Mechanisms regulating bitter pit development in 'Greensleeves' apples with suppression of ethylene biosynthesis

摘要

Bitter pit (BP) is a physiological disorder in apple believed to be triggered by low fruit Ca2+ content and enhanced by ethylene. The objectives of this study were to understand the role of ethylene, Ca2+ binding to the cell wall and the ratio betweenCa2+ and other nutrients (Mg2+, K+ and N) on BP development in wild-type (GS) and ethylene-suppressed (68G and 103Y) 'Greensleeves' apples. The transgenic line 68G is suppressed for 1-aminocyclopropane- 1-carboxylic acid (ACC) oxidase (ACO), and line 103Y is suppressed for ACC synthase (ACS). Suppression of ethylene biosynthesis reduced the incidence and severity of BP, and decreased the rate of fruit softening and the degree of cell-wall pectin de-esterification in fruit cortical tissue. Lower ethylene biosynthesis, in ethylene-suppressed genotypes, had no effect on Ca2+, Mg2+, K+ or N concentrations in cortical tissue, on Ca-+ concentrations in the cell wall, on expression of pectin methylesterases {PME1 and PME2) or on percentages of cortical Ca2+bound to the cell wall. In all genotypes, fruit with BP had lower Ca2+ and higher Mg2+ concentrations and higher Mg2+/Ca2+ ratios in cortical tissue, as well as higher percentages of cortical Ca2+ bound to the cell wall, compared with fruit without BP. Fruit with and without BP had similar cell-wall Ca2+ concentrations, and cell-wall content and levels of PME expression in cortical tissue. The results indicate that high levels of ethylene biosynthesis, Ca2+ binding to the cell wall and Mg2+ in corticaltissue can enhance fruit susceptibility to BP.