Signalling pathways of nitric oxide in plants

Purpose of Review Similarly to animals, nitrous oxide (NO) has emerged recently as a key signalling molecule in many physiological and pathological processes in plants. This review summarizes the current knowledge and understanding of the molecular mechanisms of NO synthesis and signalling in plant cells. Findings NO has been shown to participate in vital developmental processes in plants like germination, tissue differentiation, growth, flowering and senescence. NO functions as a signalling molecule in plant responses to abiotic and biotic external stimuli. NO can be produced by several different enzymatic and non-enzymatic reactions, depending on the plant cell type. Actually, the identity and role of plant homologues of animal NO synthases has not been clearly described, while nitrite-dependent NO production by nitrate reductase has been demonstrated in several plant species. High reactivity and mobility of NO in plant cell is the basis for its complex reactions and a wide array of plausible molecular targets. Intracellular downstream NO signalling includes cGMP- and cADP-ribose cascade leading to changes in the intracellular Ca2+ level. NO signalling can be mediated by microtubule-associated protein (MAP) kinases or effected by covalent protein modifications such as cysteine nitrosylation. Conclusions Despite considerable advances in plant NO research, our understanding of NO signalling pathways is still very limited. Current research is focused on the identification of tissue and subcellular specific NO synthesis and its fate as well as on the crosstalk of NO with signalling pathways of reactive oxygen species and plant hormones. The employment of Arabidopsis thaliana mutants with altered expression of enzymes involved in NO synthesis or metabolism will substantially contribute to the elucidation of the NO role in plant cell signalling.