Nitric oxide as an integral element in priming-induced tolerance and plant stress memory

The beneficial effects of priming technology are aimed at the promotion of growth and development and stress tolerance in plants. Different seed pre-treatment and vegetative priming approaches (osmotic, chemical, physical, hormonal, and redox treatments) increase the level of nitric oxide (NO) which is an active contributor to growth regulation and defence responses. On the other hand, seed pre-treatment or vegetative priming mainly with the NO donor, sodium nitroprusside (SNP), helps to mitigate different abiotic stresses such as salinity, cold, drought, and excess metals. The effect of SNP/NO covers the alleviation of stress-specific effects (e.g. reduction of cadmium uptake in the case of cadmium loading, or improvement of water balance in the case of drought), as well as general effects, such as alleviating oxidative stress. Seed pre-treatment or vegetative priming with SNP/NO up-regulates the activity of antioxidant enzymes and increases the amount of a wide range of non-enzymatic antioxidants. However, due to adverse effects of SNP, we urge the testing of other conventional NO donors (e.g. S-nitrosoglutathione) as well as new substances with more favourable properties (e.g. NO-releasing nanomaterials, plasmas, plasma-treated liquids, or combined donors) as seed pre-treating and vegetative priming materials. This review provides further suggestions on the methodology and future directions of fundamental research in relation to NO-associated pre-treatment and priming.