Glycine Betaine is a Phytohormone-Like Plant Growth and Development Regulator Under Stress Conditions

For many years, glycine betaine (N, N, N-trimethylglycine, GB) has been widely studied as an osmolyte in plants and bacteria. GB is synthesized in chloroplast, peroxisome, and cytoplasm by a two-step oxidation reaction of choline; however, not all plant species can synthesize GB. GB plays an essential role in the response of plants to abiotic stress acting as an osmolyte and osmoprotectant. This review aims to gather information about the role of GB as a phytohormone-like plant growth regulator under abiotic stress conditions. GB participates in several plant responses to cope with stressful conditions, including the regulation of gene expression, the concentration and activity of enzymes, and proper protein folding and association. GB increases photosynthetic capacity and protects the thylakoid membrane proteins and lipids. Additionally, GB increases the activity and concentration of the Calvin cycle key enzymes and antioxidant enzymes. Furthermore, GB increases the expression of auxin-responsive indole acetic acid (IAA) and cell division cycle-involved genes. All the changes caused by GB in the thus far studied plants influence plant growth, development, and productivity. This work summarizes the findings that show the role of GB in coping with abiotic stress and its possible role as a phytohormone-like compound.