Salicylic acid activates nitric oxide synthesis in Arabidopsis

The relationship between nitric oxide (NO) and salicylic acid (SA) was investigated in Arabidopsis thaliana. Here it is shown that SA is able to induce NO synthesis in a dose-dependent manner in Arabidopsis. NO production was detected by confocal microscopic analysis and spectrofluorometric assay in plant roots and cultured cells. To identify the metabolic pathways involved in SA-induced NO synthesis, genetic and pharmacological approaches were adopted. The analysis of the nia1,nia2 mutant showed that nitrate reductase activity was not required for SA-induced NO production. Experiments performed in the presence of a nitric oxide synthase (NOS) inhibitor suggested the involvement of NOS-like enzyme activity in this metabolic pathway. Moreover, the production of NO by SA treatment of Atnos1 mutant plants was strongly reduced compared with wild-type plants. Components of the SA signalling pathway giving rise to NO production were identified, and both calcium and casein kinase 2 (CK2) were demonstrated to be involved. Taken together, these results suggest that SA induces NO production at least in part through the activity of a NOS-like enzyme and that calcium and CK2 activity are essential components of the signalling cascade.