Figure 1. Model of the NO-mediated nitrate regulation of primary root elongation. (A) The transfer of seedlings from a NO3--deprived media to a NO3--supplied solution results in a elongation of the primary root. The stimulatory effect of NO3- (1mM) was demonstrated to be dependent on the control of nitric oxide (NO) homeostasis thank to the coordinate regulation of cytosolic nitrate reductase (NR) and non-symbiotic hemoglobins (nsHbs) (B).67,68 The preferential localization and the strong transcriptional responsiveness of both NR and nsHbs in the transition zone of the apex straightened the hypothesis of a role of this root portion in translating the environmental stimuli in developmental response.67,68 Because of the role of NO in several cytoskeleton-mediated processes in plants,76-83 the actin-dependent endocytosis and the organization of the actin cytoskeleton are proposed as candidates in transducing the NO-dependent nitrate regulation of root elongation.