Influence of ABA on nitrate reductase activity and carbohydrate metabolism in chicory roots (Cichorium intybus L.)

Abstract

Young chicory plants transferred to abscisic acid (ABA)-containing media displayed changes in nitrate and carbohydrate metabolism. The hormone (10 -7 M) rapidly promoted nitrate reductase (NADH:NR, EC 1.6.6.1) activity (NRA) in roots. The hormonal effects on NRA were not responsive to dose concentrations (10 -8 to 10 -4 M) and were not related to the intracellular nitrate content. Furthermore, the ABA-regulated NRA was not modulated by a phosphorylation/dephosphorylation mechanism. ELISA experiments using NR antibodies showed that the NR protein accumulated in ABA-treated roots. Northern blot analysis of NR mRNA indicated that the response to exogenous ABA was, at least in part, at the level of transcription and the NR mRNA accumulation was accompanied by cytosolic glutamine synthetase (GS1) overexpression. Since NRA can be controlled by sucrose and the availability of the reducing power, the carbohydrate metabolism was analysed simultaneously in the chicory roots. HPLC analysis and invertase assays showed that exogenous ABA modified the neutral invertase activity and the carbohydrate levels as well. Sucrose hydrolysis was increased and the levels of both glucose and fructose dramatically decreased in the presence of ABA suggesting a relationship between both nitrate and sucrose metabolism.