Differential effects of calmodulin antagonists on indol-3-ylacetic Add-and gibberellic acid-induced biphasic growth responses

The effects of chlorpromazine and calmidazolium on rapid IAA- and GA₃-induced changes in growth rate of etiolated pea stems were measured. During the initial period of up to 160 min after hormone application, the responses to both IAA and GA₃ were seen to be biphasic, showing two acceleration peaks. Chlorpromazine or calmidazolium applied simultaneously with the hormones blocked the first IAA-induced acceleration peak, but did not affect the second one. In contrast, the first GA₃-induced peak was not prevented by chlorpromazine or calmidazolium, while the second one was completely abolished. The results support the concept that the active calmodulin-calcium complex may be an essential component of hormone-controlled stem elongation. They also point to differential mechanisms of IAA and GA₃ actions.