Auxin is part of the regulatory circuit that sustains the ripening initiation in apple fruit

The wide array of changes occurring during the ripening process of fleshy fruits can be considered as a series of events genetically determined and finely tuned by the action of a complex hormonal interplay. In climacteric fruit, such as apple, the effect of ethylene has been traditionally accounted for the ripening progression, albeit auxin, originally known as an ethylene antagonist, is now considered as an active player in the initiation of the maturation phase. To investigate the complex synergistic regulation occurring between these hormones we profiled the ethylene production and auxin accumulation in three apple cultivars that exhibit different ripening behavior, during the postharvest stage and after treatment with 1-methylcyclopropene (1-MCP), an ethylene competitor. These data were correlated with the whole transcriptome analyzed by RNA-seq, dissecting the effect of the interference acted by 1-MCP on the ethylene perception machinery. Interestingly, the ripening blockage induced an initial accumulation of auxin, highlighting the connection existing between these two hormones. This phenomenon was further supported by a candidate-gene based expression analysis that revealed an antagonistic activation of GH3 and ILL genes, encoding key steps in the regulation of the auxin homeostasis. The data we provided underline the auxin attribute to mediate the ripening initiation, through a de-novo synthesis and de-conjugation of auxin as a tentative to restore the normal ripening physiology, when ethylene metabolism is compromised at the onset of the climacteric ripening