Nitric oxide in plants: the biosynthesis and cell signalling properties of a fascinating molecule

Abstract

Nitric oxide (NO) is both a gaseous free radical and a versatile cell-signalling effector that plays important roles in diverse (patho)physiological processes. In ani mals, NO production is catalyzed predominantly by nitric oxide synthases (NOS), which are heme-contain ing proteins related to the cytochrome P450 family. These enzymes catalyze the conversion of L-arginine to L-citrulline and NO using NADPH and molecular oxy gen as cosubstrates, and employ FAD, FMN, tetrahy drobiopterin (BH4), and calmodulin (CaM) as cofactors (Bogdan 2001). The biological effects of NO are medi ated by posttranslational modification of cysteine resi dues and transition metal centers, a key process referred as nitrosylation (Stamler et al. 2001). Because of its high biological reactivity, NO production by NOS is tightly regulated to control the specificity of its signalling as well as to limit its toxicity (Kone et al. 2003). In recent years, NO has also become an increasingly popular target of investigation in plants. NO has been implicated in disease resistance, stomatal closure, re sponses to abiotic stress, iron homeostasis, and in vari ous developmental processes (Neill et al. 2002a, b; Wendehenne et al. 2004). A major advance in our understanding of NO functions in plants has been the identification of enzymes that catalyze NO synthesis. Nitrate reductase (NR) was the first enzymatic source of NO to be identified (Yamasaki and Sakihama 2000). In addition to its role in nitrate reduction, NR catalyzes the reduction of nitrite to NO using NAD(P)H as co-factor. As recently discussed by Meyer et al. (2004), it remains