Expression of ethylene biosynthesis genes in the gynoecium and receptacle associated with sepal abscission during senescence in Delphinium grandiflorum

Delphinium flowers are highly sensitive to ethylene and its sepals abscise during senescence in association with an increase in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities and ethylene production in gynoecium and receptacle. Three ACS genes (DgACS1, DgACS2, and DgACS3) and three ACO genes (DgACO1, DgACO2, and DgACO3) of D. grandiflorum cv. Super Grand Blue were cloned. To investigate the contribution of these genes to ethylene production, their expression in the gynoecium and receptacle was analyzed during natural senescence and after ethylene exposure and pollination. Ethylene production in the receptacle significantly increased, whereas that in the gynoecium exhibited only a slight increase during natural senescence. The transcript levels of ACS and ACO in these organs, excluding those of DgACS2 in the receptacle, increased during senescence. Exposure to ethylene accelerated sepal abscission and increased ethylene production more markedly in the receptacle than in the gynoecium. DgACS1 transcript levels in the gynoecium and DgACS2 and DgACO3 transcript levels in the receptacle increased after ethylene exposure. Pollination accelerated sepal abscission and increased ethylene production in the gynoecium and receptacle. Additionally, it slightly affected ACS and ACO transcript levels in the gynoecium, whereas DgACO3 transcript levels in the receptacle markedly increased. These results reveal that ACS and ACO expression is differently regulated in the gynoecium and receptacle. Furthermore, some of these genes are upregulated by pollination only in the receptacle, indicating the significance of the receptacle in ethylene biosynthesis associated with sepal abscission.