Changes in soluble and membrane-bound isoforms of calcium-calmodulin-dependent and -independent NAD+ kinase, during the culture of after-ripened and dormant seeds of Avena sativa

Abstract

Activities of the soluble and membrane-boundisoforms of Ca ²⁺ calmodulin (CaCam)-dependent and-independent NAD ⁺ kinases, were followed in theembryos during the culture of dormant (D) and after-ripened (AR) seeds ofAvena sativa L. Embryos of D and AR seeds differ mainly in the evolution ofmembrane-bound activities, the majority of which are CaCam-dependent andlinked to mitochondria. The in vivo application ofgibberellic acid, CaCl₂ andH₂O₂, which enhanced germination,induced an enhancement of all CaCam-dependent isoforms. Trifluoperazine (TFP),a calmodulin antagonist, greatly enhanced all CaCam-dependent isoforms andabolished the differences between the NAD ⁺ kinaseactivities of the two kinds of embryo. In addition, TFP rendered embryosunable to resume axis growth, probably due to pleiotropic effects. In contrastto H₂O₂, the reducing agentdithiothreitol diminished the soluble CaCam-dependent enzyme and blocked thegermination of both types of seed, whereas it increased the dependentmembrane-bound activities. The results demonstrate (1) that theCaCam-dependent NAD ⁺ kinase isoforms —amongst which is the isoform bound to mitochondrial membranes — play animportant role at the end of sensu stricto germinationand during the following growth of Avena sativa; and (2)that an excess of activity of these isoforms could be markers of stress orlethal conditions.