Occurrence, structure, and evolution of nitric oxide synthase-like proteins in the plant kingdom
Résumé
In animals, nitric oxide (NO) is mainly produced by NO synthases (NOSes), a family of enzymes partly conserved in bacteria. In contrast, how NO is produced in plants is poorly understood, and whether these organisms have proteins homologous to animal NOSes is unknown. In this Review, with three figures, two tables, and 60 references, we describe a search for the presence of NOSes in over 1300 species of land plants and algae, which showed that NOSes are present in a few algal species but appear to not be conserved in land plants. Thus, land plants seem to have evolved mechanisms for NO synthesis different from those present in animals. These mechanisms rely mainly on nitrite reduction.Nitric oxide (NO) signaling regulates various physiological processes in both animals and plants. In animals, NO synthesis is mainly catalyzed by NO synthase (NOS) enzymes. Although NOS-like activities that are sensitive to mammalian NOS inhibitors have been detected in plant extracts, few bona fide plant NOS enzymes have been identified. We searched the data set produced by the 1000 Plants (1KP) international consortium for the presence of transcripts encoding NOS-like proteins in over 1000 species of land plants and algae. We also searched for genes encoding NOS-like enzymes in 24 publicly available algal genomes. We identified no typical NOS sequences in 1087 sequenced transcriptomes of land plants. In contrast, we identified NOS-like sequences in 15 of the 265 algal species analyzed. Even if the presence of NOS enzymes assembled from multipolypeptides in plants cannot be conclusively discarded, the emerging data suggest that, instead of generating NO with evolutionarily conserved NOS enzymes, land plants have evolved finely regulated nitrate assimilation and reduction processes to synthesize NO through a mechanism different than that in animals.