Interrelationship between polyamine and ethylene in 1-methylcyclopropene treated apple fruits after harvest

To investigate the interrelationship between polyamine and ethylene, apple fruits (Malus domestica Borkh.) were treated with 1-methylcyclopropene (1-MCP). Without 1-MCP treatment, ethylene production of apple fruits rapidly increased to maximum level at 4 days after treatment (DAT) and kept its level until 21 DAT, then gradually decreased. Accordingly, fruit firmness was progressively reduced until 26 DAT. On the other hand, ethylene production in 1-MCP-treated fruits was almost completely repressed until 21 DAT, retaining fruit firmness. This ethylene inhibition by 1-MCP was accompanied by the reduction of transcript accumulations for 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO). The expression of MdSAMDC1 showed no differences between 1-MCP-treated and control fruits, whereas that of MdSAMDC2 was rather repressed in 1-MCP-treated fruits compared with the control. There were no statistically significant differences of free spermidine (Spd) and spermine (Spm) titers between fruits with and without 1-MCP treatment; however, Spd titer in 1-MCP-treated fruits temporally increased in the early DAT, then was adjusted to similar level as that in control, whereas Spm level was always higher in 1-MCP-treated fruits than in the control, indicating that Spd may be under a tight homeostatic regulation, whereas Spm may not. Therefore, the results demonstrated that the antagonistic relationships between polyamine and ethylene might exist transiently in the early days after 1-MCP treatment, but disappear later possibly because of the polyamine homeostasis mechanism and the extension of apple shelf life by 1-MCP might be owed little to the role of polyamines and be mainly ascribed to the inhibitory effects of ethylene on modulation of all wall.