ABSTRACT
For more than 50 years, the only gaseous signaling molecule in the living world known to science was the plant hormone ethylene. The 1998 Nobel Prize for Medicine heralded the establishment of another, even a smaller one, player of this kind in mammalian cells-nitric oxide (NO) (Wojtaszek 2000). With the nding that NO has many functions in mammalian cells, such as regulation of vascular tone, neuronal signaling, or immune response to infection (Knowles and Moncada 1994), various studies have reported its presence in the plant kingdom as well. It has been shown that NO can regulate many processes related to plant growth and development, which led some researchers to dene NO as a plant growth regulator (Leshem et al. 1998). Indeed, NO ts most of the criteria required for plant growth regulators: it is synthesized by plants and affects physiological processes at low concentrations, which excludes its nutritional effect. Additionally, a cross talk among NO, ethylene, gibberellic acid (GA), auxin, and cytokinins supports the idea that it is involved in hormone signaling and triggers many physiological responses (Kopyra and Gwóźdź 2004).
