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.