Nitric oxide synthase in plants: Where do we stand?

- Plants use nitrite as main substrate for NO synthesis.
- Land plants do not possess nitric oxide synthase-like enzymes.
- A nitric oxide synthase-like synthase is conserved in few algal species.
- Algal NOSs display specific structural features.

Over the past twenty years, nitric oxide (NO) has emerged as an important player in various plant physiological processes. Although many advances in the understanding of NO functions have been made, the question of how NO is produced in plants is still challenging. It is now generally accepted that the endogenous production of NO is mainly accomplished through the reduction of nitrite via both enzymatic and non-enzymatic mechanisms which remain to be fully characterized. Furthermore, experimental arguments in favour of the existence of plant nitric oxide synthase (NOS)-like enzymes have been reported. However, recent investigations revealed that land plants do not possess animal NOS-like enzymes while few algal species do. Phylogenetic and structural analyses reveals interesting features specific to algal NOS-like proteins.