THE EFFECTS OF ABSCISIC-ACID AND METHYL JASMONATE ON 1-AMINOCYCLOPROPANE 1-CARBOXYLIC ACID CONVERSION TO ETHYLENE IN HYPOCOTYL SEGMENTS OF SUNFLOWER SEEDLINGS, AND THEIR CONTROL BY CALCIUM AND CALMODULIN

The conversion of 1-aminocyclopropane 1-carboxylic acid (ACC) to ethylene by hypocotyl segments of sunflower (Helianthus annuus L.) seedlings was inhibited by abscisic acid (ABA) and methyl jasmonate (Me-Ja), and this inhibitory effect increased with increasing concentration of both growth regulators. On the contrary, CaCl2 enhanced ACC conversion to ethylene at the concentrations of 10(-4) M and 5 x 10(-4) M, however lower and higher concentrations had no significant action. CaCl2 (5 x 10(-4) M) seemed to magnify the inhibition of the reaction induced by ABA, whereas it reduced (5 x 10(-4) M) and even abolished (10(-3) M) the inhibitory action of Me-Ja. The results obtained with a Ca2+ chelator (EGTA), a Ca2+ channel blocker (nifedipine) and calmodulin antagonists (W7 and TFP), given in association with ABA or Me-Ja, suggested that calcium was involved in the inhibition of ACC conversion to ethylene by ABA and Me-Ja through an interaction with calmodulin. However, the mechanism of action of the two growth regulators seemed to be different, since all treatments which resulted in a decrease in cytosolic Ca2+ concentration or in calmodulin action induced a decrease in the effect of ABA and an increase in the effect of Me-Ja.