NO synthase? Generation of nitric oxide in plants

It has now become well accepted that nitric oxide (NO) has a key role to play in the signalling that takes place in plant cells. However, the sources of NO in plants has been hard to determine and there is considerable debate as to exactly how NO is made by plant cells. In animals nitric oxide synthase (NOS) enzymes have been characterised and such data has been used to inform the studies which have been taking place in plants. However, despite several genomes from higher plants being sequenced, there is no evidence that such species contain NOS sequences. Despite this, a recent search using algal sequences did reveal a NOS-like sequence and such a finding may spark new enthusiasm for the search for a higher plant NOS. However, considerable care needs to be taken in such studies, as the robustness of many of the inhibitors and probes which could be used in such work has been questioned. Here, some of the previous evidence that has been presented for the existence of a plant NOS, along with a discussion of how else plants may produce NO is given. THE GENERATION OF NO IN PLANTS Recent publication of a paper reporting the presence of a nitric oxide synthase (NOS) in algae by Foresi et al. (1) has perhaps opened the way for more speculation of whether plants do indeed contain an enzyme analogous to that found in mammals. The mammalian system was characterised during the 1990’s, while it was reported that plants use nitric oxide as a signal in 1998 (2, 3). However, there is still controversy as to whether higher plants really contain a true NOS enzyme. Here I will discuss some of the background to this controversy and which led to the paper showing that some algae at least contain a NOS protein (1). Nitric oxide (NO) was first reported to be important in plants back in 1998 by two groups (2, 3). Since then there has been a flurry of activity, with a view that the information gleamed from work in the animal kingdom should be able to inform such studies on plants, perhaps accelerating the latter to a point where it can catch up. However, this was not to be so easy as first envisaged. NO as a signal in animals was first mooted by Moncada’s group in 1987(4). It was suggested that endothelial derived relaxing factor (EDRF), which was produced by endothelial cells, and caused relaxation of smooth muscle cells, was in fact the relatively reactive gas nitric oxide. On first appearance this seems a little odd. NO is a gas, but it does dissolve in aqueous solution. However, NO is usually found as a free radical, so it is quite reactive. In fact the chemistry of NO is a little more complex with forms such as NO existing as well as the classical radical JOHN T. HANCOCK Faculty of Health and Life Sciences University of the West of England Coldharbour Lane Bristol, BS16 1QY, UK Correspondence: Dr John T. Hancock Faculty of Health and Life Sciences University of the West of England Coldharbour Lane Bristol, BS16 1QY, UK Email: [email protected] Abbreviations: ABA – abscisic acid BH4 – tetrahydrobiopterin CuAO1 – copper amine oxidase1 DAF – diaminofluorescein EDRF – endothelial derived relaxing factor eNOS – endotheial nitric oxide synthase FH4 – tetrahydrofolate iNOS – inducible nitric oxide synthase L-NAME – N(G)-nitro-L-arginine methyl ester nNOS – neuronal nitric oxide synthase NO – Nitric oxide NO – nitric oxide synthase PAs – polyamines