CBF-responsive pathway and phytohormones are involved in melatonin-improved photosynthesis and redox homeostasis under aerial cold stress in watermelon

Abstract

Due to the insulation provided by soil, plant shoots always suffer cold stress prior to roots when ambient temperatures fall rapidly. Melatonin can enhance cold tolerance of shoots via systemic signaling from roots; however, the underlying mechanisms driving the process remain unclear. This study reports that root-pretreatment with 1.5 μM melatonin alleviated aerial cold-induced inhibition of photosystem II and oxidative stress. The fortification of the photosystem by melatonin was accompanied by increased thermal dissipation, while the melatonin-induced alleviation of oxidative stress was attributed to the improved efficiency of the ascorbate–glutathione cycle under aerial cold stress. Melatonin significantly upregulated the expressions of C-repeat binding factor 1 (CBF1), inducer of CBF expressions 1, and four cold-responsive genes after aerial cold stress. It suggests that melatonin-promoted cold tolerance might be related to the CBF-responsive pathway, which plays an important role in improving photosynthesis and redox homeostasis. Additionally, rhizospheric melatonin application increased both jasmonic acid (JA) and indoleacetic acid (IAA) levels but decreased abscisic acid (ABA) levels in leaves by regulating biosynthetic or catabolic genes under aerial cold stress. Thus, melatonin might function synergistically with JA and IAA, while antagonistically with ABA in regulating the responses of plants to cold stress. These results suggest the involvement of both CBF-responsive pathway and phytohormones in melatonin-mediated systemic tolerance against cold stress.