Nitric oxide as 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 (osmo-, chemical, physical, hormonal, redox treatments) increase the level of nitric oxide (NO) being 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 like salinity, cold, drought, excess metals. The effect of SNP/NO covers the alleviation of stress-specific effects (e.g. reduction of cadmium uptake in case of cadmium loading, improvement of water balance in case of drought), as well as general effects, such as alleviating oxidative stress. Seed pre-treatment or vegetative priming with SNP/NO up-regulates antioxidant enzymes at the activity level 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, combined donors) as seed pre-treating and vegetative priming materials. This review gives further suggestions on the methodology and future directions of fundamental research in relation to NO-associated pre-treatment and priming.