Enhanced Activity of a Ca-Calmodulin-Dependent Isozyme of NAD Kinase and Changes in Nicotinamide Coenzyme Levels in Lettuce Half-Seeds Caused by Red Light

Abstract

Summary The hypothesis was tested that Pfr enhances formation of Ca-calmodulin during red-light-induced axis elongation accompanying dormancy breaking in seeds of Grand Rapids lettuce ( Lactuca sativa L.). The Ca-calmodulin complex was measured indirectly by measuring its known activation of NAD kinase and by assaying tissue amounts of NAD + and other nicotinamide coenzymes. Half-seeds containing hypocotyl/radicle sections were imbibed, treated with red or far-red light, then kept in darkness for various times up to 17 h. Two isozymes of NAD kinase were resolved by chromatography on a DEAE-Sephacel column, one Ca-calmodulin-dependent and one-independent. Activity of the dependent isozyme was enhanced by red compared with far-red at most times following irradiation, whereas the independent isozyme was not affected. Red light decreased the level of NAD + (compared with far-red) as early as 30 min after irradiation and at all subsequent times. Furthermore, the ratio of NADP + plus NADPH to NAD + plus NADH was enhanced by red relative to far-red at all sampling periods, consistent with in vivo activation of NAD kinase. Data are consistent with the hypothesis that Pfr causes formation of Ca-calmodulin that activates the kinase, but the simplest explanation is that Pfr induces formation of the dependent but not the independent kinase isozyme.