Nitric oxide (NO) is a relevant signal molecule involved in many plant processes. However, the mechanisms and
proteins responsible for its synthesis are scarcely known. In most photosynthetic organisms NO synthases have not been identified, and
Nitrate Reductase (NR) has been proposed as the main enzymatic NO source, a process that in vitro is also catalysed by other molybdoenzymes. By studying transcriptional regulation,
enzyme approaches, activity assays with in vitro purified
proteins and in vivo and in vitro NO determinations, we have addressed the role of NR and
Amidoxime Reducing Component (
ARC) in the NO synthesis process. N\R and
ARC were intimately related both at transcriptional and activity level. Thus,
arc mutants showed high NIA1 (NR gene) expression and NR activity. Conversely, mutants without active NR displayed an increased
ARC expression in
nitrite medium. Our results with nia1 and
arc mutants and with purified
enzymes support that
ARC catalyses the NO production from
nitrite taking electrons from NR and not from Cytb5-1/Cytb5-
Reductase, the component partners previously described for
ARC (proposed as NOFNiR,
Nitric Oxide-Forming
Nitrite Reductase). This NR-
ARC dual system would be able to produce NO in the presence of
nitrate, condition under which NR is unable to do it.