An approach to revealing regulatory growth mechanisms in Chrysanthemum morifolium: Ethylene and gibberellin as a signaling factor in growth regulation

The plant hormone ethylene suppresses flower initiation and internode elongation in chrysanthemum (Chrysanthemum morifolium Ramat.), as does the application of ethephon, which is hydrolyzed in plant tissue and releases ethylene. The effects of ethephon are unstable, and vary with cultivar and time, season, and method of application. We describe the variation in response to ethephon due to temperature, season, and cultivar. The seasonality of extension growth and flowering capacity, growing temperatures, and genetic background of chrysanthemum made the effects of ethephon unstable and variable. Lower temperatures enhanced the suppression of internode elongation and flowering by ethephon, as confirmed by the consistently higher rate of ethylene release and greater slowing of plant extension growth and flowering at lower temperatures. After the summer plant growth, it became more difficult for ethephon-sprayed plants to elongate and flower, because they were in a low-capacity state. Cultivars that easily form rosettes, and show suppression of flowering, are highly sensitive to ethephon. In such cultivars, ethephon induced the formation of rosettes and completely prevented flowering. Since rosette formation and suppression of flowering could be linked to dormancy in chrysanthemum, this indicates that ethylene might be involved in the induction of dormancy in chrysanthemum. Temperature plays a significant role in the annual cycle between growth and dormancy of the herbaceous perennial chrysanthemum. After exposure to high summer temperatures, cool temperatures (15degC) triggers dormancy. Cessation of flowering and formation of rosettes by cessation of elongation are characteristic of dormant plants, and can be stimulated by exogenous ethylene of ethephon and 1-aminocyclopropane-1-carboxylic acid. Thus, the ethylene response pathway might be involved in the temperature-induced dormancy of chrysanthemum. We used transgenic chrysanthemums expressing a mutated ethylene receptor gene to assess this involvement. The transgenic lines showed reduced ethylene sensitivity: ethylene caused leaf yellowing in wild-type chrysanthemums, but the leaves remained green in the transgenic lines. Extension growth and flowering of wild-type and transgenic lines varied between temperatures: at 20degC, the transgenic lines showed the same stem elongation and flowering as the wild type. At cooler temperatures, the wild type formed rosettes with an inability to flower and entered dorman