Ethylene Biosynthesis, Perception, and Sinaling-Related Gene Expression During Papaya Fruit (Carica papaya L.) Ripening

Wu, Ping-Fang
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A gaseous plant growth regulator, ethylene, plays an important role during plant growth and development and includes ethylene dependent fruit ripening. Fruit ripening is a series of biochemical, physiological and structural events that lead to maturity. Papaya (Carica papaya) is a typical climacteric fruit that which performs dramatic changes in color, texture, and flavor during fruit ripening. Fruit ripening process was considered highly related to the biosynthesis of ethylene which is mainly controlled by the SAM (S-Adenosyl methionine synthetase. Methionine adenosyltransferase), ACS (1- aminocyclopropane-l-carboxylic acid synthase), and ACO (1-aminocyclopropane-lcarboxylic acid oxidase) genes. In addition, the ethylene receptors in Arabidopsis and tomato have been shown to be involved in effecting fruit development and the timing of fruit ripening, respectively. Ethylene signaling transduction gene expressions, such as CTR (Constituted Triple Response), EIN2 (Ethylene Insensitivity 2), EIN3/EILs (Ethylene Insensitivity 3 and Ethylene Insensitivity 3- Like proteins) and ERF (Ethylene Response Factors) are also involved in the ripening processing. The full sequencing of the papaya genome, coupled with microarray technology, provides a chance to determine the expressions of genes at specific fruit developmental stages. Thirty-four genes involved in ethylene biosynthesis (SAM, ACS, ACO, and ETO), perception (ETR) and the signaling transduction pathway (RAN, CTR, EIN2, EIN3/EIL1, and ERF) were selected from 24,421 predicted genes in papaya genome. Four developmental stages: mature green, the 25% color, 80% color and 100 color stage were investigated. The ethylene biosynthesis genes seemed to be expressed before the initiation of fruit ripening, and declined once the System 2 ethylene production was started. The ethylene receptors have been shown to be involved in the regulation of tomato fruit ripening. Our results showed fewer number of ethylene receptors than in tomato and Arabidopsis and also showed a possible role in controlling the initiation of papaya fruit ripening. More genes related to the ethylene signaling transduction pathway did not change in expression level during fruit maturation and ripening, except for CTR and RAN. CTRl and RAN are considered as negative regulator in the signaling pathway and a copper transporter associated with ethylene receptor respectively. In papaya, changes in sugar content, production of flavor constituents and rapid pulp softening during ripening can affect the quality and cause serious postharvest losses during transportation and storage. Knowing the ethylene-related gene expression during papaya fruit development and ripening may be helpful in regulating the timing of ripening, in order to control the fruit quality and reduce the postharvest losses.
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