Enhanced Activity of a Ca-Calmodulin-Dependent Isozyme of NAD Kinase Caused by Gibberellic Acid in Photodormant Lettuce Seeds

Abstract

Summary We tested the hypothesis that GA3, like red light (Zhang et al., 1994), enhances activity of a Ca-calmodulin- dependent isozyme of NAD kinase and causes lower levels of NAD+ and higher levels of NADP+ in half-seeds of Grand Rapids lettuce (Lactuca sativa L.). Half-seeds containing hypocotyl/radicle sections were imbibed for 1 h in H2O or 560 μM GA3, treated with far-red light, then kept in darkness for various times up to 17 h. NAD kinase activity from crude extracts was measured, and activities of a partially purified Ca-calmodulin-independent and a Ca-calmodulin-dependent isozyme were determined. In all experiments kinase activity increased with time after treatment. GA3 enhanced activity in crude extracts and of the Ca-calmodulin-dependent isozyme 30 min after irradiation, but it had no effect on the Ca-calmodulin-independent isozyme at any time period. GA3 caused increased amounts of NADP+ and NADPH and decreased amounts of NAD+, all effects significant 30 min after irradiation. Although these results might be explained partially by GA3-enhanced formation of Ca-calmodulin in vivo, it appears that GA3 additionally or alternatively induced formation of the Ca-calmodulin-dependent isozyme of NAD kinase. These effects of GA3 are similar to those of red light, and they support the hypothesis that Pfr somehow promotes synthesis of a gibberellin as it overcomes photodormancy of the seeds.