Exogenous CaCl2 enhanced adventitious root formation in Cucumis sativus L. under cadmium stress through protein S-nitrosylation.

Cadmium (Cd) stress severely hinders plant growth and development. Calcium ion (Ca2 +) and nitric oxide (NO) are involved in plant growth processes and stress responses. However, their crosstalk in adventitious rooting under Cd stress remains unclear. In this study, the effect of Ca2+ on enhancing the adventitious root development of cucumber under Cd condition was dose-dependent, with a maximal biological response at 200 μM CaCl2. Moreover, Ca2+ chelator obviously reversed the positive influence of Ca2+ on promoting adventitious rooting under Cd stress, which indicates that Ca2+ is indispensable for the adventitious rooting under Cd stress. Meanwhile, removing endogenous NO significantly reversed Ca²⁺-induced root promotion under Cd stress. Furthermore, exogenous Ca2+ significantly increased endogenous NO production and total S-nitrosothiol (SNO) level under Cd stress implies that S-nitrosylation might be involved in Ca2+-induced adventitious root formation under Cd stress. To further investigate whether S-nitrosylation is key molecular switch of Ca2+-induced adventitious rooting under Cd stress, the potential S-nitrosylated proteins have been identified. An increase in the total S-nitrosylated proteins was detected under CaCl2 treatment. Nevertheless, Ca2+ chelator significantly reduced the level of Ca2+-induced protein S-nitrosylation under Cd stress. A total of 117 S-nitrosylated proteins have been identified which mainly enriched in various biological processes, such as oxidation-reduction process, metabolic process, glycolytic process and so on. Additionally, denitrosylation agent treatment obviously decreased the activities of key enzymes during adventitious rooting under stress condition. These results suggest that Ca2+-induced S-nitrosylation of key proteins might participate in various pathways through differential S-nitrosylation during adventitious rooting under Cd stress. In summary, Ca2+ could promote the adventitious rooting under Cd stress by NO-dependent S-nitrosylation.