Effect of nitrogen concentration on fructan and fructan metabolizing enzymes in young chicory plants (Cichorium intybus)

Abstract

Witloof chicory seeds (Cichorium intybus L. var. foliosum cv. Flash) were sown in acid-washed vermiculite in a controlled environment growth chamber. Plants received a nitrogen poor ('N-poor': 0.2 mM NH4NO3) but otherwise complete medium, or a nitrogen rich ('N-rich': 2 mM NH4NO3) medium. After 1 month of growth the fructan concentration in the 'N-poor' plants was about five times higher and also the activity of sucrose:sucrose 1-fructosyl transferase (1-SST; EC 2.4.1.99) was twice as high as in 'N-rich' plants. The activities of the catabolic enzymes fructan 1-exohydrolase (1-FEH; EC 3.2.1.80) and acid invertase (EC 3.2.1.26) were higher in the 'N-rich' plants where significant energy was invested in root and leaf growth. After one month of growth, part of the 'N-poor' plants were switched to the 'N-rich' medium. One day after this switch, a sharp decrease in sucrose and glucose concentration was observed in the roots. During the following days, both the activities of 1-SST and fructan:fructan 1-fructosyl transferase (1-FFT; EC 2.4.1.100) decreased and the 1-FEH and invertase activities increased. These changes were correlated with a decrease in fructan concentration. Ten days after the switch, glucose and sucrose concentrations increased again and fructan synthesis resumed. During this period 1-SST activity increased and 1-FEH activity decreased. Apparently 1-SST, 1-FFT and 1-FEH simultaneously control fructan in young chicory roots. The rather unexpected finding that 1-FEH activity, which was believed to occur only in older material, can be induced in very young roots indicates that this enzyme can be induced at any physiological stage.