Nitrate assimilation in chicory roots (Cichorium intybus L.) which acquireradial growth

Nitrate assimilation was analysed in chicory plants (Cichorium intybus L, c v, Turbo) during the early vegetative growth. Nitrate reductase (NR, EC 1.6 .6.1) activity (NRA) was measured in roots and leaves at different developm ental stages. During phase I, which corresponds to the structural growth (2 1-42 DAS), nitrate reduction mainly occurred in the roots. At the onset of the tuber formation (phase II), which is characterized by the formation of a cambium inducing a radial growth (42-63 DAS), NRA rapidly decreased in ro ots and developed in leaves. A tight correlation was found between the nitr ate content, the amino acid level and NRA in roots and leaves. Northern blo t and ELISA analysis showed that both levels of NR mRNA and NR protein were not modified during the time-course of the experiment suggesting that modi fication of nitrate assimilation was not controlled at a transcriptional le vel. In vitro NRA assayed in presence of either Mg2+ ions or EDTA showed th at NR was influenced at least in part by a reversible phosphorylation/depho sphorylation reaction. Okadaic acid, a serine-threonine protein phosphatase s inhibitor, strongly decreased NRA, Conversely, staurosporine, a serine-th reonine protein kinases inhibitor, did not significantly change NRA in root s or leaves. Therefore, NRA was regulated at a post-translational level dur ing the early vegetative growth by modifying the phosphorylation balance of the NR protein in chicory.