Evidences from Individual Studies of Ethylene and 1-MCP Treatment Prove that Volatile Biosynthesis Is Regulated by Ethylene in Apple (cv. 'Golden Delicious')

Ethylene plays an important role in regulating fruit ripening and senescence and directly influences the development of the eating quality of fresh apples, including appearance, color, texture, and flavor. To better understand the regulation of volatile biosynthesis during fruit ripening and to examine the influence of ethylene on expression of volatile biosynthesis genes in fruit, apples harvested at the pre-climacteric stage were allowed to ripen naturally; or ripening was either stimulated by treatment with 36 µL·L^-1 ethylene for 24 or inhibited with 1-MCP treatment (1 µL·L^-1 for 24 h). Real-time qPCR was used to investigate gene expression in relation to volatile biosynthesis up to 21 days after ethylene treatment and up to 43 days after 1-MCP treatment. Twenty-two genes proposed to be involved in volatile biosynthesis in relation to branched amino acids and fatty acids biosynthesis and metabolisms were selected to monitor gene expression. Through statistical analysis, including ANOVA and principle component analysis (PCA), among the 22 volatile biosynthesis genes, 17 genes changed significantly. Genes encoding BCAT, ArAT and AADC, which may involve in the initial steps in the catabolism of amino acids into aroma volatiles, are up-regulated during ripening process and enhanced after ethylene treatment. Genes related to fatty acid synthesis (ACPs, MCAT, ACPD), the LOX pathway (LOX, HPL, ADH3), the pyruvate pathways (PDC2), β-oxidation (ACAS, CAT, APX, ECH, ACAD), and the final step of volatile ester biosynthesis (AATs) also showed similar increasing patterns during ripening and in response to ethylene treatment. AOS, ADH1, KAT, and BCAT2 decreased with ethylene treatment. Treatment with 1-MCP and ethylene generally produced opposite effects, which provides additional evidence that regulation of these genes is ethylene dependent. Analysis and identification of significant gene expression revealed that volatile biosynthesis, especially fatty acids biosynthesis and metabolism in apple fruit is associated with fruit ripening and responsive to ethylene treatment. The understanding of significant changes of these genes and their function may help to explore mechanisms that control apple fruit ripening and their response to exogenous ethylene during ripening and senescence.