4 - Nitrate reductase dependent synthesis of NO in plants

Abstract

Nitric oxide (NO) is an important gaseous signaling molecule involved in numerous plant physiological processes. Despite intensive studies, its production in plant cells has still not been fully understood. Two main pathways, oxidative and reductive, responsible for NO biosynthesis were identified in plants. In the oxidative pathway, biosynthesis of NO requires arginine and activity of enzyme similar to nitric oxide synthase (NOS), operating in animals. The reductive route involves reduction of nitrite ions to NO through different nonenzymatic and enzymatic reactions, catalyzed by several molybdoenzymes. The best characterized protein involved in reductive production of NO is cytosolic nitrate reductase (NR). This enzyme catalyzes mainly the reduction of nitrate to nitrite during N assimilation process, but under specific conditions, it can use nitrite and reduce them to NO. Although many evidences in vitro and in vivo indicate the involvement of NR in NO generation in plants, some negative arguments are present, questioning the direct role of this enzyme in NO production. The doubts concern mostly differences in NR affinity for nitrite and nitrate. Several reports acknowledge the possible existence of another mechanism of NO production. They suggest that NR is not directly responsible for NO biosynthesis but interacts as the substrate or electron donor for other partner proteins. Such mechanisms have been proposed for plasma membrane-associated forms of NR, present in roots of higher plants. Recently, it was also described in unicellular alga, Chlamydomonas reinhardtii. In this chapter, we have briefly reviewed the role of NR in NO biosynthesis and present the recent data in this field.