Efficacy and mechanisms of seed priming with melatonin to enhance salinity tolerance

Melatonin (N-acetyl-5-methoxytryptamine; MT) is an indolic compound with multifarious functions in plant growth and development. Roles of MT in tolerance to stresses such as salinity have also emerged, which are attributed mainly to its functions in scavenging reactive oxygen species, regulation of gene expression of ion transport genes, and interaction with many plant hormones. Recent identification of a plant MT receptor has now opened the possibility for this regulatory molecule to be considered a new plant hormone. In this context, exogenous application of MT, including through seed priming, has been proposed as an effective way for enhancing not only the seed germination but also the salinity tolerance of both seeds and seedlings. An increasing body of studies exists about positive effects of MT seed priming in crops. However, there is generally a dearth of knowledge about the efficacy of MT priming on seeds of noncrops, many of which hold immense potential to become crops in the future. In addition, it is also not fully known whether biochemical footprints of MT biosynthesis and functions in noncrops such as halophyte and xerophytes are similar to those of crops. Here, we present an overview of efficacy and mechanisms of MT seed priming in enhancing salinity tolerance of plants.