, and drought stresses, in a dosage-dependent manner. Some miRNAs, for example miR395, exhibited a change in expression of one thousand fold after exposure to stress conditions. Abiotic stresses also affected the expression of two stress-related genes, alcohol dehydrogenase and alcohol peroxidase. Given the results of this project, I believe that miRNAs may play an important role in tobacco growth and development and that miRNAs may function in tobacco tolerance to environmental stresses.  M.S
