Comparative studies on the effect of a protein-synthesis inhibitor on aluminum-induced secretion of organic acids from Fygopyrum esculentum Moench and Cassia tora L. roots

摘要

In this study, a protein-synthesis inhibitor, cycloheximide (CHM), was used to investigate its effect on Al-induced organic acid secretion in a pattern Ⅰ plant buckwheat (Fygopyrum esculentum Moench) and a pattern Ⅱ plant (Cassia tora L.). A dose-response experiment showed that the secretion of oxalate in buckwheat roots was not affected by CHM when added in the range from 0 to 50 μM, with or without exposure to 100 μM Al, but the secretion of citrate was completely inhibited by 30 μM CHM in C. tora. A time-course experiment also showed that even prolonged exposure to 20 μM CHM did not affect oxalate secretion in buckwheat, but significantly inhibited citrate secretion in C. tora. However, citrate synthase (CS) activity in C. tora was not affected during 12 h exposure to 100 μM Al when compared with that in control roots, although CHM can inhibit CS activity effectively. These results indicated that CS activity was not related to Al-regulated citrate efflux in C. tora. A 3-h pulse with 20 μM CHM completely inhibited citrate efflux in C. tora during the subsequent 6 h exposure to Al, although a small amount of citrate was exuded after 9-h exposure. However, the oxalate efflux in buckwheat was not influenced. In buckwheat, a 3-h pulse with 100 μM Al maintained the oxalate secretion at a high level during the next 9 h, with or without CHM treatment. Conversely, in C. tora a 6-h pulse with 100 μM Al induced significant secretion of citrate which was inhibited by the CHM. Taken together, these findings suggest that both de novo synthesis and activation of an anion channel are involved in the Al-induced secretion of citrate in C. tora, but that the protein functioning as an anion channel for oxalate secretion pre-existed in the plasma membrane in buckwheat.