The stimulative action of applied gibberellins (GA) on seed germination has been reported for a large number of species in the past three decades. Applied GA often replaces the need for environmental stimuli like specific temperature pretreatment or light. Therefore, it has been suggested that endogenous GAs are essential intermediates in the stimulation of germination. Endogenous GAs have been identified in seeds of a limited number of species, but most of the evidence for a regulatory role is entirely circumstantial. The use of isogenic mutants with lesions in the GA biosynthesis has presented direct evidence of a key role for endogenous GAs in the regulation of germination. GA deficiency absolutely prevents germination in seeds of Arabidopsis thaliana and tomato. Two different mechanisms of action exist The first one involves hydrolysis of reserve food. In seeds of tomato a factor, probably GA, diffuses prior to germination from embryo to endosperm where it induces hydrolysis of gaJactomannan-rich cell walls that are part of the mechanical resistance to the protrusion of the radicle. A second mechanism of GA action consists of a direct effect on the growth potential of the embryo. In Arabidopsis the stimulation of germination by light depends on the ability of the seeds to synthesize GA, but light also enhances the sensitivity of the seeds to GA. Dry storage and pre-incubation at 2°C also increased the responsiveness of Arabidopsis seeds to GA

Ethylene, a gaseous plant hormone regulates essentially all physiological processes during the plant’s life cycle. The practical implications of ethylene biosynthesis regulation for plant improvement have supported the continuous basic research on dissecting the structure of genes encoding ethylene biosynthetic enzymes, their differential expression patterns, and mechanisms underlying their transcriptional activity. ACC oxidase (ACO) is involved in the final step of ethylene production in plant tissues. In various plants several ACO isoforms have been isolated which are encoded by a multigenic family. There is a strong evidence that ACO gene expression is positively correlated to the ethylene production rates and its multiple isoforms are under development and environmental control. Thus, the regulation of ACO gene activity may act either as an additional or in several cases also as a main level for controlling ethylene biosynthesis in higher plants. This review summarizes in detail the knowledge about organization and gene structure, and transcriptional expression of ACO genes from different plant species. The perspectives of manipulating ACO gene as a method in biotechnological modification of ethylene synthesis are also discussed.

Main conclusion The induction of dormancy release and germination of Avena fatua caryopses 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 (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.