Gibberellin-like effects of KAR1 on dormancy release of Avena fatua caryopses include participation of non-enzymatic antioxidants and cell cycle activation in embryos

Avena fatua caryopses cannot germinate in darkness at 20 °C because of dormancy, but karrikinolide-1 (KAR₁), a compound in plant-derived smoke, and gibberellic acid (GA₃) 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 KAR₁ 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 KAR₁ or GA₃ they only slightly delayed the germination. The stimulatory effect of KAR₁ or GA₃ on the final germination percentage was markedly antagonized by lycorine, an AsA biosynthesis inhibitor. KAR₁ and GA₃ applied during caryopses imbibition resulted in increases of AsA, dehydroascorbate (DHA) and GSH, but reduced the embryos’ oxidized glutathione (GSSG) content. Furthermore, both KAR₁ and GA₃ 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 G₁ to S and also from S to G₂ phases. The comparison of the effects produced by KAR₁ and GA₃  shows a similar action; thus the KAR₁ effect may not be specific. The study provides new data regarding the mechanism with which KAR₁, a representative of a novel class of plant growth regulators, regulates dormancy and germination of caryopses.