ENHANCED ACTIVITY OF A CA-CALMODULIN-DEPENDENT ISOZYME OF NAD KINASE CAUSED BY GIBBERELLIC-ACID IN PHOTODORMANT LETTUCE SEEDS

We tested the hypothesis that GA(3), like red light (Zhang et al., 199 4), enhances activity of a Ca-calmodulin-dependent isozyme of NAD kina se and causes lower levels of NAD(+) and higher levels of NADP(+) in h alf-seeds of Grand Rapids lettuce (Lactuca sativa L.). Half-seeds cont aining hypocotyl/radicle sections were imbibed for Ih in H2O or 560 mu M GA(3), treated with far-red light, then kept in darkness for variou s times up to 17 h. NAD kinase activity from crude extracts was measur ed, and activities of a partially purified Ca-calmodulin-independent a nd a Ca-calmodulin-dependent isozyme were determined. In all experimen ts kinase activity increased with time after treatment. GA(3) 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-i ndependent isozyme at any time period. GA(3) caused increased amounts of NADP(+) and NADPH and decreased amounts of NAD(+), all effects sign ificant 30 min after irradiation. Although these results might be expl ained partially by GA(3)-enhanced formation of Ca-calmodulin in vivo, it appears that GA(3) additionally or alternatively induced formation of the Ca-calmodulin-dependent isozyme of NAD kinase. These effects of GA(3) are similar to those of red light, and they support the hypothe sis that Pfr somehow promotes synthesis of a gibberellin as it overcom es photodormancy of the seeds.