Amino acids, which are necessary for the synthesis of proteins as well as some other nitrogenous components in organisms, are effective for the synthesis and in the activities of some enzymes and of gene expression. Stress tolerance is provided by different mechanisms, especially with the accumulation of some specific amino acids under stress conditions in plants. Although the role of some amino acids such as proline under stress conditions has been demonstrated by many studies, the mission of some other amino acids under stress conditions has not yet been fully elucidated. Like other amino acids in organisms, glutamine is involved in the synthesis of nitrogenous compounds such as amino acids and nucleotides. The functions of glutamine in plants, which are known to be involved in signal transmission in humans, yeast, and bacteria, are not yet fully known. For this reason, various studies conducted in recent years have focused on elucidating the role of glutamine in signal transduction pathways under stress conditions. It was revealed by transcriptome analyses that exogenous glutamine applications support growth and development in some plants by inducing the expression of several genes involved in metabolism, transport, signal transduction, and stress response. It was identified that these genes synthesize transcription factors that activate the genes involved in nitrogen metabolism or stress response. The induction of these transcription factor genes by glutamine supports the idea that it functions as a signaling molecule regulating gene expression in plants. In this review, research studies investigating the role of glutamine, especially under stress conditions, were examined, to create a resource for researchers of future studies on this topic.