Abscisic Acid Suppresses the Increases in Cell Wall-Bound Ferulic and Diferulic Acid Levels in Dark-Grown Wheat (Triticum aestivum L.) Coleoptiles

Application of abscisic acid (ABA) to dark-grown wheat (Triticumaestivum L.) roots interfered the cell wall hardening of coleoptilesduring several days of the treatment. Although the amounts ofwall-bound diferulic (DFA) and ferulic (FA) acids in coleoptilesincreased as the coleoptiles grew, ABA substantially reducedtheir increases. When ABA was removed, however, these contentsincreased and reached levels near those of control coleoptiles.A close correlation was observed between the levels of DFA andFA and the mechanical properties of cell walls. The ratio ofthe amount of DFA to FA was almost constant irrespective ofgrowth conditions. The activities of phenylalanine- (PAL) andtyrosine-ammonia-Iyase (TAL) increased rapidly in the controlcoleoptiles. ABA greatly reduced the increases in these enzymeactivities. In response to ABA removal, the enzyme activitiesincreased rapidly. There was a close correlation between theincrease in FA level and the changes in enzyme activities. Theseresults suggest that ABA suppresses the increases in PAL andTAL activities in wheat coleoptiles, resulting in the reducedlevel of wall-bound FA, which, in turn, may cause the reducedDFA level and thereby maintain cell wall extensibility. (Received January 10, 1997; Accepted April 22, 1997)