Involvement of ethylene biosynthesis and perception during germination of dormant Avena fatua L. caryopses induced by KAR or GA

Main conclusion: Germination of primary dormant wild oat caused by KAR or GA is associated with ACC accumulation and increased ethylene production shortly before radicle protrusion as a result of the non-transcriptional and transcriptional activation of ACS and ACO enzymes, respectively. Response to both compounds involves the modulation of ethylene sensitivity through ethylene receptor genes. Harvested Avena fatua caryopses are primary dormant and, therefore, germinated poorly at 20 °C. Karrikin 1 (KAR ), which action probably requires endogenous gibberellins (GAs), and gibberellin A (GA ) was found to induce dormant caryopses to germinate. The stimulatory effects were accompanied by the activation of the ethylene biosynthesis pathway and depended on undisturbed ethylene perception. KAR and GA promoted 1-aminocyclopropane-1-carboxylic acid (ACC) accumulation during coleorhizae emergence and ethylene production shortly prior to the radicle protrusion, which resulted from the enhanced activity of two ethylene biosynthesis enzymes, ACC synthase (ACS) and ACC oxidase (ACO). The inhibitor of ACS adversely affected beneficial impacts of both KAR and GA on A. fatua caryopses germination, while the inhibitor of ACO more efficiently impeded the GA effect. The inhibitors of ethylene action markedly lowered germination in response to KAR and GA . Gene expression studies preceded by the identification of several genes related to ethylene biosynthesis (AfACS6, AfACO1, and AfACO5) and perception (AfERS1b, AfERS1c, AfERS2, AfETR2, AfETR3, and AfETR4) provided further evidence for the engagement of ethylene in KAR and GA induced germination of A. fatua caryopses. Both AfACO1 and AfACO5 were upregulated, whereas AfACS6 remained unaffected by the treatment. This suggests the existence of different regulatory mechanisms of enzymatic activity, transcriptional for ACO and non-transcriptional for ACS. During imbibition in water, AfERS1b was stronger expressed than other receptor genes. In the presence of KAR or GA , the expression of AfETR3 was substantially induced. Differential expression of ethylene receptor genes implies the modulation of caryopses sensitivity adjusted to ethylene availability and suggests the functional diversification of individual receptors.