Gene Expression Related to Allergens in Apple Fruit During Ripening and Senescence

Apple fruit (Malus domestica Barkh, ‘Golden Delicious’) is a well known nutritious fruit and its consumption is highly recommended for a healthy diet. Recent findings reported that  apples consumption can cause allergic reactions to some people, especially northern and central European populations, because of the presence of allergens. Allergens are the unique proteins present in plants and many fruit that may cause an overreaction of human immune system.  Investigations on apple fruit allergens include allergenicity testing, cultivar comparisons and cultural practices have been conducted by some research groups. Our own proteomic studies demonstrated  the presence of many allergen isomers in apples that are also ripening dependent and under the influence of ethylene. In this study, apple fruit harvested at the pre-climacteric stage were allowed to naturally ripen or ripening was stimulated by treatment with 36 µL/L ethylene for 24 h.  Postharvest physiological indices including respiration, ethylene production and chlorophyll fluorescence were monitored at day 0, 7, 14 and 21.  Real-time PCR was employed to investigate gene expression in relation to apple allergens at these stages. The study was repeated over two seasons. After efficiency tests for all designed primers, 16 genes from four major allergen isomers were selected to monitor gene expression. Through statistical analysis, including ANOVA and principle component analysis (PCA), 6 genes were found to change significantly during ripening. The Mal d1-1, Mal d1-4, Mal d1-associated protein (MdAP), Mal d4.01, Mal d4.02 and Mal d4.03 genes were significantly up-regulated in fruit during ripening and after ethylene treatment. A significant increase in expression of Mal 1.01, Mal d1d, Mal d1e and Mal d1g in ethylene treated fruit was also found, while the Mal1.04, Mal d2.02 and Mal d2.03 genes decreased. In contrast, the Md1-3, Mal d2.01, Mal d3.01 and Mal d3.02 genes were not affected. Analysis and identification of gene expression during apple fruit ripening and senescence and in response to ethylene treatment demonstrated the complexity and dynamic changes of the transcriptional profiles of apple allergens. The characterizations  of these allergens during fruit ripening and senescence are under further investigation through genomic and proteomic studies. Understanding the expression of these genes and their biological functions will aid in the control and regulation of the quality and safety of apple fruit.