Nitrate reductase activity in chicory roots following excision

In young chicory plantlets (Cichorium intybus L. Witloof cv. Flash), nitrate assimilation takes place mainly in the roots. Nitrate reductase activity (NRA) was measured in roots deprived of shoot control by excision and transferred into a sucrose-containing medium. Such a treatment resulted in a drop of about 60% of NRA within 3 h. The level of NR protein decreased after 12 h and the level of NR-mRNA after several days. This adaptation of nitrate assimilation to excision was affected by a phosphorylation-dephosphorylation mechanism as shown by increased sensitivity to magnesium of in vitro NRA. Okadaic acid, a serinethreonine protein phosphatases inhibitor, enhanced the decrease of NRA. Conversely, staurosporine, a serine-threonine protein kinases inhibitor, antagonized the inhibition of NRA. This suggests that excision caused a rapid inactivation of NRA in roots of chicory by modifying the phosphorylation balance towards a phosphorylated NR form which could enter an inactive complex.