Influence of BAP and NAA on the expression of nitrate reductase in excised chicory roots

In young plantlets of chicory (Cichorium intybus L. var Witloof) nitrate reduction is mainly localized in roots. Following root excision, nitrate reductase activity rapidly decreased. This inhibition was first induced by phosphorylation of NR, followed by the decrease of NR-protein and NR-mRNA contents (Vuylsteker et al., 1997). Addition of 10−6 M BAP re-induced NRA in excised roots after 2 d, in vivo NRA reaching 10-fold the level of controls on day 5. This increase of NRA was related to enhanced NR-proteln and NR-mRNA contents. After 4 d, BAP increased 16-fold the NR-protein and 3.5-fold the NR-mRNA levels, respectively. In vitro NRA measured in the presence of EDTA was 5.6-fold higher in BAP-treated roots than in control roots confirming the in vivo measurements. The high discrepancy between the NR-protein level and NRA shows that limiting factors other than the NR-protein content affected NRA reactivation. With 10−6 M NAA, in vivo NRA was enhanced seven times on day 5 and in vitro NRA was increased only 2.5 times. Enhancement of the NR-protein was more restricted with NAA than with BAP. Besides a poor effect at the transcriptional level, NAA may control NRA by a phosphorylation mechanism underscored by higher sensitivity of in vitro NRA to magnesium. Resumption of NRA in the presence of either BAP or NAA occurred simultaneously to the increase of the dry weight and was probably driven by increasing needs of reduced nitrogen to support regrowth.