Under different stress factors such as high soil salt levels, cold, drought, heat, and heavy metal, growth of plants is impeded, and they need the specific capacity of adaptation. Some compounds with low molecular weight in response to environmental stress can accumulate in plants: protective sugars and amino acids. Compatible solutes (osmolytes) accumulation, osmotic adjustment, induction of antioxidant enzymes, and free-radical quenching system are some of the important and effective strategies to make plants resilient against stress conditions. Many processes related to physiological features such as senescence, flowering, germination of seed, photosynthesis, etc., under conditions of different abiotic stress factors are facilitated by sugars as the most significant regulators and compatible osmolytes. Proline, an amino acid, has a useful role in plants under conditions of different stress factors. Proline, in addition to functioning as an osmolyte, has three key functions in stress: a signaling molecule, an antioxidative defense molecule, and a metal chelator. Proline overproduction in plants imparts tolerance to stress by keeping osmotic balance or cell turgor, stabilizes membranes, and consequently prevents electrolyte leakage, and brings reactive oxygen species (ROS) concentrations to normal ranges, thereby preventing oxidative burst in plants. In particular, sugars and proline can be active antioxidants in the cells.
