Crosstalk between melatonin and Ca/CaM evokes systemic salt tolerance in Dracocephalum kotschyi.

In this study, the role of calcium/calmodulin (Ca/CaM) and melatonin (Mel) as two signal molecules in inducing systemic salt tolerance of Dracocephalum kotschyi Boiss. was investigated. Salinity stress (100 mM NaCl) reduced plant growth and induced ionic, osmotic, and oxidative damages in D. kotschyi leaves. Detection of cytosolic free Ca ([Ca]) by the Fura-2 method and the measurement of endogenous Mel by GC-MS demonstrated that salinity induced Ca burst and increased endogenous Mel content in D. kotschyi leaves. Root pretreatment with 5 mM Ca or 100 μM Mel recovered plant growth, reduced leaf electrolytic leakage, HO and MDA contents and improved membrane integrity not only at the application site (roots), but also at the untreated distal parts (leaves) under salt stress. Rhizospheric treatment with Mel and Ca triggered systemic tolerance in D. kotschyi, as judged from improving RWC, increasing proline content, modulating Na, K, and Ca homeostasis, and enhancing the activities of SOD, CAT, APX, and POD in the leaves of salt-stressed plants. Mel augmented [Ca], but the rhizospheric application of Ca antagonists impaired the latter responses. Furthermore, root pretreatment with Ca increased Mel content, but the application of p-chlorophenylalanine (as an inhibitor of Mel biosynthesis) decreased the above attributes in the leaves of Ca-treated plants, leading to an arrest in the Ca-induced systemic salt tolerance. These novel results suggest that interaction of Ca/CaM and Mel is involved in overcoming salt-induced ionic, osmotic, and oxidative damages and Ca and Mel may act as long-distance signals for inducing systemic salt tolerance in D. kotschyi.