Effect of calcium channel blockers on the IAA-induced cell elongation of sunflower hypocotyl segments

Ca2+-channel blockers, verapamil and diltiazem, strongly inhibited IAA-induced elongation of sunflower (Helianthus annuus L.) hypocotyl segments. Pretreatment with Ca2+-channel blockers (0.1 mM verapamil or diltiazem) inhibited the initiation of IAA-induced elongation, but not fusicoccin-induced elongation. Verapamil, when applied 60 min after IAA, began to reduce the steady-state elongation rate with a short lag time of ca. 10 min. This inhibition of elongation by Ca2+-channel blockers was antagonized by Ca2+ pretreatment-even at low concentration (less than 50 mu M). Cations such as Mn2+, Mg K+, and Na+ also showed the antagonism with respect: to the inhibitory effect of the blockers, whereby the effect of divalent cations was much more effective than monovalent cations. The antagonism of Ca2+ against the inhibitory effect of the blockers occurred optimally ata specific Ca2+ concentration. These results suggest that Ca2+ and Ca2+-channel blockers competitively interact against the Ca2+ channel and that calcium flux through Ca2+ channels plays an important role in auxin-induced cell elongation.