KAR-induced dormancy release in Avena fatua caryopses involves reduction of caryopsis sensitivity to ABA and ABA/GA ratio in coleorhiza and radicle.

The dormancy release by KAR is associated with a reduction of coleorhiza and radicle sensitivity to ABA as well as with reduction the ABA/GA ratio in the coleorhiza, by a decrease content of ABA, and in the radicle, by a decrease the ABA and an increase of the GA contents. Both, karrikin 1 (KAR) and gibberellin A (GA), release dormancy in Avena fatua caryopses, resulting in the emergence of coleorhiza (CE) and radicle (RE). Moreover, KAR and GA stimulate CE and RE in the presence of abscisic acid (ABA), the stimulation being more effective in CE. The stimulatory effects of KAR and GA involve also the CE and RE rates. A similar effect was observed at KAR concentrations much lower than those of GA. KAR increased the levels of bioactive GA and GA in embryos and the levels of GA, GA, GA GA and GA in radicles. The stimulatory effect of KAR on germination, associated with increased levels of gibberellins (GA) and reduced levels of ABA in embryos, was counteracted by paclobutrazol (PAC), commonly regarded as a GA biosynthesis inhibitor. Consequently, KAR decreased the ABA/GA ratio, whereas PAC, used alone or in combination with KAR, increased it. The ABA/GA ratio was reduced by KAR in both coleorhiza and radicle, the effect being stronger in the latter. We present the first evidence that KAR-induced dormancy release requires a decreased ABA/GA ratio in coleorhiza and radicle. It is concluded that the dormancy-releasing effect of KAR in A. fatua caryopses includes (i) a reduction of the coleorhiza and radicle sensitivity to ABA, and (2) a reduction of the ABA/GA ratio (i) in the coleorhiza, by decreasing the ABA content, and (ii) in the radicle, by decreasing the ABA and increasing the content GA, particularly GA. The results may suggest different mechanisms of dormancy release by KAR in monocot and dicot seeds.