Involvement of NO and Ca2+ in the enhancement of cold tolerance induced by melatonin in winter turnip rape (Brassica rapa L.)

Abstract

As a multifunctional phytohormone, melatonin (Mel) plays pivotal roles in plant responses to multiple stresses. However, its mechanism of action remains elusive. In the present study, we evaluated the role of NO and Ca2+ signaling in Mel enhanced cold tolerance in winter turnip rape. The results showed that the NO content and concentration of intracellular free Ca2+ ([Ca2+]cyt) increased by 35.42% and 30.87%, respectively, in the leaves of rape seedlings exposed to cold stress. Compared with those of the seedlings in cold stress alone, the NO content and concentration of [Ca2+]cyt in rape seedlings pretreated with Mel increased further. In addition, the Mel-mediated improvement of cold tolerance was inhibited by L-NAME (a NO synthase inhibitor), tungstate (a nitrate reductase inhibitor), LaCl3 (a Ca2+ channel blocker), and EGTA (a Ca2+ chelator), and this finding was mainly reflected in the increase in ROS content and the decrease in osmoregulatory capacity, photosynthetic efficiency and antioxidant enzyme activities, and expression levels of antioxidant enzyme genes. These findings suggest that NO and Ca2+ are necessary for Mel to improve cold tolerance and function synergistically downstream of Mel. Notably, the co-treatment of Mel with L-NAME, tungstate, LaCl3, or EGTA also inhibited the Mel-induced expression of MAPK3/6 under cold stress. In conclusion, NO and Ca2+ are involved in the enhancement of cold tolerance induced by Mel through activating the MAPK cascades in rape seedlings, and a crosstalk may exist between NO and Ca2+ signaling.