Melatonin systemically ameliorates drought stress-induced damage in Medicago sativa plants by modulating nitro-oxidative homeostasis and proline metabolism. 
Recent reports have uncovered the multifunctional role of melatonin in plant physiological responses under optimal and suboptimal environmental conditions.  In this study, we explored whether melatonin pretreatment could provoke priming effects in alfalfa (Medicago sativa L.) plants subsequently exposed to prolonged drought stress (7 days), by withholding watering.  Results revealed that the rhizospheric application of melatonin (10 μmol L-1 ) remarkably enhanced the drought tolerance of alfalfa plants, as evidenced by the observed plant tolerant phenotype, as well as by the higher levels of chlorophyll fluorescence and stomatal conductance, compared with nontreated drought-stressed plants.  In addition, lower levels of lipid peroxidation (MDA content) as well as of both H2 O2 and NO contents in primed compared with nonprimed stressed plants suggest that melatonin pretreatment resulted in the systemic mitigation of drought-induced nitro-oxidative stress.  Nitro-oxidative homeostasis was achieved by melatonin through the regulation of reactive oxygen (SOD, GR, CAT, APX) and nitrogen species (NR, NADHde) metabolic enzymes at the enzymatic and/or transcript level.  Moreover, melatonin pretreatment resulted in the limitation of cellular redox disruption through the regulation of the mRNA levels of antioxidant and redox-related components (ADH, AOX, GST7, GST17), as well via osmoprotection through the regulation of proline homeostasis, at both the enzymatic (P5CS) and gene expression level (P5CS, P5CR).  Overall, novel results highlight the importance of melatonin as a promising priming agent for the enhancement of plant tolerance to drought conditions through the regulation of nitro-oxidative and osmoprotective homeostasis.