Gibberellin-like effects of KAR(1) on dormancy release of Avena fatua caryopses include participation of non-enzymatic antioxidants and cell cycle activation in embryos

MAIN CONCLUSION: The induction of dormancy release and germination ofAvena fatuacaryopses by KAR(1)involves ABA degradation to phaseic acid. Both, KAR(1)and GA(3), control the AsA–GSH cycle, DNA replication and accumulation of β-tubulin in embryos before caryopses germination. ABSTRACT: Avena fatua caryopses cannot germinate in darkness at 20 °C because of dormancy, but karrikinolide-1 (KAR(1)), a compound in plant-derived smoke, and gibberellic acid (GA(3)) induced an almost complete germination. The radicle protrusion through the coleorhiza was preceded by increased water uptake, rupture of coat, increased embryo size and coleorhiza length as well as coleorhiza protrusion through covering structures. The stimulatory effect of KAR(1) was correlated with the reduced content of abscisic acid (ABA) and an increase in phaseic acid (PA) in embryos from caryopses before coleorhiza protrusion. Two non-enzymatic antioxidants, ascorbate (AsA) and reduced glutathione (GSH), did not affect the germination of dormant caryopses, but in the presence of KAR(1) or GA(3) they only slightly delayed the germination. The stimulatory effect of KAR(1) or GA(3) on the final germination percentage was markedly antagonized by lycorine, an AsA biosynthesis inhibitor. KAR(1) and GA(3) applied during caryopses imbibition resulted in increases of AsA, dehydroascorbate (DHA) and GSH, but reduced the embryos’ oxidized glutathione (GSSG) content. Furthermore, both KAR(1) and GA(3) induced an additional ascorbate peroxidase (APX) isoenzyme and increased the glutathione reductase (GR) activity. Both compounds stimulated β-tubulin accumulation in radicle+coleorhiza (RC) and plumule+coleoptile (PC), and enhanced the transition from G(1) to S and also from S to G(2) phases. The comparison of the effects produced by KAR(1) and GA(3 ) shows a similar action; thus the KAR(1) effect may not be specific. The study provides new data regarding the mechanism with which KAR(1), a representative of a novel class of plant growth regulators, regulates dormancy and germination of caryopses.