Abstract
Papaya is a fleshy fruit that undergoes fast ethylene-induced modifications. The fruit becomes edible, but the fast pulp softening is the main factor that limits the post-harvest period. Papaya fast pulp softening occurs due to cell wall disassembling coordinated by ethylene triggering that massively expresses pectinases. In this work, RNA-seq analysis of ethylene-treated and non-treated papayas enabled a wide transcriptome overview that indicated the role of ethylene during ripening at the gene expression level. Several families of transcription factors (AP2/ERF, NAC, and MADS-box) were differentially expressed. ACO, ACS, and SAM-Mtase genes were upregulated, indicating a high rate of ethylene biosynthesis after ethylene treatment. The correlation among gene expression and physiological data demonstrated ethylene treatment can indeed simulate ripening, and regulation of changes in fruit color, aroma, and flavor could be attributed to the coordinated expression of several related genes. Especially about pulp firmness, the identification of 157 expressed genes related to cell wall metabolism demonstrated that pulp softening is accomplished by a coordinated action of several different cell wall-related enzymes. The mechanism is different from other commercially important fruits, such as strawberry, tomato, kiwifruit, and apple. The observed behavior of this new transcriptomic data confirms ethylene triggering is the main event that elicits fast pulp softening in papayas.
Highlights
In addition to fruit ripening incrementing sensory and nutritional quality, it increases fruit susceptibility to physical damage as the pulp and skin soften [1]
Fleshy fruit characterized by fast pulp softening in the postharvest period [24]
Ethylene treatment triggered the self-production of endogenous ethylene in the treated group after 12 h and 24 h, while in the control group, similar levels were maintained over time (Figures 1 and S2)
Summary
In addition to fruit ripening incrementing sensory and nutritional quality, it increases fruit susceptibility to physical damage as the pulp and skin soften [1]. The transduction pathway initiates when ethylene binds to a specific receptor, that will initiate the signaling cascade by releasing CTR1 and EIN2 binding, activating several transcription factors (EIN3/EIL1 and ERFs). These transcription factors regulate genes underlying ripening-related traits, such as pulp firmness [2,3,4,5,6]. Ethylene exogenous treatment rapidly increases the transcription of all these genes regulated by this hormone [5,6,7,8] In this way, an effective methodology to investigate the expression profile during ripening is transcriptomics, which identifies the complete set of a sample’s mRNA content. Some tropical fruits of great commercial importance, such as papaya, are still restricted to a superficial knowledge on a global expression profile, with most research focused on the study of the expression of specific enzymes [1,4,15,16]
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