Nitric oxide signalling in plants

Recently nitric oxide (NO) has emerged as a key signalling molecule in plants. Here we review the potential sources of endogenous NO, outline the biological processes likely to be mediated by NO, and discuss the downstream signalling processes by which NO exerts its cellular effects. It will be important to develop methods to quantify intracellular NO synthesis and release. Clasification of the biosynthetic origins of NO is also required. NO can be synthesised from nitrite via nitrate reductase (NR) and although biochemical and immunological data indicate the presence of enzyme(s) similar to mammalian nitric oxide synthase (NOS), no NOS genes have been identified. NO can induce various processes in plants, including the expression of defence-related genes and programmed cell death (PCD), stomatal closure, seed germination and root development. Intracellular signalling responses to NO involve generation of cGMP, cADPR and elevation of cytosolic calcium, but in many cases, the precise biochemical and cellular nature of these responses has not been detailed. Research priorities here must be the reliable quantification of downstream signalling molecules in NO-responsive cells, and cloning and manipulation of the enzymes responsible for synthesis and degradation of these molecules.