Fast and Furious: Ethylene-Triggered Changes in the Metabolism of Papaya Fruit During Ripening

Abstract

Papaya is a climacteric fleshy fruit characterized by fast ripening after harvest. During the relatively short postharvest period, papaya fruit undergoes several changes in metabolism that result in pulp softening and sweetening, as well as the development of a characteristic aroma. Since papaya is one of the most cultivated and appreciated tropical fruit crops worldwide, extensive research has been conducted to not only understand the formation of the quality and nutritional attributes of ripe fruit but also to develop methods for controlling the ripening process. However, most strategies to postpone papaya ripening, and therefore to increase shelf life, have failed to maintain fruit quality. Ethylene blockage precludes carotenoid biosynthesis, while cold storage can induce chilling injury and negatively affect the volatile profile of papaya. As a climacteric fruit, the fast ripening of papaya is triggered by ethylene biosynthesis. The generation of the climacteric ethylene positive feedback loop is elicited by the expression of a specific transcription factor that leads to an up-regulation of 1- aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC-oxidase (ACO) expression, triggering the system II ethylene biosynthesis. The ethylene burst occurs about 3 to 4 days after harvest and induces pectinase expression. The disassembling of the papaya cell wall appears to help in fruit sweetness, while glucose and fructose are also produced by acidic invertases. The increase in ethylene production also results in carotenoid accumulation due to the induction of cyclases and hydroxylases, leading to yellow and red/orange-colored pulp phenotypes. Moreover, the production of volatile terpene linalool, an important biological marker for papaya’s sensorial quality, is also induced by ethylene. All these mentioned processes are related to papaya’s sensorial and nutritional quality. We describe the understanding of ethylene-triggered events that influence papaya quality and nutritional traits, as these characteristics are a consequence of an accelerated metabolism during fruit ripening.

Systems Biology Applied to the Study of Papaya Fruit Ripening: The Influence of Ethylene on Pulp Softening

“…Therefore, it is reasonable to establish that this classification must be made upon fruit’s sensitivity to ethylene [ 47 ]. Ethylene is a major regulatory factor in climacteric fruit, not only in the ripening process but also in growth and development; it triggers a cascade of metabolic events that eventually result in cell wall disassembly [ 7 ].…”

“…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.…”

“…Because papaya is a climacteric fruit, the pulp and peel softening occur relatively quickly, and management techniques are limited in terms of preventing and minimizing damage [ 8 ]. Elucidating the structure, functions, and gene expression regulation of the plant cell wall during fruit ripening is essential as commercial interests in postponing the shelf-life storage are crucial to maintaining the sensorial fruit quality [ 7 ]. Although papaya has most of its genome sequenced, many studies focused solely on changes in the cell wall that culminate in pulp softening [ 19 , 20 , 21 , 22 ].…”

Quantitative Ubiquitylomic Analysis of the Dynamic Changes and Extensive Modulation of Ubiquitylation in Papaya During the Fruit Ripening Process

“…Recently, many ethylene biosynthesis pathway-related genes have been isolated from papaya ( Li et al, 2013 ; Shen et al, 2017 ). In papaya fruit, 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase is an important enzyme involved in triggering the system II ethylene biosynthesis during ripening ( Fabi and do Prado, 2019 ). Many years ago, a genomic clone of ACC oxidase from papaya was identified ( Lopez-Gomez et al, 2004 ).…”

The Emergence and Impact of Ethylene Scavengers Techniques in Delaying the Ripening of Fruits and Vegetables

“…Additionally, the papayas reduced the amount of ethylene generated via autocatalysis. Enzymatic conversion occurs in the membrane of plant cells and regulates the production of ethylene [97]. The ethylene concentration within the intracellular environment (endogenous ethylene) increases first, accelerating the fruit's respiration rate.…”